SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES ? NUMBER 21
Schreibersite Growth and Its
Influence on the Metallography
of Coarse-Structured
Iron Meteorites
Roy S. Clarke, Jr.
and Joseph I. Goldstein
SMITHSONIAN INSTITUTION PRESS
City of Washington
1978
ABSTRACT
Clarke, Roy S., Jr., and Joseph I. Goldstein. Schreibersite Growth and Its
Influence on the Metallography of Coarse-Structured Iron Meteorites. Smithson-
ian Contributions to the Earth Sciences, number 21, 80 pages, 28 figures, 20 tables,
1978. ?The role that schreibersite growth played in the structural development
process in coarse-structured iron meteorites has been examined. The availability
of many large meteorite surfaces and an extensive collection of metallographic
sections made it possible to undertake a comprehensive survey of schreibersite
petrography. This study was the basis for the selection of samples for detailed
electron microprobe analysis. Samples containing representative structures
from eight chemical Groups I and IIAB meteorites were selected.
Electron microprobe traverses were made across structures representative of
the observed range of schreibersite associations. Particular emphasis was placed
on schreibersite-kamacite interface compositions. An analysis of these data has
led to a comprehensive description of the structural development process.
Massive schreibersite, one of the four major types of schreibersite encoun-
tered, may be accounted for by equilibrium considerations. Subsolidus nuclea-
tion and growth with slow cooling from temperatures at least as high as 850? C,
and probably much higher, explain the phase relationships that one sees in
meteorite specimens. The retention of taenite in the octahedrites establishes
that bulk equilibrium did not extend as low as 550? C. Schreibersite undoubtedly
continued in equilibrium with its enclosing kamacite to lower temperatures.
A second type of schreibersite to form is homogeneously nucleated rhabdite.
It nucleated in kamacite in the 600? C temperature range, either as a conse-
quence of low initial P level or after local P supersaturation developed following
massive schreibersite growth.
A third type of schreibersite is grain boundary and taenite border schreiber-
site. It formed at kamacite-taenite interfaces, absorbing residual taenite. Nuclea-
tion took place successively along grain boundaries over a range of temperatures
starting as high as 500? C or perhaps slightly higher. Grain boundary diffusion
probably became an increasingly important factor in the growth of these
schreibersites with decreasing temperature.
The fourth type of schreibersite is microrhabdite. These schreibersites
nucleated homogeneously in supersatuated kamacite at temperatures in the 400?
C range or below.
P diffusion controlled the growth rate of schreibersite. The Ni flux to a
growing interface had to produce a growth rate equal to that established by the
P flux. This was accomplished by tie line shifts that permitted a broad range of
Ni growth rates, and these shifts account for the observed range of Ni
concentrations in schreibersite. Equilibrium conditions pertained at growth
interfaces to temperatures far below those available experimentally. Kinetic
factors, however, restricted mass transfer to increasingly small volumes of
material with decreasing temperature.
OFFICIAL PUBLICATION DATE is handstamped in a limited number of initial copies and is recorded inthe Institution's annual report, Smithsonian Year. SERIES COVER DESIGN: Aerial view of Ulawun
Volcano, New Britain.
Library of Congress Cataloging in Publication DataClarke, Roy S., Jr.
Schreibersite growth and its influence on the metallography of coarse-structured iron meteorites.(Smithsonian contributions to the earth sciences; no. 21)
Bibliography: p.1. Schreibersite. 2. Metallography. 3. Meteorites, Iron. I. Goldstein, Joseph I., joint author.
II. Title. III. Series: Smithsonian Institution. Smithsonian contributions to the earth sciences;no.21.
QEl.S227no. 21 [QE395] 550'.8s [552] 77-10840
Contents
Page
Introduction 1
Acknowledgments 2
Historical Background 2
Introduction 2
19th-century Studies of Schreibersite 3
Early 20th-century Studies of Iron Meteorites 4
Modern Work on Metallic Phases of Iron Meteorites 5
Related Materials 8
Experimental 9
Results 11
Coahuila 13
Bellsbank 16
Ballinger 18
Santa Luzia 21
Lexington County 25
Bahjoi 29
Goose Lake 30
Balfour Downs 31
Discussion 37
Equilibrium Considerations of Phase Growth 37
Coahuila 40
Ballinger 42
Santa Luzia 43
Lexington County and Bahjoi 44
Goose Lake and Balfour Downs 45
Bellsbank 46
Low Temperature Phase Growth and the Equilibrium Diagram 48
Diffusion-Controlled Schreibersite Growth 54
Cooling Rate Variations 58
Interface Data and Schreibersite Distribution 59
Interface Data and the a/a + Ph Boundary 66
Schreibersite with Cohenite Borders 70
Summary and Conclusions 73
Appendix 75
Literature Cited 77
Schreibersite Growth and Its
Influence on the Metallography
of Coarse-Structured
Iron Meteorites
Roy S. Clarke, Jr. and Joseph I. Goldstein
Introduction
Meteorites are currently understood to be the
oldest rocks available for scientific study, contain-
ing components and structures that span the pe-
riod from the final stages of solar nebula conden-
sation to the present (Anders, 1962, 1971; Gross-
man and Larimer, 1974; Wasson, 1974). They are
fragments of parent bodies that accreted from
preexisting aggregations of material during the
period of planetary system formation some 4.6
billion years ago. These parent bodies were subse-
quently disrupted into smaller objects that then
had independent existences in space. Individual
fragments eventually intercepted the earth and
landed as recoverable meteorites. Meteorite struc-
tures and compositions have undergone varying
degrees of modification while resident in their
parent bodies, as small bodies in space, on their
passage through the atmosphere, and on landing
on the earth's surface. Further changes result
from long residence time on the ground and may
continue during storage in collections. Bearing
evidences of these complex histories, meteorites
are samples not only from a far distant place but
also from a far distant time, having been preserved
in a remarkably gentle environment when com-
Roy S. Clarke, Jr., Department of Mineral Sciences, National Museum
of Natural History, Smithsonian Institution, Washington, D. C.
20560. Joseph I. Goldstein, Department of Metallurgy and Materials
Sciences, Lehigh University, Bethlehem, Pennsylvania 18015.
pared to terrestrial or lunar rocks. As a conse-
quence, meteorites have a unique place in the
study of the development of the planetary system,
yielding information that is available from no
other source.
Iron meteorites represent only a small fraction
of the meteorites that have been observed to fall,
about five percent. Fortunately, many specimens
from ancient falls have been preserved under such
conditions that deterioration has not been severe
and are available for study (Buchwald, 1976). If
this were not the case, our view of iron meteorite
compositions and structures would be much nar-
rower than it is. Only one of the meteorites used
in this study is an observed fall.
The spectacular metallographic structures re-
vealed on prepared surfaces of iron meteorites
have fascinated scientist and dilettante alike for
more than 160 years (Perry, 1944). The Widman-
statten pattern is the historical distinguishing char-
acteristic of the populous octahedrite classes of
iron meteorites (Goldstein and Axon, 1973). At an
early date this structure was understood to result
from very slow cooling at relatively low tempera-
tures in the meteorite parent body. The tempera-
ture range through which this structure develops
is now believed to be from 700? to 350? C, temper-
atures well below those where silicate systems un-
dergo change. Interpretation of metallographic
structures, therefore, gives information on a late,
low temperature period in the history of a parent
1
SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
body, information that is not readily available
from other types of meteorite studies.
Modern interpretations of iron meteorite struc-
tures have been based on the Fe-Ni equilibrium
diagram (Goldstein and Olgivie, 1965a), as it was
the only applicable system known with sufficient
accuracy in the low temperature range required.
The Widmanstatten pattern, therefore, resulted
simply from taenite transforming to kamacite with
decreasing temperature as a solid state, diffusion
controlled reaction. Recently the iron-rich corner
of the Fe-Ni-P equilibrium system has been deter-
mined experimentally down to 550? C, leading to
new possibilities for investigating meteorite struc-
tures (Doan and Goldstein, 1970). In this study
the metallography of a group of schreibersite-con-
taining coarse-structured iron meteorites has been
examined using this low temperature Fe-Ni-P dia-
gram. It will be shown that equilibrium growth at
higher temperatures combines with diffusion-con-
trolled growth at lower temperatures to explain
observed structure more comprehensively than
was possible using the simpler system. Schreiber-
site growth becomes an integral part of the overall
structural development process, and schreibersite
may no longer be considered an "inclusion" that
can be safely ignored when structural development
is discussed.
ACKNOWLEDGMENTS
A number of colleagues and associates have
contributed invaluable assistance throughout the
course of this study. The support and encourage-
ment of departmental chairman William G. Mel-
son and his predecessor Brian Mason are acknowl-
edged with sincere thanks. Erik Randich provided
penetrating discussion of many of the problems
studied here, and his analysis of ternary diffusion
as applied to schreiberate growth and meteorite
cooling rates was made available to us prior to
publication. A. D. Romig, Jr., of Lehigh Univer-
sity made available to us data on the low tempera-
ture Fe-Ni-C system prior to publication. Eugene
Jarosewich and Charles R. Obermeyer III fur-
nished unstinting help with the electron micro-
probe work. Grover C. Moreland and Richard
Johnson prepared many excellent polished sec-
tions. Ann Garlington and Kathy P. Porter were
scrupulous in their attention to the details of
manuscript preparation. Dante Piacesi, Jr., of the
Office of Computer Services was responsible for
the preparation of data reduction programs. A
grant from the Secretary's Fluid Research Fund
supported a brief visit to Cambridge University,
Cambridge, England, in the fall of 1974, where a
serious beginning was made on the interpretation
of the data. The support of the Geochemistry
Program, Division of Earth Sciences, National Sci-
ence Foundation, through Grant No. DES 74-
22518 is also acknowledged. The acronym USNM
(for the former United States National Museum)
is used for catalog numbers in the National Mu-
seum of Natural History, Smithsonian Institution.
This paper was presented to the George Wash-
ington University, Washington, D.C., by R. S.
Clarke, Jr. in partial fulfillment of the require-
ments for the Ph.D. degree.
Historical Background
INTRODUCTION
Meteorites, with their dramatic arrivals on earth,
exotic origins, and unusual structures, have at-
tracted much more than routine scientific interest
ever since they became respectable objects of study
at the end of the 18th century. Many of the leaders
in the development of 19th-century physical sci-
ence made significant contributions to their study,
and several of them will be mentioned below. The
older literature is extensive, but it is mainly obso-
lete for other than descriptive and historical pur-
poses. A critical review of this material was not
attempted, but a survey of selected early develop-
ments will be given. Scientific interest in schreiber-
site dates from near the beginning of meteorite
studies, and it is interesting to place modern work
in this perspective.
The first major development in the metallogra-
phy of meteorites was the discovery of the phe-
nomenon that has become known as the Widman-
statten pattern. This structure was first discovered
by William Thomson, an Englishman living in
exile in Naples. He published a French version of
his findings along with a good illustration in a
Swiss journal in 1804. What appears to be an
identical paper, only in Italian, dated 6 February
1804, was published in Siena four years later (G.
Thomson, 1804, 1808). Modern writers on meteor-
itics have been unaware of the earlier paper by
NUMBER 21
Thomson, and this has led to confusion as to
whether Thomson or Von Widmanstatten really
had priority. The record seems to be unambigu-
ously in favor of Thomson. Marjorie Hooker of
the U.S. Geological Survey retrieved this 1804
paper, and its existence was mentioned in a foot-
note in a biographic study of Thomson by C. D.
Waterston (1965: 133). The writer ran across Wa-
terston's paper among unindexed material in the
F. A. Paneth meteorite literature collection that
was deposited in the Smithsonian Institution in
1974.
Alois von Widmanstatten, Director of the Impe-
rial "Fabrik-Produkten-Cabinett" (Industrial Prod-
ucts Collection), Vienna, discovered the phenom-
enon that now bears his name independently in
1808. He devoted many years to its study and
circulated his observations privately, drawing the
phenomenon to the attention of students of mete-
orites. Von Widmanstatten's observations were fi-
nally published in 1820 by his co-worker, Carl von
Schreibers (1820). The history of the discovery of
the Widmanstatten pattern has been reviewed by
Paneth (1960) and Smith (1960, 1962). Smith's
review contains excellent reproductions of early
prints and a lengthy translation of Schreibers'
description of Widmanstatten's work. These early
reproductions of iron meteorite structures by Wid-
manstatten not only represent an advance in the
science of meteoritics but also one in the art of
printing.
During the 19th century descriptive studies of
meteorites advanced with the broadening base of
understanding in mineral chemistry and related
physical science. Many of the iron meteorites
known today were already represented in collec-
tions, and a number of them had been carefully
described in the literature. The major phases of
iron meteorites had been characterized and given
mineral names: kamacite (low-nickel Ni-Fe, fer-
rite, a-iron, b.c.c), taenite (high-nickel Ni-Fe,
austenite, y-iron, f.c.c), plessite (fine-grained in-
tergrowth of kamacite and taenite), troilite (FeS),
cohenite (Fe3C), and schreibersite ((Fe,Ni)3P) were
all known and their bulk compositions understood.
Accessory minerals such as daubreelite (FeCr2S4),
chromite (FeCr2O4), and graphite (C) were also
known. The Widmanstatten pattern was under-
stood to have formed by slow cooling under crys-
tallographicly controlled conditions, kamacite
plates forming parallel to the faces of an octahe-
dron, giving the name octahedrite to those meteor-
ites that displayed this pattern on polished and
etched surfaces.
19TH-CENTURY STUDIES OF SCHREIBERSITE
Schreibersite was first characterized in 1832 by
the great Swedish chemist J. J. Berzelius (1832a,b,
1833, 1834). He isolated brittle, silver-white grains
of a magnetic material from an acid insoluble
residue of the B?humilitz, Bohemia, coarse octa-
hedrite. Berzelius' elaborate analytical procedure
yielded a surprisingly good analysis for the time,
demonstrating that his material was an iron-nickel-
phosphorus compound containing about 14 per-
cent phosphorus and considerably more nickel
than the bulk of the meteorite. In 1834 he reported
similar analyses for schreibersite from the Elbo-
gen, Bohemia, medium octahedrite and the Kras-
nojarsk, Siberia, pallasite (the historic Pallas Iron).
It is also interesting to note that Berzelius' concept
of the Widmanstatten pattern included the ele-
ment phosphorus, as is indicated by the following
quotation:
Es mochte wahrscheinlich seyn, dass die sogenannten Widman-
stddtschen Figuren einer, der hier analysirten Schuppen analo-
gen, Verbindung zwischen Eisen, Nickel und Phosphor, zuzu-
schreiben seyen, welches zu erforschen ich aus Mengel an
Material verhindert bin.
Berzelius (1832b:297)
Early descriptions of schreibersite occurrences
were confounded by inadequate separation tech-
niques, poor analytical methods, and complica-
tions arising from both the varied morphology
and the wide range in composition that is typical
of this mineral. C. U. Shepard (1846, 1853) ana-
lysed schreibersite and attempted to assign a min-
eral name to it. His suggestion, however, was
supported by inadequate data. Credit for naming
schreibersite belongs to A. Patera, whose recom-
mendation was reported by Haidinger (1847). The
name honors Professor Karl von Schreibers (1775-
1852), director of the Imperial Cabinet, Vienna, a
pioneer worker on meteorites and colleague of
Von Widmanstatten.
The writings of J. Lawrence Smith, a distin-
guished chemist of the middle part of the last
century (Silliman, 1886; Phillips, 1965), include
numerous examples of early work on meteorites.
SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
His description of the Tazewell, Tennessee, finest
octahedrite contains a lengthy discussion of his
observations (Smith, 1855). At this early date he
recognized the name schreibersite and applied it
to the correct mineral. He reports having identi-
fied schreibersite by visual inspection in a number
of iron meteorites from "the Yale College Cabi-
net," where it had previously been thought to be
pyrite. Smith's low nickel value for Tazewell
schreibersite is suspect and his attempt at a for-
mula was unsuccessful, but he clearly recognized
the species and understood which elements were
its major constituents. This work was undoubtedly
done, at least in part, in the chemical laboratory
of the Smithsonian Institution. This laboratory
was organized by Professor Smith in 1854, and
Secretary Joseph Henry reported that during the
year "he also made a series of analyses of meteor-
ites, among which were fourteen specimens from
the cabinet of James Smithson, the founder of the
Institution" (Goode, 1897:614).
Confusion in nomenclature, however, persisted
in the literature for many years. Von Reichenbach
(1861) discussed schreibersite under the descriptive
term "Ganzeisen" and also used the name lamper-
ite. Both of these names are now part of the
synonymy of schreibersite. Rose (1865) introduced
the synonym rhabdite into the literature, a name
derived from the Greek word for "rod." This
term is still in common use by meteoriticists, but
only when morphological distinctions are of inter-
est. Rhabdite is the term used for the small schrei-
bersite crystals occurring in the kamacite of iron
meteorites. In polished sections these small schrei-
bersite crystals have rhombohedral cross sections
that in some cases are elongated, suggesting rods
or needles.
The early literature on meteoritic phosphides,
schreibersite and rhabdite, was reviewed in detail
by Cohen (1894). Included were a number of
analyses of schreibersite and rhabdite isolated
from various meteorites by Cohen and his co-
workers, as well as selected analyses from the
literature. This work established that schreibersite
and rhabdite are morphological variations of the
same mineral species, the rhabdite form having a
somewhat higher nickel concentration than that
observed in large schreibersite inclusions. A simi-
lar review was given a few years later by Farrington
(1915), using much of the same data Cohen used
but with several later analyses.
The 19th-century work on meteoritic phos-
phides that culminated with Cohen's and Farring-
ton's reviews was limited by the capabilities of
classical descriptive mineralogy and classical ana-
lytical chemistry. A plateau had been reached in
descriptive iron meteorite studies in general that
became an accepted norm that was exceeded by
few workers during the first half of the 20th
century. Most iron meteorite descriptions pub-
lished during this period employed neither con-
cepts nor techniques beyond those available to
Cohen. The descriptive literature grew in volume
and in quality, but new interpretations lagged.
Powerful petrographic techniques were being de-
veloped and used by metallurgists and ore micro-
scopists, but they were not much used by students
of iron meteorites. The availability of good trans-
mitted light microscopes in the hands of petrogra-
phers who were accustomed to working with sili-
cate minerals resulted in stony meteorites being
preferred subjects of investigation.
EARLY 20TH-CENTURY STUDIES OF
IRON METEORITES
The emergence of modern physical chemistry
and physical metallurgy during the decades adja-
cent to the turn of the century had a marked
effect on the future course of iron meteorite
investigations. Descriptive studies following the
19th-century pattern continued to comprise the
bulk of the published literature and, indeed, con-
tinue to be published today as essential documen-
tation. More modern experimental approaches,
however, became significant, resulting in a body
of knowledge more readily interpretable in terms
of origins and developmental histories of meteor-
ites than was previously possible.
Actually, the roots of this work go back to a
much earlier period. Pioneering synthetic experi-
ments were made by Michael Faraday as early as
1820. He prepared metals of meteoritic composi-
tions in the course of an investigation to improve
steel (Stodart and Faraday, 1820). At a later pe-
riod, Daubree (1868) described synthetic experi-
ments related to meteorites. Sorby (1877, 1887),
the father of metallography, made astute observa-
tions, some of which were based on artificial alloys
of meteoritic compositions. Unfortunately, Sorby's
pioneering efforts in the metallography of mete-
orites were not pursued. The following quotation
NUMBER 21
from his 1877 paper reflects his perceptive under-
standing of the structures he observed:
These facts clearly indicate that the Widmanstatt's Figuring is
the result of such a complete separation of the constituents
and perfect crystallisation as can occur only when the process
takes place slowly and gradually. . . . Difference in the rate of
cooling would serve well to explain the difference in the
structure of some meteoritic iron which do not differ in
chemical composition; ... we are quite at liberty to conclude
that they may have been melted ....
During the same period the well-known French
mineralogist, Meunier (1880), reported the results
of his synthetic work on meteoritic systems. A
paper by Brezina (1906) is an excellent summariz-
ing statement of this early period. It contains a
number of good photomicrographs of iron mete-
orites, an unusual feature for papers of the time,
and it discusses the development of these struc-
tures in terms of exsolution phenomena (solid
state transformation). He also gave a succession
for the formation of the constituents of iron mete-
orites, starting with olivine in pallasites: olivine,
daubreelite, troilite, graphite, schreibersite, co-
henite, chromite, swathing kamacite, kamacite
bands, taenite, and plessite. This is a remarkably
informative listing for the time, demonstrating
careful petrographic observation and keen insight.
One of the perennial problems of meteorite
research has been that of drawing the attention of
those with the knowledge and facilities to make
significant contributions to the meteorite speci-
mens and the important and interesting scientific
problems they represent. Despite Sorby's enthusi-
asm for meteorite studies, metallurgists showed
little interest and had to be attracted into the
field. An example of this process is an invited
paper read before the Vienna meeting of the Iron
and Steel Institute in 1907. Professor Frederick
Berwerth's (1907) paper, "Steel and Meteoritic
Iron," was based on his knowledge of the meteor-
ite collection of the Imperial Natural History Mu-
seum, Vienna. It was an open invitation to metal-
lurgists to become involved in this field of study.
During the early decades of this century, syn-
thetic studies were combined with analyses based
on equilibrium phase diagrams. The work of Os-
mond and Cartaud (1904), Benedicks (1910), and
Belaiew (1924) are examples. Unfortunately, the
tools were not available at the time to determine
sufficiently accurate equilibrium diagrams. As a
result, many of the conclusions drawn were seri-
ously flawed. These studies, however, pointed the
way for future workers.
The introduction of X-ray diffraction analysis
into metallurgical and mineralogical research in
the 1920s had an important influence on meteorite
studies. Young (1926) published an early study
establishing that kamacite precipitates from tae-
nite, not the converse as many had previously
thought. X-ray examination of the Widmanstatten
pattern continued for over a decade with deepen-
ing understanding and some controversy (Mehl
and Derge, 1937; Derge and Komnel, 1937; Owen,
1938; Smith and Young, 1938, 1939).
Rudolf Vogel's contributions to metallurgical
studies in meteorites deserves special mention, his
published papers having covered the period 1925
to 1967. They treated a broad range of topics
related to meteoritics, eight of them being of
particular interest in this context (Vogel, 1927,
1928, 1932, 1951, 1952, 1957, 1964; Vogel and
Baur, 1931). These papers deal with the role of
phosphorus in the development of meteoritic
structures. His 1928 paper discusses schreibersite
and rhabdite precipitation in terms of a hypothet-
ical ternary Fe-Ni-P system. His 1932 and 1952
papers discuss the same material in terms of exper-
imentally derived ternary diagrams.
A watershed publication that represents the cul-
mination of this earlier period of iron meteorite
research is Perry's (1944) monograph on the met-
allography of meteoritic iron. Although many
photographs of iron meteorites had been pub-
lished in the older literature, most of these were
external views of complete specimens or macro-
photographs of meteorite slices prepared for ex-
hibit purposes. Perry's publication was the first
comprehensive and systematic collection of iron
meteorite photomicrographs. The variety of me-
tallic structures in iron meteorites was presented
to the scientific public in a way that has had
significant impact up to the present. Sorby had
pointed out the power of the metallographic ap-
proach in 1877, but it was not seriously pursued
until E. P. Henderson drew the publisher and
meteorite collector S. H. Perry into this field in
the 1930s (Henderson and Perry, 1958: ii).
MODERN WORK ON METALLIC PHASES OF
IRON METEORITES
Modern work on the metallic phases of iron
meteorites includes studies that may be grouped
SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
into the following categories for convenience: (1)
interpretations of the Widmanstatten pattern in
terms of cooling histories of meteoritic parent
bodies, (2) compositional studies utilizing trace
elements and structure as a means of identifying
parent bodies, (3) schreibersite growth as a key to
structure development and cooling history, (4)
descriptive studies and literature reviews. These
studies are interrelated and provide essential back-
ground for the work to be discussed below. Brief
mention of major contributions in these areas will
be given here.
Progress in developing accurate low tempera-
ture phase diagrams has played an essential role
in modern iron meteorite studies. The Fe-Ni dia-
gram of Goldstein and Ogilvie (1965a) has been
used by meteoriticists for over 10 years (Figure 1).
Electron microprobe techniques were employed
by these workers to improve upon earlier equilib-
rium diagrams (Owen and Sully, 1939; Owen and
Lui, 1949) and to reconcile the Fe-Ni diagram
with observations on meteorites (Agrell et al.,
1963). Buchwald (1966) studied the Fe-Ni-P system
at low temperature using X-ray techniques and
proposed a diagram that he used to discuss mete-
orite structure development. More extensive work
on the Fe-Ni-P system based on longer cooling
periods and electron microprobe measurements
was reported by Doan and Goldstein (1970), and
their diagram is basic to the interpretation of the
experimental observations discussed below.
The modern literature on the growth of the
Widmanstatten pattern is extensive. Massalski and
Park (1962) used the Fe-Ni equilibrium diagram to
calculate initial temperatures of kamacite precipi-
tation and final equilibrium temperatures, using
areal distribution of kamacite and taenite and a
lever rule calculation. Wood (1964) and Goldstein
and Ogilvie (1965b) considered the nonequilibrium
nature of iron meteorite structures, and combined
phase diagram and kinetic considerations to derive
cooling rates and to estimate parent body sizes.
Both papers recognize Widmanstatten pattern
growth as diffusion controlled and not subject to
bulk equilibrium consideration. Wood (1964) uses
the relationship between Ni buildup at the centers
of taenite bands and kamacite band widths to
calculate cooling rates. The Goldstein and Ogilvie
(1965b) approach used a detailed analysis of diffu-
sion gradients within taenite at taenite-kamacite
interfaces. Both approaches gave comparable cool-
ing rates within reasonable limits of error. A
number of other papers were published during
this period that contributed significantly to our
knowledge of the structural detail of meteoritic
iron (Short and Anderson, 1965; Goldstein, 1965;
Reed, 1965b; Axon and Boustead, 1967). Cooling
rate and thermal history studies were pursued by
Goldstein and co-workers (Short and Goldstein,
1967; Goldstein and Short, 1967a, b; Fricker et al.,
1970). Goldstein and Doan (1972) discussed the
effect of phosphorus on the formation of the
Widmanstatten pattern and reported the first lab-
oratory production of an artificial Widmanstatten
pattern. They conclude that cooling rate calcula-
tions are not seriously affected by phosphorus. A
comprehensive review of Widmanstatten pattern
studies has been published by Goldstein and Axon
(1973).
Compositional studies of iron meteorites have
traditionally gone hand-in-hand with classification,
bulk Ni values and metallographic structure being
the primary considerations in assigning classifica-
tion categories. Modern versions of this type of
classification have been given by Buchwald and
Munck (1965) and Goldstein (1969). The grouping
of iron meteorites on the basis of trace element
content was introduced by Goldberg et al. (1951)
and Lovering et al. (1957). Wasson and co-workers
in a series of papers (Wasson, 1974; Scott and
Wasson, 1975, 1976) have extended this approach
to define genetic groups in a large number of iron
meteorites, based on Ni, Ga, Ge, and Ir contents
and structural considerations. Sixteen iron mete-
orite groups have been characterized, represent-
ing 12 genetically related meteorite groups and a
number of individual meteorite parent bodies.
iRecent work on schreibersite growth in iron
meteorites may be dated from the observations of
Henderson and Perry (1958). Using conventional
chemical analysis, they observed unusually low Ni
concentrations in swathing kamacite bordering the
large low-Ni (12%) schreibersite in the Tombigbee
River meteorite, a meteorite they also showed to
contain low-Ni (20%) rhabdite. They explained
their observations in part on the migration of Fe
and Ni atoms in the solid state and suggested that
much of the phosphide may have separated from
the liquid. Somewhat later, the first electron mi-
croprobe measurements of schreibersite composi-
NUMBER 21
900 1
800
700
600
500
400
300
-
0
-
-
i i i i
\
\
\
\
11
11
1w
11
1 1 1 1 1 1
a+y
1
r
i 1
10 20 30 40
ATOMIC % NICKEL
FIGURE 1. ?LOW temperature Fe-Ni phase diagram of Goldstein and Ogilvie (1965).
50
tions were reported by Adler and Dwornik (1961)
on schreibersite and individual rhabdite crystals
from the Canyon Diablo meteorite.
Goldstein and Ogilvie (1963) reported electron
microprobe analyses of the composition of various-
sized schreibersites in the Canyon Diablo, Breece
and Grant meteorites. They observed a size-com-
position correlation, the smaller the schreibersite
the higher the Ni content, and showed the pres-
ence of a zone of Ni depletion (swathing zone)
even around very small schreibersites (rhabdites).
They suggested that most schreibersite grew by
solid state precipitation; although massive schrei-
bersites showing no relation to the Widmanstatten
pattern probably formed directly from the liquid
state. Size and composition of the schreibersites
were explained as dependent upon nucleation
temperature and time available for growth of the
precipitate. A growth analysis based on diffusion
kinetics demonstrated that larger schreibersites
nucleated between 700? and 500? C, and that small
schreibersites (rhabdites) formed between 500? and
400? C. They also demonstrated that P solubility
was greater in kamacite than in taenke.
Reed (1965a) emphasized the important role P
plays in the formation of iron meteorite structures.
He determined the range of Ni values in schreiber-
sites and rhabdites in a large number of meteor-
ites, finding Ni values as low as 14% in large
schreibersites and as high as 50% in small taenite-
border schreibersites. Ni depletion at kamacite-
schreibersite interfaces was also demonstrated.
The precipitate size and Ni content relationships
were confirmed, and Reed pointed out that size-
for-size rhabdite grains in the Canyon Diablo me-
teorite are 7% higher in Ni than those in the
Coahuila meteorite. The author recognized the
approximate nature of chemical analysis values
8 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
for total P, suggesting that in most cases the actual
values are higher than the measured values. He
discussed the sequence of phase formation in me-
teorites based on Vogel and Baur's (1931) phase
diagram and stressed the need for more precise
information on phase equilibria.
In a subsequent paper, Reed (1967) measured P
distribution in the Mount Edith meteorite, dem-
onstrating the practicality of measuring Ni and P
distribution profiles in kamacite, taenite, and
schreibersite. Later Reed (1969) discussed the role
of P in relation to Widmanstatten pattern forma-
tion and cooling rate calculations, and reported
measurements of P content in the kamacite of 61
meteorites.
Doan and Goldstein (1969) discussed the forma-
tion of phosphides in the various meteorite classi-
fication groups on the basis of their newly deter-
mined Fe-Ni-P phase diagram. They estimated
total P values from examination of the largest
available polished sections of a number of meteor-
ites, and used them as the basis of a discussion of
schreibersite precipitation reactions in various me-
teorite composition ranges. They inferred thermal
equilibrium in iron meteorites down to 650? C,
with schreibersites forming at higher temperatures
while rhabdites form from kamacite at low temper-
atures when Ni and P diffusion are severely lim-
ited.
Comerford (1969) discussed schreibersite and
cohenite occurrences in iron meteorites and
pointed out that rhabdites, schreibersites that nu-
cleated within kamacite, may be more reliable for
diffusion analysis than grain boundary schreiber-
sites. Axon and co-workers have discussed schrei-
bersite occurrences in a broad range of meteorites
(Axon and Faulkner, 1970; Axon and Waine, 1971,
1972; Axon and Smith, 1972). The papers of Axon
and Waine (1971, 1972) discuss schreibersite mor-
phology, mode of occurrence, and relationship to
other minerals in great detail. Hornbogen and
Kreye (1970) have reported detailed metallo-
graphic studies on the Coahuila and Gibeon mete-
orites and have discussed Ni and P solid state
reactions in some detail. Reed (1972) has reported
analyses of schreibersite in the Oktibbeha County
meteorite, the highest Ni schreibersite known in
meteorites (65.1%), and De Laeter et :al. (1973)
have reported on schreibersite in the Redfields
meteorites, the lowest Ni schreibersite on record
(7.0%).
Modern reviews that are particularly important
to the investigations at hand have been mentioned
previously. The paper by Goldstein and Axon
(1973) is a comprehensive review of the develop-
ment of the Widmanstatten pattern. The recent
book by Wasson (1974) is a broad treatment of
meteorites and includes an excellent summary of
the work on chemical groupings of iron meteorites
with extensive listings of individual iron meteorite
analyses and classification. The treatise on iron
meteorites by Buchwald (1976) has been of great
help. Much of the descriptive material on individ-
ual meteorites had been available to us in manu-
script, and Dr. Buchwald spent many hours teach-
ing the senior author the rudiments of iron mete-
orite metallography during his two-year stay
(1968-1970) at the Division of Meteorites, Smith-
sonian Institution.
RELATED MATERIALS
Schreibersite occurrences are not restricted
solely to iron meteorites. It is an accessory mineral
in pallasites, mesosiderites, and enstatite chon-
drites, and has been reported in several ordinary
chondrites, and a few carbonaceous chondrites
and achondrites (Ramdohr, 1973; Powell, 1971;
Malissa, 1974). Schreibersite is also known as a
product of combustion in the coal mines of Com-
mentry and Cranzac, France (Palache et al., 1944:
125), as a cavity mineral in iron slags (Spencer,
1916), and associated with metallic iron in the
Disko Island, Greenland, basalts (Pauly, 1969).
Schreibersite has been observed by a number of
workers in metal particles in lunar soil and rocks.
Individual particles have been described in great
detail, and Ni/Co ratios have been used to identify
metal of meteoritic origin (Goldstein et al., 1970;
Goldstein and Yakowitz, 1971; Axon and Gold-
stein, 1972; Goldstein et al., 1972). Nickel concen-
tration in metal and schreibersite in contact with
each other have also been used to suggest final
equilibrium temperatures and to imply cooling
histories. As lunar studies have continued, it has
become apparent that a greater proportion of
schreibersite-containing lunar metal than had pre-
viously been thought has been derived from proc-
esses other than simple disruption of impacting
meteorites (Axon and Goldstein, 1973; El Goresy
et al., 1973; Brown et al., 1973; Carter and Pado-
vani, 1973; McKay et al., 1973; Goldstein and
Axon, 1973; Gooley et al., 1973).
NUMBER 21 9
A second phosphide mineral, barringerite
(Fe,Ni,Co)2P, has been reported from the Ollague
pallasite (Buseck, 1969). This material is probably
a secondary product of some type and not an en-
dogenous meteoritic mineral (Buchwald, 1976:
105).
The phosphide mineral, schreibersite, is the
only phosphorus mineral that is observed as a
minor phase in most iron meteorites, and it is the
only phosphorus-containing mineral that will be
considered in this paper. There are, however,
nine phosphate minerals known in meteorites,
and seven of these have been identified as trace
constituents in iron meteorites (Fuchs, 1969; Bild,
1974). They are chlorapatite, Ca5(PO4)3Cl; grafton-
ite, (Fe,Mn)3(PO4)2; sarcopside, (Fe,Mn)3(PO4)2;
whitlockite, Ca3(PO4)2; brianite, Na2MgCa(PO4)2;
panethite, Na2Mg2(PO4)2; and farringtonite,
Mg3(PO4)2. These minerals are observed in a vari-
ety of associations: as inclusions? in graphite-troi-
lite-silicate nodules, in simple troilite nodules or
in troilite-chromite nodules, as inclusions in or in
contact with schreibersite, and associated with
other minor minerals or isolated in metal. The
phosphate-phosphide association has been used to
calculate equilibrium oxygen fugacity in iron me-
teorites and pallasites (Olsen and Fredriksson,
1966; Olsen and Fuchs, 1967). In the iron meteor-
ites considered in this paper, phosphate minerals
are either absent or isolated within inclusions that
represent a higher temperature stage in the devel-
opment of the meteorite structure. No evidence
has been developed in this work to indicate that
the observed phosphide-metal equilibria have been
affected by the presence of phosphates.
Experimental
Two types of data are needed in order to inter-
pret iron meteorite structural development as a
consequence of cooling through the subsolidus
Fe-Ni-P system. Accurate bulk compositions are
the first requirement, and good bulk Ni values
are available from the literature. The problem of
obtaining good bulk P values has not been solved
satisfactorily, and estimated values were found to
be useful. This aspect of the problem will be
discussed in detail below. The second type of data
required are P and Ni gradients within kamacite
and schreibersite, and particularly P and Ni values
at kamacite-schreibersite interfaces. Experimental
procedures for obtaining these values were devel-
oped in the electron microprobe laboratory of the
Department of Mineral Sciences, National Mu-
seum of Natural History, Smithsonian Institution.
Reed (1967, 1969) had reported traverse data on
the Mount Edith meteorite and had demonstrated
the practicality of measuring the low P levels in
kamacite with the microprobe. Doan and Gold-
stein's (1970) experience in their phase diagram
studies was also encouraging. Ideally, one re-
quired step-by-step traverses over many meteorite
structures, each traverse consisting of a large num-
ber of individual analyses for P and Ni. Ni concen-
trations were in a range that presented no unusual
analytical problems, but the expected low levels of
P required special care.
An A.R.L. EMX electron microprobe (Applied
Research Laboratory, Sunland, California) was
used in this work. Simultaneous measurements
were made for Ni Ka (LiF crystal) and P Ka (ADP
crystal) radiation at an X-ray takeoff angle of
52.5?. The instrument was operated at an acceler-
ating voltage of 20 KV, with an approximately 0.1
fia sample current on 100% Fe and a beam size of
approximately 1 /xm. Average count rates mea-
sured on 50% Ni and calculated to 100% Ni were
30,000 cts/sec, with a peak to background ratio of
230. Count rates for 100% P measured on schrei-
bersite were 25,000 cts/sec with a peak to back-
ground ratio of 915. Standard statistical assump-
tions suggest that for the 20 second counting
periods employed, a detectability limit for P of
150 ppm for a single determination would be
achieved, within a satisfactory range for the prob-
lem at hand (Ziebold, 1967). Neighboring P deter-
minations within a given traverse generally agreed
within ?50 ppm.
The standards used were 100% Fe, 100% Ni, a
series of Fe-Ni alloys of known composition, and a
fragment of a large schreibersite from the Canyon
Diablo meteorite. The 100% Fe was used for deter-
mining both Ni and P background counts. The
Fe-Ni standards were prepared by R. E. Ogilvie
and obtained from J. I. Goldstein. They were
used to derive an empirical curve that was incor-
porated into a data reduction program to correct
Ni determinations. The schreibersite standard
contained 12.9% Ni, 0.26% Co, and.was assumed
to contain 15.5% P. A linear relationship between
P counts in schreibersite and concentration in an
unknown was assumed. This was thought to cause
10 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
no serious error in low level P measurements, but
it does introduce errors in P determination in
schreibersite. In this study, P values for schreiber-
site were only used to identify the mineral, and
for this purpose corrections seemed unnecessary.
Sections were prepared for study by standard
metallographic procedures. Pieces of meteorite
were mounted in one-inch diameter Bakelite,
ground on a graded series of sandpapers, and
polished with 3 /xm diamond paste and finally
Linde B alumina compound. The sections were
then lightly etched with 0.5% nital (0.5 ml nitric
acid in 100 ml of 95% ethanol). Light etching was
essential to reveal structures that were examined
and to permit their being relocated and analyzed
in the microprobe. It would have been impossible
to obtain the mass of data required using unetched
sections. While deep etching is known to cause
errors in microprobe measurements at phase inter-
faces, the light etching used here did not seem to
cause a problem. A schreibersite-kamacite-taenite
structure in the Carleton meteorite was selected to
test this point. As nearly identical traverses as
possible, considering that the section had to be
removed from the microprobe, were made before
and after etching. The results were identical within
reasonable experimental error. Areas selected for
microprobe analysis were photographed in detail,
permitting the recording of the exact location of
microprobe traverses.
The electron microprobe data was accumulated
using a step-scan procedure. Ni and P counts
were measured simultaneously and recorded on
punch cards along with step length. The data
were computer processed, resulting in a second
set of cards containing calculated P values, empir-
ically corrected Ni values, and step length. These
cards in turn were used to obtain computer gen-
erated plots. Photographs of the data plots as
received from the computer are given in Figures 2
and 3. The only additions to these diagrams are
the vertical lines that have been added to help
identify the structural elements. Figure 2 is a
traverse across a structure in the Ballinger, Texas,
meteorite, catalog number USNM 824. The label-
ing indicates that this is the fifth traverse made
over structures in that meteorite, the data having
been taken on 8 September 1972. Two symbols
are used to indicate P concentration, the ordinate
scale being zero to 0.7% P for low values and zero
to 70% P for high values. The traverse started in
kamacite and crossed a very thin, elongated phos-
phide (4 /mm wide) at the edge of a taenite lamella.
It passed into kamacite and after traveling 275 /Am
entered a rhombohedral phosphide (rhabdite) 30
/u,m across and then passed into kamacite again.
The two unusually high P values in the kamacite
are undoubtedly due to unseen phosphides that
were included in the excitation volume of the
microprobe beam. Figure 3 is a similar plot for
the Lexington County Meteorite. In this case only
a portion of the complete traverse is reproduced
(700 out of 1600 fim). The traverse started in
kamacite, entered a phosphide at a taenite border,
passed into kamacite, a second phosphide, and a
second taenite lamella. Interface concentration
values and concentration gradient shapes and
lengths were read from these plots.
Data obtained in this way are subject to several
sources of error beyond those that apply to an
individual point analysis in a homogeneous mate-
rial. Stability of the microprobe over the long
periods required to obtain lengthy traverses was a
problem. Due to the survey nature of this investi-
gation, compromises were made in favor of large
numbers of traverses and for lengthy traverses.
The microprobe beam current was monitored and
kept within narrow limits, but this does not neces-
sarily prevent all instrumental drift. Data were
rejected when drift became an obvious problem,
but normally measurements on standards or re-
peated measurements on the same structures gave
identical results within reasonable limits of error
(? 0.005% P, ?0.1% Ni).
The schreibersite-kamacite interface measure-
ments are subject to errors due to both finite
beam size and to interface geometry. The finite
beam produces measured interface profiles that
are less sharp than the actual profile. It is unlikely
that this is a serious error in determining the
interface value of P in kamacite, as this is only
read to the nearest 0.01%. The interface value of
Ni in kamacite is probably increased slightly by
the proximity of high Ni in the adjoining schrei-
bersite. More serious errors may result from inter-
face geometry, as there is no reason to assume
that the interfaces measured were perpendicular
to the surface of the section. The small size of
most of the structures measured, combined with
their large number, made it impractical to attempt
measuring slopes and applying corrections. Repro-
ducibility of measurements on the same structure
NUMBER 21 11
BRLLINGER 824 05 8 SEPT 72
ELEMENT
rn = P (0-.7)
? = P (0-70)
* = NI (0-70)
B 5 a
me a
50 100 150 200 250 300 350 400 450 500 550
DISTflNCE (MICRONS)
FIGURE 2. ?Computer plotted Ni-P traverse across structure in Ballinger meteorite.
and on similar structures within the same meteor-
ite indicated that this is not too serious a problem
for the study at hand.
Results
Eight coarse-structured iron meteorites covering
a range of bulk Ni and bulk P contents were
selected for Ni and P measurements at kamacite-
schreibersite interfaces (Table 1). Four structural
and five chemical classification categories are rep-
resented in the group. The Bellsbank meteorite
contains very low total Ni, while Balfour Downs is
one of the highest Ni members of Group IA.
Coahuila contains only about 0.3 wt. %P, while
Bellsbank contains exceptionally high P. The other
meteorites in the group range between these limits
in Ni and P contents. The data in Table 1 were
taken from Wasson (1974) and Buchwald (1976).
Buchwald's values combine, where appropriate,
chemical analytical values with the results of plan-
ometric analyses of large surfaces. Contributions
of Ni and P due to large schreibersite inclusions
are included in this approach, resulting in esti-
mates more representative of the meteorite as a
whole than uncorrected chemical values. Wasson's
Ni values are chemical values determined on small
samples taken for trace element analysis. A com-
plete review of the literature of each of these
meteorites and a discussion of their detailed met-
allography has been given by Buchwald (1976).
Microscopic examination of metallographic sec-
tions of these eight meteorites led to the selection
of representative schreibersite-containing struc-
12 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
LEXINGTON COUNTY 3334 03 30 SEPT 72
ELEMENT
m = P (0-.7)
O = P (0-70)
A = NI t0-70)
HO O a
100 ISO 200 250 300 360 400 450 600 550
DISTRNCE (MICRONS)
FIGURE 3. ?Computer plotted Ni-P traverse across structure in Lexington County meteorite.
tures for detailed study by the electron microprobe
step-scan procedure described above. Several mi-
croprobe traverses were made for each meteorite,
and the data obtained on Ni and P values at
kamacite-schreibersite interfaces and on Ni and P
gradients in kamacite are summarized below. A
separate table accompanies the description of trav-
erses for a given meteorite, the format used being
the same in each case (Tables 2-9). The first two
columns give the specimen number, which in-
cludes the USNM catalog number, and the
traverse number. Column three contains two types
of information. The first entry for each traverse is
enclosed in square brackets and gives the sequence
of phases traversed and the length of the traverse.
The abbreviations used here are Ph for the various
morphologies of schreibersite, a for kamacite and
y for taenite. The following entries for a given
traverse identify the specific schreibersites mea-
sured and give the length of schreibersite tra-
versed. The next column, headed %NiSch, gives
the weight percent Ni in schreibersite either at a
kamacite-schreibersite interface or within a large
schreibersite. Where an interface measurement
has been made, the schreibersite Ni value will be
followed in the next two columns by the weight
percent Ni in kamacite at the interface (%Niot) and
the weight percent P at the interface (%Pa). The
next two sets of two columns each indicate the
length of observed Ni and P gradients away from
the interface and the Ni and P values at which a
gradient was no longer distinguishable. Factors
NUMBER 21 13
TABLE 1. ? Composition and classification of selected meteorites
Meteorite
Coahuila
Bellsbank
BaTMnger
Santa Luzia
Lexington County ....
Bahjol
Goose Lake
Balfour Downs
?Buchwald (1976)
**Wasson (1974)
Weight Percent
%P*
ChemicalClassification** StructuralClassification*
0.3
2
0.4
0.9
0.3
0.4
0.3
5.6
5.3
6.5
6.6
-7.0
7.7
8.3
8.4
5.49
4.13
6.19
6.3
6.69
7.95
8.00
8.39
IIA
IRANOM
IA-AN
I IB
IA
IA-AN
IA-AN
IA
H
H
Og
Ogg
0g
0g
0m
Og
such as impingement and termination of a partic-
ular traverse influence these final observed Ni
and P values. In the final three columns the Ni
and P values from columns three through six have
been converted to atomic percent for use later.
COAHUILA
The Coahuila, Mexico, meteorite is an example
of an ordinary hexahedrite (IIA), similar in com-
position and structure to a number of other hexa-
hedrites. It consists of single crystal kamacite con-
taining profuse small schreibersites generally of
rhabdite morphology, and occasional larger
schreibersites. The larger schreibersites sometimes
border troilite or troilite-daubreelite inclusions,
and in rare instances are associated with cohenite.
Section 3298 is dominated by six troilite-dau-
breelite inclusions, ranging in size from 5 to 0.3
mm in diameter. Half to three-quarters of the
length of the borders of these sulfides are rimmed
with schreibersite. One small sulfide is bordered
for most of its circumference with schreibersite,
the remaining border being rimmed with cohenite.
Kamacite in the vicinity of these inclusions contains
many subgrain boundaries and is free of schreiber-
site. Microrhabdites measuring less than a few
microns in maximum length and generally consid-
erably smaller are present in profusion away from
these inclusions. Somewhat larger rhabdites are
present in several areas aligned along Neumann
bands. Several lamellar schreibersites, 5 to 10 /am
wide and extending to lengths that are a major
part of a millimeter or more, are also present. A
summary of the traverses of this section is given
below and specific measurements are listed in
Table 2.
3298, traverse 1: crossed a 150 /xm wide schreibersite bordering
a 2 x 2 mm troilite-daubreelite inclusion and then extended
into the surrounding kamacite for nearly 1 mm.
3298, traverse 2: crossed a 40 yum wide schreibersite bordering a
0.5 x 0.5 mm troilite-daubreelite inclusion and then ex-
tended 350 (im into the surrounding kamacite.
3298, traverse 3: crossed a 100 /u.m wide schreibersite bordering
the 5x5 mm troilite-daubreelite inclusion and then ex-
tended for 350 /xm into surrounding kamacite. This traverse
is reproduced at the upper left in Figure 4.
Section 1641(1) primarily contains regions of
kamacite with evenly distributed rhabdite, the in-
dividual rhabdites averaging 200 fim2 in cross-
sectional area. Also present in areas of clear kam-
acite are several large schreibersite inclusions and
schreibersite-cohenite inclusions. The largest
schreibersite inclusion has an area of 0.25 mm2. A
0.1 x 0.6 mm schreibersite is bordered by cohen-
ite, and an adjacent, slightly smaller schreibersite
is partially surrounded by cohenite. One cohenite
area, 0.2 x 0.3 mm, contains six small schreiber-
sites. A 0.4 x 1.4 mm cohenite area contains a
number of small schreibersites and surrounds a
central schreibersite measuring 0.04 x 0.6 mm.
An 80 X 150 fim daubreelite is surrounded by
schreibersite. Two lamellar schreibersites, each
approximately 0.02 X 1.0 mm, are present, and
one of these is partially bordered with cohenite.
1641(1), traverse 4: crossed a large skeletal schreibersite enclos-
ing a kamacite area. The traverse first crossed 450 fjun of
kamacite, then 120 /urn of schreibersite, 70 jxm of included
kamacite, a second 70 (im schreibersite crossing, and termi-
nated after passing through 360 /tm of kamacite. The schrei-
bersite was of uniform composition, and the measurements
14 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
TABLE 2. ?Coahuila schreibersite-kamacite interface measurements
Specimen
No.
3298
3298
3298
1641(1)
1641(1)
1641
1641
1641
1641
Traverse
No.
1
2
3
4
5
6
7
8
9
Structure traversed & schreibersites measured
[Ph border of troilite-daubreelite inciusion-a,
1.1 mm]
Schreibersite, 150 pm wide
[Ph border of troilite-daubreelite inclusion-a,
400 ym]
Schreibersite, 40 ym wide
[Ph border of troilite-daubreelite inclusion-a,
450 ym]
Schreibersite, 100 um wide
[a-Ph-a-Ph-a, 1 mm]
Schreibersite 260x500 ym with 60x170 um
included kamacite
120 ym traverse
70 ym traverse
[a-Ph-a, 350 um]
Rhabdite, 25 ym traverse
Enter
Exit
[a-Ph-a, 750 um]
Rhabdite (45x75 um), 45 um traverse
[a-Ph-a, 800 um]
Rhabdite (50x70 um), 70 ym traverse
Exit
[o-Ph-a-Ph-a, 850 ym]
Rhabdite (50x70 ym), 50 um traverse
Rhabdite (25x30 ym), 25 ym traverse
[a-Ph-a, 200 ym]
Rhabdite (20x40 um), 25 ym traverse
Weight
*NiSch
23.5
25.0
25.0
23.5
23.5
34.5
34.5
27.5
27.0
27.0
29.0
33.0
Percent
3SNi %Pa a
3.4
3.6
3.7
3.5
3.5
4.9
4.9
3.7
4.0
4.1
3.7
4.5
0.07
0.07
0.08
0.08
0.08
0.06
0.06
0.07
0.07
0.07
0.06
0.05
Ni Gradient
Length
(ym) Wt.%Ni
600
250
250
200
250
50
50
175
100
150
200
50
5.5
5.3
5.3
5.3
5.3
5.3
5.3
5.3
5.3
5.3
5.3
5.3
P Gradient
Length
(um) Wt.%P
800
350
300
50
200
Flat
100
100
Flat
Flat
100
Flat
0.16
0.15
0.15
0.10
0.10
0.06
0.07
0.08
0.07
0.07
0.08
0.05
Atomic
*NiSch
20.1
21.4
21.4
20.1
20.1
29.7
29.7
23.6
23.1
23.1
24.9
28.4
Percent
%N1 %Pa a
3.2
3.4
3.5
3.3
3.3
4.7
4.7
3.5
3.8
3.9
3.5
4.3
0.13
0.13
0.14
0.14
0.14
0.11
0.11
0.13
0.13
0.13
0.11
0.09
in Table 2 are for the initial and final schreibersite-kamacite
interfaces.
1641(1), traverse 5: crossed 100 fim of kamacite, a 25 fim wide
rhabdite and terminated after another 200 fim of kamacite.
This traverse is reproduced at the lower left in Figure 4.
Section 1641 is a neighboring specimen to
1641(1) and is similar in metallography. Evenly
distributed rhabdites averaging about 200 jam2 in
area are the primary feature. Random larger
schreibersites, schreibersite-daubreelite, and
schreibersite-cohenite inclusions similar to those
described above are present, although they are
neither as numerous nor as large as the largest
mentioned above. Two unusually large rhabdites
and two smaller ones were selected for measure-
ment.
1641, traverse 6: crossed 400 fim of kamacite, a 45 fim wide
rhabdite, and 300 fim of kamacite. Values reported in Table
2 are an average of the two similar interface measurements.
This traverse is reproduced upper right in Figure 4.
1641, traverse 7: crossed 230 fim of kamacite, entered the
rhabdite from an area of disturbed kamacite, traversed 70
fim of rhabdite and then 0.5 mm of kamacite. Only the
measurements on leaving the rhabdite were usable.
1641, traverse 8: crossed 370 fim of kamacite, 50 fim of rhabdite,
25 fim of kamacite, 25 fim of rhabdite, and 350 fim of
kamacite. The large rhabdite traversed here is the same one
traversed in number 7 but from a near perpendicular direc-
tion. Values reported in Table 2 are averages of two similar
interface measurements.
1641, traverse 9: crossed a smaller rhabdite approximately 1
mm from traverses 6 through 8. 90 fim of kamacite were
crossed, followed by 25 fim of rhabdite and 80 fim of
kamacite. Average interface values are listed in Table 2.
This traverse is reproduced at the lower right in Figure 4.
Ni and P concentration-distance profiles typical
of the Coahuila data outlined above are given in
Figure 4. These diagrams were prepared from
tracings of the computer plotted step-scan data.
Ni concentrations are indicated by the solid lines,
and P concentrations multiplied by 100 by dashed
lines. The Ni profiles are normally readily repro-
duced from the data charts without ambiguity.
The low level P data contains more scatter; draw-
ing the appropriate line, and particularly selecting
-NUMBER 21 15
5O
45-
40-
om
UJ
0.
? 3O
O
UJ
S
25
20-
15-
,0
25
100 (Jm
0.08
3.7
P x 100
Ni
3296
28
0.07
3.7
1641
400
MICRONS
N4.9
300
MICRONS
0.05,,4.5
FIGURE 4. ?Ni and P profiles at kamacite-schreibersite interfaces in Coahuila: upper left, traverse
3; upper right, 6; lower left, 5; lower right, 9.
16 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
the kamacite interface P level, is more subjective.
A smoothing procedure relying on eyeball exami-
nation was used. Ni values for schreibersite and
Ni and P concentrations in kamacite at the kama-
cite-schreibersite interface are indicated at an ap-
propriate place on the diagrams. The bulk Ni
content of the meteorite is also given and indicated
by a short dashed line.
The four profiles (Figure 4) illustrate the ob-
served range of Ni in Coahuila schreibersite and
the shape and level of the Ni and P gradients in
the surrounding kamacite. They are given from
upper left to lower right in order of decreasing P
concentration at the kamacite-schreibersite inter-
face . Three of the profiles are of large rhabdites
(traverses 6, 5, 9, Table 2), while the one at the
upper left is of a 100 /xm wide schreibersite border-
ing a large troilite-daubreelite inclusion (traverse
3, Table 2). The high P interface value and the
high level in the surrounding kamacite are of
particular interest. The three rhabdites appear to
increase in Ni, both with decreasing cross-sectional
area and with increasing interface Ni in kamacite
value. The interface P values do not seem to
correlate with either Ni in schreibersite or with
interface Ni in kamacite values. Ni and P gradients
extend for several hundred microns around the
larger schreibersites but appear to be much more
restricted in extent around the smaller ones. The
33% Ni schreibersite has an essentially flat P con-
centration at 0.05%. For these four cases, the
length and steepness of the P gradient decreases
with decreasing P at the interface.
BELLSBANK
The Bellsbank, South Africa, meteorite is a
phorphorus-rich hexahedrite. Massive and large
skeletal schreibersite crystals, with individual faces
over a centimeter in length and areas larger than
2 cm2, dominate polished surfaces and appear to
be distributed throughout this meteorite (photo-
graph in Henderson, 1965). The kamacite matrix
contains a network of lamellar schreibersite. These
rarely intersecting planar crystals may exceed a
centimeter in length, but are normally only 5 to 10
/u,m thick. Occasional large rhabdites and subgrain
boundary schreibersites are present. Microrhab-
dites are present in profusion in kamacite areas
away from the other forms of schreibersite. The
Bellsbank meteorite has experienced severe terres-
trial weathering. Many of the lamellar schreiber-
sites have been replaced with corrosion products,
and corrosion along kamacite-schreibersite inter-
faces is common.
Section 2162 contains parts of two massive
schreibersites surrounded by schreibersite-free
kamacite areas. Two lengthy subgrain boundaries
TABLE 3. ?Bellsbank schreibersite-kamacite interface measurements
Specimen
No.
2162
2162
2162
2162
2162
2162
Traverse
No.
1
2
3
4
5
6
Structure traversed & schreibersites measured
[a-Ph, 3.6 mm]
Massive schreibersite (7x2 mm), 1 mm traverse
[a-Ph-a, 900 um]
Large rhabdite along sub-grain boundary,
40x50 um, diagonally across body.
[a-Ph-a, 200 um]
Same rhabdite as traverse 2, parallel to 50 pm
direction
Enter
Exit
[a-Ph-a, 300 um]
Rhabdite along sub-grain boundary, 15x120 um
[a-Ph-a, 300 um]
Rhabdite along sub-grain boundary, 15x120 um,
repeat of traverse 2162-4
[a-Ph-a, 130 ym]
Large rhabdite Isolated in micro-rhabdite
area of a, 30x60 pm
Weight
*NiSch
12.4
22.0
22.0
22.0
22.6
22.8
18.6
Percent
SNi
a
1.6
3.2
3.3
2.7
2.6
2.6
2.8
XP
a
0.09
0.09
0.09
0.05
0.05
0.05
0.09
Ni Gradient
Length
(um)
900
20
40
40
80
90
40
Wt.SSNI
3.9
4.5
4.2
4.2
4.3
4.3
4.2
P Gradient
Length
(um)
1000
50
Flat
60
100
100
30
Wt.JSP
0.20
0.18
0.09
0.12
0.20
0.18
0.20
Atomic
*N1Sch
10.6
18.8
18.8
18.8
19.3
19.5
15.9
Percent
XN1
a
1.5
3.1
3.1
2.5
2.5
2.5
2.7
%P
a
0.16
0.16
0.16
0.09
0.09
0.09
0.16
NUMBER 21 17
MICRO-RHABOITES
400
MICRONS
Px 100
\
400
MICRONS
FIGURE 5. ?Ni and P profiles at kamacite-schreibersite interfaces in Bellsbank: top, traverse 1;
lower left, 6; lower middle, 3; lower right, 4 and 5. (Dashed line indicates bulk Ni value.)
18 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
cross the kamacite and are sites of small lamellar,
irregular-shaped, and rhabdite-shaped schreiber-
sites. The kamacite matrix contains profuse micro-
rhabdites. Oxidation products are present at sites
previously occupied by lamellar schreibersite, and
along some Neumann bands and possibly cleavage
planes.
2162, traverse 1: crossed 2.6 mm of kamacite and entered a
large schreibersite and continued for 1 mm. The traverse
was perpendicular to a straight kamacite-schreibersite inter-
face, and the Ni content of the schreibersite was uniform for
the length of the measurement (reproduced at top of Figure
5). Numerical data for this and succeeding Bellsbank trav-
erses are given in Table 3.
2762, traverse 2: crossed 650 fim of kamacite, a rhabdite at a
subgrain boundary and continued into surrounding kamacite
for 200 /urn. Average values for the two interface values are
given in Table 3.
2162, traverse 3: crossed 70 fim of kamacite, the same rhabdite
measured above, and passed into 70 fim of kamacite.
Traverse 2 crossed diagonally, while this one passed through
the middle parallel to the 50 ^im edge. Two different
interface values for Ni and P in kamacite were obtained,
perhaps influenced by the presence of the subgrain boundary
(Table 3 and lower middle of Figure 5).
2162, traverse 4: crossed 170 fim of kamacite, the narrow
direction of an elongated rhabdite situated along a subgrain
boundary, and 100 fim of kamacite (lower right in Figure 5).
2162, traverse 5: a remeasurement of traverse 4.
2762, traverse 6: crossed a large rhabdite surrounded by a
region of clear kamacite within a microrhabdite area (lower
left in Figure 5).
An attempt was made to determine Ni in other
small schreibersites in Bellsbank. A number of
measurements were made on schreibersites rang-
ing down to a minimum of a few microns in
width. Twelve measurements were obtained where
the P value indicated that the microprobe beam
had been completely within a schreibersite. The
Ni values ranged from 22% to a maximum of
32%. Interface measurements were not attempted
on these small bodies.
The four profiles in Figure 5 represent the
traverses described above and illustrate the shapes
and levels of the Ni and P gradients in the sur-
rounding kamacite. The profile at the top is the
interface portion of traverse 1. The low value for
Ni in the schreibersite combines with an unusually
low interface value for Ni and a high interface
value for P in kamacite. The rhabdite profile at
the lower left has the same P interface value, but
higher Ni in both kamacite and schreibersite. The
P gradient in this case is particularly severe. The
rhabdite at the lower right has similar Ni values to
the one on the left, but the P interface value is
considerably lower. The middle profile has uni-
form Ni in the schreibersite but has two different
sets of P and Ni kamacite interface values, the
only such observation made in this study.
BALLINGER
The Ballinger, Texas, meteorite is a low-Ni
coarse octahedrite. Figure 6 is a photograph of
the slice studied, USNM 824. The area marked PS
shows where material was taken for section prepa-
ration. Section 824 is from the back of this surface
and contains a large schreibersite similar in size
and shape to the one at the lower right in Figure
6. Section 824(1) is the outlined surface after repol-
ishing. The overall structure consists of large
kamacite grains and three areas of heiroglyphic
schreibersite. A Widmanstatten pattern is sug-
gested in some areas of the structure, but it is
certainly not well developed. The large schreiber-
site on the left is completely surrounded by rims
of partially decomposed cohenite. The center
group of schreibersite crystals is partly bordered
by partially decomposed cohenite, while the schrei-
bersite on the right is completely free of cohenite.
Areas of this surface containing the best Widman-
statten pattern development are well away from
the large schreibersite inclusions. It is interesting
to note that the lower left-hand edge of this slice
contains a rim of kamacite heat altered to a2, a
remnant from ablation heating. This structure has
not been previously observed in the Ballinger
meteorite.
Section 824 contains a large schreibersite 0.8
mm long and averaging 1 to 2 mm in width,
surrounded by a narrow area of schreibersite-free
kamacite. The kamacite matrix contains a profu-
sion of both Neumann bands and rhabdites. Neu-
mann band bending, particularly at the interfaces
with the large schreibersite, suggests mild mechan-
ical distortion in Ballinger. The kamacite contains
numerous grain boundaries that are the sites of
grain boundary schreibersites. Terrestrial oxida-
tion has penetrated areas of this section.
824, traverse 1: crossed three areas of the large schreibersite.
500 fim of kamacite were crossed, followed by 350 fim of
schreibersite, 800 fim of kamacite, 400 fim of schreibersite,
450 fim of kamacite, 670 fim of schreibersite, and finally 500
fim of kamacite. The three schreibersite areas traversed are
essentially free of Ni gradients and they all have approxi-
NUMBER 21 19
PS
1 cmI 1
FIGURE 6. ?Macrostructure of a slice of Ballinger meteorite (large dark inclusions are schreibersite,
the one on the left enclosed in partially decomposed cohenite; area marked PS was used for
microstructure examination [824(1)]; and edge marked a2 contains a rim of ablation recrystallized
kamacite).
mately the same Ni value. Numerical data for this and
succeeding Ballinger traverses are given in Table 4.
824, traverse 2: crossed two rhabdites along a grain boundary 2
mm away from the massive schreibersite. 400 fim of kamacite
were crossed, followed by two 40 fim rhabdites separated by
25 fim of kamacite, and finally 300 fim of kamacite. Because
of impingement effects, the interface values in Table 4 are
an average of the two similar external measurements.
824, traverse 3: crossed two areas of a 1 x 0.4 mm skeletal
schreibersite 3.5 mm from the massive schreibersite. The
values given in Table 4 are averages of the two external
interface values.
Section 824(1) is from a typical Widmanstatten
area of the Ballinger meteorite, the area outlined
in Figure 6. The kamacite matrix contains Neu-
mann bands and rhabdites in a wide range of
sizes. Kamacite lamellae are separated by grain
boundaries that are alternately sites of grain
boundary schreibersites and taenite-plessite bands.
824(1), traverse 4: crossed 230 fim of kamacite and entered a
small schreibersite embedded in a taenite border, passed
through a narrow band of taenite, and then 220 fim of
kamacite.
824(1), traverse 5: crossed 150 fim of kamacite, a schreibersite
embedded in taenite, 20 fim of taenite, 270 fim of kamacite,
a large rhabdite, and finally 120 /AITI of kamacite (see Figure
2 in experimental section).
20 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
TABLE 4. ?Ballinger schreibersite-kamacite interface measurements
Specimen
No.
824
824
824
824(1)
824(1)
824(1)
824(1)
824(1)
Traverse
No.
1
2
3
4
5
6
7
8
Structure traversed & schreibersites measured
[a-Ph-a-Ph-a-Ph-a, 3.7 mm]
Massive schreibersite, 350 ym
Enter
Exit
Massive schreibersite, 400 ym
Enter
Exit
Massive schreibersite, 670 ym
Enter
Exit
[a-Ph-o-Ph-a, 850 ym]
Two large rhabdites, 40x40 um each,
separated by 25 um. Values recorded
are averages of similar values.
[a-Ph-a-Ph-o, 700 um]
Skeletal schreibersite 1x0.4 mm. Values
recorded are average of similar values
from the two exterior Interfaces.
[a-Ph-Y-o? 480 um]
Rhabdite embedded in taenite border,
15x20 um
[a-Ph-Y-a-Ph-a, 650 ym]
Rhabdite embedded In taenite border, 5 um
wide
Large rhabdite (35x25 um), 250 ym into a
from above
[ct-Ph-Y-a, 300 um]
Rhabdite embedded in taenite border,
10x15 um
[a-Ph-a-Ph-a-Ph-a-Ph-a, 1.4 mm]
Grain boundary schreibersite in sequence
with taenite, 20x15 ym
Rhabdite 50 ym from above, 40x50 ym
Rhabdite 400 ym from above, 40x40 ym
Rhabdite 600 ym from above, 30x50 ym
[a-Ph-a, 1.0 mm]
Grain boundary schreibersite in sequence
with taenite, 15x140 ym
Weight
*N1Sch
19.5
19.5
19.5
19.3
19.7
19.5
34.5
35.8
49.8
49.5
41.5
51.0
47.0
44.5
40.0
40.0
48.8
Percent
a
2.9
3.0
2.6
2.6
...
2.6
4.2
4.8
6.0
6.0
5.2
6.3
5.7
5.7
5.0
5.0
6.0
%P
a
0.06
0.06
0.06
0.06
0.06
0.06
0.04
0.05
0.03
0.03
0.05
0.03
0.03
0.04
0.05
0.05
0.03
N1 Gradient
Length
(ym)
500
300
200
150
150
500
200
150
50
50
50
50
50
100
100
100
100
Wt.%N1
5.2
4.3
4.3
4.0
4.0
5.0
6.0
6.8
7.2
7.2
7.2
7.2
7.2
7.2
7.2
7.2
7.0
P Gradient
Length
(um)
150
200
200
150
100
150
100
200
50
50
50
50
50
100
200
200
300
Wt.SP
0.10
0.09
0.08
0.08
0.08
0.08
0.09
0.08
0.05
0.05
0.07
0.06
0.06
0.07
0.09
0.09
0.08
Atomic
*NiSch
16.6
16.6
16.6
16.5
16.8
16.6
29.7
30.8
43.1
42.9
35.8
44.2
40.7
38.4
34.5
34.5
42.3
Percent
XN1a
2.8
2.9
2.5
2.5
2.5
2.5
4.0
4.6
5.7
5.7
5.0
6.0
5.4
5.4
4.8
4.8
5.7
XP
a
0.11
o.n
o.n
o.n
o.n
0.11
0.07
0.09
0.05
0.05
0.09
0.05
0.05
0.07
0.09
0.09
0.05
824(1), traverse 6: crossed 80 fj.m of kamacite, a taenite border
schreibersite, 15 fim of taenite, and 150 fim of kamacite.
824(1), traverse 7: crossed 140 /u,m of kamacite, a grain boundary
schreibersite in sequence with taenite, 50 fim of kamacite, a
large rhabdite, 300 (im of kamacite, a large rhabdite, 620
jum of kamacite, a large rhabdite, and finally 80 /um of
kamacite.
824(1), traverse 8: crossed 90 jxm of kamacite, a grain boundary
schreibersite in sequence with taenite, and 1 mm of kamacite.
The massive schreibersite in Ballinger contains
higher Ni than similar sized schreibersite in Bells-
bank, and the Ni in kamacite interface values are
higher while the P interface values are lower.
Large rhabdites in Ballinger contain more Ni than
similar ones in Coahuila or Bellsbank, and their
interface values are generally higher in Ni and
lower in P. Grain boundary schreibersite tends to
contain more Ni than kamacite matrix rnabdites,
and taenite border schreibersite contains the high-
est Ni values observed. As the Ni values in the
schreibersite go up, interface values in kamacite
tend to go up for Ni and down for P. Ni and P
gradients extend over relatively large distances
around the massive schreibersite and over much
shorter distances in the higher Ni forms of schrei-
bersite .
NUMBER 21 21
SANTA LUZIA
The Santa Luzia, Brazil, meteorite is a P-rich
coarse octahedrite, a chemical Group IIB meteor-
ite. The structure of a typical section is illustrated
in Figure 7. Large, hieroglyphic schreibersites with
broad areas of swathing kamacite dominate the
structure. Two such schreibersite areas are shown
in the figure, the central one bordering a large
troilite. The second schreibersite area could also
be associated with a lens-shaped troilite nucleus
that lies outside the plane of this section. The
areas of structure between the troilite-schreiber-
site-swathing kamacite areas contain a coarse Wid-
manstatten pattern, particularly well developed in
the top of Figure 7. These Widmanstatten areas
are reminiscent of the schreibersite-free areas of
the Ballinger meteroite. Weathering has pene-
trated deeply into Santa Luzia, particularly along
the major grain boundaries separating the swath-
ing kamacite areas from the areas of Widmanstat-
ten pattern.
Figure 8 is a sketch of the section of the photo-
graph (Figure 7), indicating how this slice was
sampled for detailed metallographic and electron
microprobe analysis. Four metallographic sections
were prepared from the areas indicated by dashed
lines in the sketch. Section 1618 is from an area of
Widmanstatten pattern, 1618P from the schreiber-
site-swathing kamacite area, and 1618PS and
1618PS(1) from the sulfide-schreibersite-swathing
kamacite area. The numbered arrows in the sketch
indicate the location of the traverses described
below. Lengthy traverses were made in each of
the two hieroglyphic phosphide areas, with short
ones being made in the Widmanstatten pattern
area.
Section 1618P contains massive schreibersite and
swathing kamacite (Figure 8). Clear kamacite sur-
rounds the large schreibersite, grading into kama-
cite containing a profusion of microrhabdites.
Neumann bands are most obvious in the transition
areas between clear kamacite and microrhabdite
areas. Occasional subgrain boundaries are present
within the kamacite, and small schreibersites of
various morphologies are associated with them.
1618P, traverse 1: crossed 1.3 mm of massive schreibersite, 2.0
mm of kamacite, 0.7 mm of massive schreibersite, 0.5 mm of
kamacite, 0.2 mm of massive schreibersite, and 2.6 mm of
kamacite. Numerical data for this and succeeding Santa
Luzia traverses are given in Table 5.
1618P, traverse 2: crossed 1.0 mm of kamacite, 0.4 mm of
massive schreibersite, 1.9 mm of kamacite, 0.8 mm of massive
schreibersite, 0.9 mm of kamacite, and 0.2 mm of massive
schreibersite.
1618P, traverse 3: crossed 0.2 mm of massive schreibersite, 1.0
mm of kamacite, and 0.2 mm of massive schreibersite.
Section 1618PS contains troilite, massive schrei-
bersite and areas of swathing kamacite (Figure 8).
Its metallographic characteristics are similar to
those of section 1618P.
1618PS, traverse 4: crossed 0.2 mm of troilite, 2.9 mm of
kamacite, 140 /tm of massive schreibersite, 0.6 mm of kama-
cite, 2.9 mm of massive schreibersite, and 0.5 mm of kama-
cite.
1618PS, traverse 5: crossed 0.4 mm of troilite, 0.7 mm of
massive schreibersite, 4.6 mm of kamacite, 2.5 mm of massive
schreibersite, and 1.8 mm of kamacite. A representative
interface profile for this traverse is reproduced in Figure 9.
1618PS, traverse 6: crossed 0.6 mm of massive schreibersite and
1.9 mm of kamacite.
Section 1618PS(1) contains an outer edge of the
massive schreibersite and spans the swathing kam-
acite zone, containing a segment of its exterior
boundary. A detailed electron microprobe traverse
was not made on this sample, but sufficient mea-
surements were taken to establish the Ni and P
concentration patterns for the swathing zone. In
the direction away from the schreibersite and
toward the exterior grain boundary, the Ni in
kamacite increased from slightly more than 2% at
the schreibersite interface to approximately 5%
within the first 0.5 mm. Over the next 7 mm the
Ni concentration increased smoothly to approxi-
mately 7%. It is in the first 0.2 mm of kamacite
surrounding the schreibersite that the Ni concen-
tration increases most rapidly, and it is this zone
that is free of microrhabdites. The P concentration
increases to approximately 0.1% within this same
region of approximately 0.2 mm bordering the
interface. Upon entering the zone of microrhab-
dite precipitation, P concentration falls off in kam-
acite to a uniform level of approximately 0.05%
for the remainder of the swathing zone. Late-
stage microrhabdite precipitation seems to have
effectively removed the P gradient that must have
been produced within the swathing zone during
the massive schreibersite growth, while apparently
only slightly modifying the Ni gradient.
The clear kamacite surrounding the central
schreibersite within the swathing zone is 100 to
22 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
FIGURE 7. ?Etched surface of Santa Luzia meteorite, USNM 1618: center, troilite surrounded by
schreibersite and swathing kamacite; left, schreibersite area in swathing kamacite.
FIGURE 8. ?Sketch of Santa Luzia meteorite slice in Figure 7, (coarse pattern, troilite; fine pattern,
schreibersite; lines enclose areas of swathing kamacite; broken lines, locations of metallographic
sections; numbered arrows, positions of electron microprobe traverses; stars, positions of residual
taenite areas).
200 /xm wide. This clear kamacite grades into
kamacite containing profusion of microrhabdites,
and it is in this transitional area that Neumann
bands are frequently best developed. Microrhab-
NUMBER 21 23
TABLE 5. ? Santa Luzia schreibersite-kamacite interface measurements
Specimen
No.
1618P
1618P
1618P
1618PS
1618PS
1618PS
1618
1618
1618
Traverse
No.
1
2
3
4
5
6
7
8
9
Structure traversed & schreibersites measured
[Ph-a-Ph-a-Ph-a, 7.3 mm]
Massive schreibersite,
Start
Exit
Massive schreibersite,
Enter
Exit
Massive schreibersite,
Enter
Exit
[a-Ph-a-Ph-a-Ph, 5.3 mm]
Massive schreibersite,
Enter
Exit
Massive schreibersite,
Enter
Exit
Massive schreibersite,
Enter
Stop
[Ph-a-Ph, 1.4 mm]
Massive schreibersite,
Start
Exit
Massive schreibersite,
Enter
Stop
[Troilite-a-Ph-a-Ph-a, 7.4
Massive schreibersite,
Enter
Exit
Massive schreibersite,
Enter
Exit
[Troll1te-Ph-a-Ph-a, 1 cm]
Massive schreibersite,
Start
Exit
Massive schreibersite,
Enter
Exit
[Ph-a, 2.5 nin]
Massive schreibersite,
Start
Exit
[a-Ph-a, 350 ym]
Traverses across grain
site, 12x170 ym
Tip
Center
Tip
[a-Y-Ph-a, 400 ym]
1.3 mm traverse
0.7 mm traverse
250 ym traverse
430 ym traverse
800 ym traverse
200 ym traverse
220 ym traverse
200 ym traverse
mm]
140 ym traverse
2.9 mm traverse
0.7 mm traverse
2.5 mm traverse
0.6 mm traverse
boundary schreiber-
Taenite border schreibersite, 10x20 ym
[a-Y-Ph-a, 350 ym]
Taenite border schreibersite, 15x40 ym
Weight
*N1Sch
16.0
16.5
16.5
16.5
17.9
17.9
16.0
16.0
16.2
16.2
16.1
16.1
16.0
16.0
16.2
16.2
17.3
17.3
16.6
17.1
16.6
16.6
16.4
16.6
16.6
16.6
45.5
44.7
45.7
48.0
48.5
Percent
a
2.4
2.3
2.4
2.4
2.4
2.3
2.3
2.2
2.4
2.3
2.3
2.3
2.6
2.3
2.4
2.5
2.5
2.5
2.5
2.3
5.8
6.0
6.2
6.4
6.5
%Pa
0.07
0.06
0.07
0.07
0.07
0.07
0.07
0.07
0.07
0.07
0.07
0.07
0.08
0.06
0.07
0.08
0.07
0.07
0.07
0.07
0.03
0.04
0.04
0.03
0.03
Ni Gradient
Length
(ym)
500
600
200
200
1000
900
1000
600
400
400
200
300
600
300
300
500
900
500
600
600
50
100
50
100
70
Wt.?N1
4.9
4.9
3.8
3.8
5.3
4.0
3.7
3.8
4.2
4.2
4.0
4.0
4.8
3.5
3.5
5.0
4.6
4.6
5.2
4.8
7.3
7.4
7.4
7.4
7.5
P Gradient
Length
(ym)
500
400
100
Flat
800
500
300
400
300
200
300
300
800
300
200
250
700
800
800
500
100
100
100
150
150
Wt.*P
0.15
0.15
0.08
0.07
0.16
0.09
0.09
0.09
0.09
0.09
0.11
0.11
0.16
0.08
0.08
0.14
0.14
0.14
0.18
0.13
0.10
0.10
0.10
0.10
0.10
Atomic
%N1Sch
13.6
14.1
14.1
14.1
15.3
15.3
13.6
13.6
13.8
13.8
13.7
13.7
13.6
13.6
13.8
13.8
14.8
14.8
14.4
14.6
14.2
14.2
14.0
14.2
14.2
14.2
39.4
38.6
39.6
41.5
42.0
Percent
a
2.3
2.2
2.3
2.3
2.3
2.2
2.2
2.1
2.3
2.2
2.2
2.2
2.5
2.2
2.3
2.4
2.4
2.4
2.4
2.2
5.5
5.7
5.9
6.1
6.2
vn
#r
a
0.13
0.11
0.13
0.13
0.13
0.13
0.13
0.13
0.13
0.13
0.13
0.13
0.14
0.11
0.13
0.14
0.13
0.13
0.13
0.13
0.05
0.07
0.07
0.05
0.05
24 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
TABLE 5?continued
Specimen
No.
772
772
772
772
Traverse
No.
10
11
12
13
Structure traversed & schreibersites measured
[a-Ph-a-Ph-a, 3.6 Itm]
Massive schreibersite, 650 um traverse
Enter
Exit
Massive schreibersite, 600 um traverse
Enter
Exit
[a-Ph-a, 2.3 mm]
Grain boundary schreibersite, 400x450 um
Enter
Exit
[a-Ph, 750 um]
Grain boundary schreibersite, 70 um traverse
Enter
Stop
[a-Ph-Y-a, 1.4 mm]
Taenite border schreibersite, 15x20 um
a-border
Y-border
Weight
*N1Sch
19.3
19.3
19.7
19.7
28.0
28.0
36.2
36.2
47.0
47.5
Percent
%N1a
2.8
2.8
2.8
2.8
3.7
3.9
4.6
6.0
%Pa
0.07
0.07
0.07
0.07
0.06
0.06
0.05
0.03
N1 GradientLength
(um)
800
250
250
80
1400
500
500
200
Wt.%N1
5.8
4.7
4.7
5.8
6.9
6.3
7.2
7.4
P Gradient
Length
(um)
500
250
250
600
1000
300
500
200
Wt.%P
0.14
0.10
0.10
0.15
0.11
0.12
0.09
0.07
Atomic
*N1Sch
16.5
16.5
16.8
16.8
24.0
24.0
31.2
31.2
40.7
41.1
Percent
SN1a
2.7
2.7
2.7
2.7
3.5
3.7
4.4
5.7
0.13
0.13
0.13
0.13
0.11
0.11
0.09
0.05
dites appear to increase in number per unit area
for the first few hundred microns away from the
clear kamacite. As distance increases further, there
appears to be a slight increase in both concentra-
tion and coarseness of microrhabdites. Coarser
microrhabdites are occasionally associated with
subgrain boundaries within the kamacite. There
are also occasional aligned microrhabdites, as if
they had precipitated along Neumann bands that
are no longer present. Rare lamellar schreibersites
2 fxm or less in width and as long as 200 ju,m are
present. Near the outer edge of the swathing zone
occasional segments of grain boundary type schrei-
bersite protrude into the swathing kamacite or are
observed enclosed within it. Four areas that con-
tain residual taenite as well as associated schreiber-
site were observed within the two swathing zones
(Figure 8), and close to their exterior borders.
The swathing zone boundary has suffered se-
vere terrestrial weathering and now contains sec-
ondary iron oxides that have penetrated into the
adjoining kamacite. Remnant grain boundary
schreibersite is present, but it is encased in oxides
precluding interface measurements. The amount
of remnant schreibersite suggests that originally
as much as 50% of the length of the grain bound-
ary may have been occupied by schreibersite. A
Ni determination on one of these schreibersites
gave a value of 29%.
Section 1618 is from an area of the large section
containing coarse-structured Widmanstatten pat-
tern (Figure 8). Parts of four kamacite grains 4 to
5 mm wide are present. The general metallogra-
phy of the section is similar to that described for
Ballinger, with the exception that only microrhab-
dites are present within kamacite areas. Measure-
ments described below were made along a 1.5 mm
grain boundary containing taenite and taenite-
plessite areas in sequence with grain boundary
schreibersite.
1618, traverse 7: combines data from three short (350 fim)
traverses perpendicular to the long direction of a single
grain boundary schreibersite. It measured 170 fim in length
and was separated from taenite by 10 to 20 fim at either end.
Measurements were made near the two ends and in the
middle of the schreibersite. The interface profile for the
center traverse is given in Figure 9.
1618, traverse 8: crossed 200 fim of kamacite, 10 fim of schrei-
bersite, 15 fim of taenite, and 170 fim of kamacite. This
taenite border schreibersite was embedded in the taenite
lamella at one end of the schreibersite in traverse 7, approx-
imately 200 fim away. The schreibersite interface profile is
given in Figure 9.
1618, traverse 9: crossed 200 fim of kamacite, 15 fim of schrei-
bersite, 25 fim of taenite, and 120 fim of kamacite. The
location was 200 fim farther along the same taenite measured
in traverse 8.
Section 772 is from a separate small specimen of
the Santa Luzia meteorite. It combines both the
swathing kamacite and Widmanstatten pattern
NUMBER 21 25
400
MICRONS
FIGURE 9. ?Ni and P profiles at kamacite-schreibersite interfaces in Santa Luzia meteorite, USNM
1618: traverses 5, 7, and 8 as indicated.
areas measuYed above on separate sections in one
microprobe section of 2 cm2. Figure 10 is a scale
drawing indicating the areas of interest. A massive
schreibersite approximately 7 mm in length is
surrounded by clear kamacite that in turn is en-
closed in microrhabdite-containing kamacite. A
major grain boundary containing grain boundary
schreibersite is within 3 mm of one end of the
massive schreibersite. Leading off of that grain
boundary is another that contains taenite as well
as grain boundary schreibersite, within 6 mm of
the massive schreibersite. Measurements have
been made in these areas as indicated in Figure
10.
772, traverse 10: crossed 0.9 mm of kamacite, 0.6 mm of massive
schreibersite, 0.6 mm of kamacite, 0.6 mm of massive schrei-
bersite, and 0.9 mm of kamacite. Part of this traverse
including the first kamacite-schreibersite interface is given in
Figure 11.
772, traverse 11: crossed 1.4 mm of kamacite, 0.4 mm of grain
boundary schreibersite, and 0.5 mm of kamacite. The kama-
cite-schreibersite interface on leaving the schreibersite is
given in Figure 11.
772, traverse 12: crossed 0.7 mm of kamacite and 50 fim of
schreibersite. The Ni and P profiles are reproduced in
Figure 11.
772, traverse 13: crossed a taenite boundary schreibersite (see
inset in Figure 10). The major features of this traverse are
given in Figure 11.
The data on the Santa Luzia meteorite may be
of particular significance from the standpoint of
classification considerations. The schreibersite re-
lationships observed for the large swathing zones
and their included schreibersite are in many ways
similar to the Bellsbank meteorite. The schreiber-
site relationships in the Widmanstatten areas of
Santa Luzia are similar to those observed in Ballin-
ger. Could the differences in these two structures
(Figures 6, 7) be due primarily to differences in
initial P concentration?
LEXINGTON COUNTY
The Lexington County, South Carolina, mete-
orite is a coarse octahedrite of intermediate Ni
26 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
r
1-2
mm
772
FIGURE 10. ?Scale drawing of schreibersites in an area of Santa Luzia meteorite section, USNM 772
(location of electron microprobe traverses indicated; traverse 13 is of a taenite border schreibersite,
indicated in more detail in the inset).
NUMBER 21 27
0.06
3.8
400
MICRONS
50-
45-
40-
ZiJ 35-
E
L
r 30-s>
uS
25-
20-
15-
10-
5'
PxlO
772 *I2
36
0.054.6 N
772 * 13
47
1
?
6.0
0.0
-
-
-
-
FIGURE 11. ? Ni and P profiles at kamacite-schreibersite interfaces in Santa Luzia meteorite, USNM
772: traverses 10 through 13 as indicated. (Dashed line indicates bulk Ni value.)
28 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCE ES1,
3334
FIGURE 12. ?Sketch of Lexington County section, USNM 3334, prepared from photomosaic of area
studied (arrows, electron microprobe traverses; large structure at left, massive schreibersite
surrounded by cohenite; subgrain boundaries, grain boundary schreibersites, taenite, and taenite
border schreibersite are indicated).
content. Polished and etched surfaces reveal large
areas of well-developed Widmanstatten pattern.
The kamacite of these areas contains rhabdites of
various sizes, grain boundary schreibersites, grain
boundary and residual taenite and plessite, and
Neumann bands. Oxidation has severely pene-
trated the exterior surface of the specimen and
invaded the interior of the meteorite along major
grain boundaries. Occasional skeletal schreiber-
sites in the millimeter size range are present,
normally completely surrounded by cohenite.
These large schreibersites tend to be surrounded
by areas of swathing kamacite that interrupt the
regularity of the Widmanstatten pattern. Isolated
patches of cohenite are also present.
Section 3334 contains a typical Lexington County
Widmanstatten pattern area and a large schreiber-
site surrounded by cohenite (6 x 0.3 mm). Edges
of the section and major grain boundaries have
been invaded by oxidation, but the areas selected
for microprobe analysis appear fresh. The paths
of the traverses listed below are indicated in Figure
12 and numerical data are given in Table 6.
3334, traverse 1: crossed 1.4 mm of kamacite, a 30 fim schreiber-
site partially embedded in cohenite at a cohenite-kamacite
interface, 60 fim of cohenite, 90 fim of schreibersite embed-
ded in cohenite, 0.3 mm of cohenite, 0.2 mm of massive
schreibersite, 0.1 mm of cohenite, 0.2 mm of kamacite, 50
fim of cohenite, 0.2 mm of massive schreibersite, 80 fim of
cohenite, and 0.8 mm of kamacite.
3334, traverse 2: crossed 1.0 mm of kamacite, 50 fim of a large
grain boundary schreibersite, and 1.5 mm of kamacite. ?
3334, traverse 3: crossed 0.8 mm of kamacite, a 15 fim wide
taenite border schreibersite, 25 ^im of taenite, 0.3 mm of
kamacite, a 15 fim wide schreibersite in sequence with
taenite, 10 fim of kamacite, a 40 fim taenite, and 0.4 mm of
kamacite. The data for an abbreviated section of this traverse
are given in Figure 3.
3334, traverse 4: crossed 0.1 mm of kamacite, a 10 fim wide
taenite border schreibersite, 25 fim of taenite, and 50 fim of
kamacite.
3334, traverse 5: crossed 50 fim of kamacite, a 5 fim wide
taenite-border schreibersite, 20 /xm of taenite, and 60 fim of
kamacite.
Figure 12 is a tracing of a photomosaic of the
studied area in Lexington County, section 3334.
The path of the traverses, the phases crossed, and
the nature of the structure are indicated diagram-
matically. This section illustrates a general prob-
lem encountered in obtaining kamacite-schreiber-
site interface data in the more carbon-rich coarse-
structured octahedrites. The large, low-Ni schrei-
bersites in these meteorites are frequently com-
pletely surrounded by cohenite. Their Ni values
may be determined, but kamacite interfaces are
not present. Traverse 1, however, illustrates an
interesting schreibersite-cohenite relationship that
will be observed later in other meteorites. Cohenite
borders around large schreibersites occasionally
include small schreibersites, and more frequently
have small schreibersites at cohenite-kamacite in-
NUMBER 21 29
TABLE 6. ?Lexington County schreibersite-kamacite interface measurements
Specimen
No.
3334
3334
3334
3334
3334
Traverse
No.
1
2
3
4
5
Structure traversed & schreibersites measured
[a-Ph-cohen1te-Ph-coheni te-Ph-coheni te-a-
cohen1te-Ph-cohen1te-a, 3.5 mm]
Schreibersite embedded in cohenite at a-
Interface, 25x120 ym
Schreibersite embedded in cohenite, 140x280
ym
Enter
Exit
Massive schreibersite surrounded by cohen-
ite (6x0.3 mm), 200 vm traverse
Enter
Exit
Massive schreibersite surrounded by cohen-
ite (6x0.3 mm), 170 ym traverse
Enter
Exit
[a-Ph-a, 2.5 mm]
Grain boundary schreibersite, 500x40 ym
[a-Ph-Y-a-Ph-a-Y-a, 1.6 mm]
Taenite border schreibersite, 15x40 ym
Schreibersite near taenite, 10x20 ym
[a-Ph-Y-a, 200 ym]
Taenite border schreibersite, 5x10 ym
[a-Ph-Y-a, 140 ym]
Taenite border schreibersite, 5x10 ym
Weight
*NiSch
34.8
33.0
32.5
25.0
24.6
24.1
24.1
39.3
50.0
52.5
50.5
51.0
Percent
XN1a
4.4
5.0
6.0
6.2
6.2
6.3
%
0
0
0
0
0
0
.04
.04
.03
.03
.03
.03
Ni GradientLength
(ym)
150
300 .
50
50
50
30
Wt.XNi
6.0
7.0
7.2
7.4
7.3
7.4
P Gradient
Length
(ym)
200
200
150
100
50
30
Wt.%P
0.07
0.06
0.06
0.05
0.05
0.05
Atomic
*NiSch
29.9
28.3
27.9
21.4
21.1
20.6
20.6
33.9
43.3
45.6
43.6
44.2
Percent
%N1a
4.2
4.8
5.7
5.9
5.9
6.0
XP
a
0.07
0.07
0.05
0.05
0.05
0.05
terfaces. The Ni values in these schreibersites
increase away from the large schreibersite. In this
case, the large schreibersite contains approxi-
mately 25% Ni, the small one completely embed-
ded in cohenite has an apparent Ni gradient from
32.5% to 33.0% increasing in the direction toward
kamacite, and the kamacite interface schreibersite
contains 35'% Ni. The measurements in Table 6
are otherwise comparable to those reported above
for similar structures.
BAHJOI
The Bahjoi, India, meteorite is an observed fall
of 1934, the only one included in this study. It is a
typical Group I meteorite of above average Ni
content. Bahjoi contains complex silicate-troilite-
graphite-chromite nodules surrounded by rims of
schreibersite, and, in turn, cohenite. Grain bound-
ary schreibersites, rhabdites of various sizes, and
taenite border schreibersites are present. Taenite
and plessite are present in greater abundance than
in the previously examined meteorites, and pearl-
itic and martensitic forms are common. An area
of the carbide haxonite was observed within a
pearlitic plessite area.
Section 1807(Sil) contains part of a complex
silicate-containing inclusion surrounded by schrei-
bersite and cohenite, and small areas of kamacite.
1807(Sil), traverse 1: started within the complex inclusion, cross-
ing first an island of kamacite surrounded by cohenite. The
phases in sequence were: 0.1 mm of cohenite, 0.2 mm of
kamacite, 0.1 mm of cohenite, 1.5 mm of schreibersite, 0.5
mm of cohenite, 0.1 mm of schreibersite containing a slight
Ni gradient, and finally 1.0 mm of kamacite. The central
part of this traverse is illustrated in Figure 13 and the data is
given in Table 7.
Section 1807 is of a typical Widmanstatten area
of Bahjoi, free of large inclusions. Rhabdites,
grain boundary schreibersites, and taenite border
schreibersites are present in abundance. Many
taenite-plessite areas are present in a variety of
morphologies. One of these martensitic areas con-
tains a 0.1 x 0.1 mm area of haxonite.
1807, traverse 2: crossed 30 /xm of grain boundary schreibersite,
0.7 mm of kamacite, a 15 fim wide taenite border schreiber-
site, 20 /xm of taenite, and 10 fim of kamacite.
The observations made on the Bahjoi meteorite
30 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
TABLE 7. ?Bahjoi schreibersite-kamacite interface measurements
SpecimenNo.
1807(Sil)
1807
TraverseNo.
1
2
Structure traversed & schreibersites measured
[Cohenite-a-cohenite-Ph-cohenite-Ph-a, 3.5 mm]
Massive schreibersite, 10x4 mm, surroundedby cohenite and containing -30% silicate
Enter
Exit
Schreibersite 100 ym wide embedded incohenite at a-border
Enter
Exit
[Ph-a-Ph-a, 750 um]
Grain-boundary schreibersite, 35x250 um
Taenite-border schreibersite, 20x60 wm
Weight
*N1Sch
18.8
18.9
32.3
33.5
38.8
49.0
Percent
%M
4.2
5.2
6.2
%P
a
0.04
0.04
0.03
Ni Gradient
Length
( m)
600
150
30
Wt.XNI
6.5
7.2
6.2
P Gradient
Length
( m)
200
150
100
Wt.SP
0.06
0.06
0.06
Atomic Percent
%Ni,. . XN1 %PSch a a
16.0
16.1
27.7
28.8
33.4 5.0 0.07
42.4 5.9 0.05
are mainly similar in character to examples previ-
ously described. Figure 13 is a composition profile
based on data from a part of traverse 1. Part of
the massive schreibersite and its cohenite border
are included. Ni values are low in cohenite and
increase slightly upon moving away from the large
schreibersite. The small exterior schreibersite is
enclosed on three sides by cohenite and contains a
measurable Ni gradient. This type of association
is common in high C Group I meteorites.
GOOSE LAKE
The Goose Lake, California, meteorite is a high
Ni member of Group I with a medium octahedrite
structure. Polished and etched surfaces reveal
many large cohenite bordered, skeletal schreiber-
site inclusions, and occasional complex silicate -
troilite-graphite-schreibersite-cohenite associa-
tions. The cohenite has undergone partial decom-
position to kamacite and graphite. Grain boundary
schreibersites, rhabdites in a range of sizes, and
taenite border schreibersites are present. Taenite-
plessite areas are more abundant than in the
meteorites examined previously and are present
in a variety of forms. Weathering has invaded the
structure along grain boundaries and is present in
cracks within the large schreibersites.
Section 1332 is dominated by two areas of cohen-
ite bordered schreibersite of approximately 5 X 10
mm each (photograph in Doan and Goldstein,
1969:765). One smaller cohenite-schreibersite in-
clusion is also present. Kamacite areas around
these inclusions are comparatively free of other
structures for distances up to 2 mm. Beyond this,
normal Widmanstatten areas are present.
1332, traverse 1: began at the border of a taenite area, crossed
2.4 mm of kamacite, and entered a 30 /xm wide grain
boundary schreibersite near its contact with a taenite border
of a plessite area. Numerical data for this and succeeding
Goose Lake traverses are given in Table 8.
1332, traverse 2: crossed 430 /xm of kamacite, 50 fim of schrei-
bersite embedded in a cohenite rim of a large schreibersite,
90 fim of cohenite, 0.2 mm of schreibersite, 50 pm of
cohenite, and 0.9 mm of kamacite.
1332, traverse 3: began by crossing a 40 fim wide schreibersite
associated with a taenite-plessite area, crossed 2.5 mm of
kamacite, entered a 40 /xm wide schreibersite embayed in
cohenite, 150 fim of cohenite, and 0.3 mm of massive
schreibersite.
1332, traverse 4: started near the initial point of traverse 3
within the taenite border, crossed 0.2 mm of kamacite, a 50
fim wide grain boundary schreibersite, and 0.7 mm of
kamacite.
Section 1332(2) is similar to number 1332 in that
it also contains a 5 X 10 mm skeletal schreibersite-
cohenite area. This section contains an unusually
large scrfreibersite that is only partially surrounded
by cohenite, thus permitting kamacite-schreiber-
site interface measurements. Its longest dimension
is 1.4 mm and has a maximum thickness of 0.6
mm. Morphology suggests that it is an unusually
large grain boundary schreibersite rather than a
massive schreibersite of the skeletal variety. The
areas away from these inclusions contained normal
Widmanstatten pattern.
1332(2), traverse 5: crossed 0.1 mm of kamacite, 0.1 mm cohen-
ite, 1.8 mm of massive schreibersite, 170 fxm cohenite, 2.1
mm of kamacite, 20 /tin of cohenite, 0.1 mm of large grain
boundary schreibersite, and 0.8 mm kamacite.
NUMBER 21 31
4.2
0.04
400
MICRONS
FIGURE 13. ?Ni and P profile for Bahjoi meteorite, USNM 1807: left, massive schreibersite
bordered by cohenite; right, small schreibersite partially enclosed in cohenite, but also with an edge
in contact with kamacite.
1332(2), traverse 6: crossed 0.4 mm of kamacite, 25 fim of
cohenite, 1.2 mm of schreibersite containing a small Ni
gradient over part of its distance, and 0.5 mm of kamacite.
This traverse crosses the length of the large schreibersite in
number 5.
1332(2), traverse 7: crossed 40 /*m tip of large schreibersite, 1.2
mm of kamacite, 0.2 mm of second tip of same schreibersite,
and 0.4 mm of kamacite. This is the same large schreibersite
traversed in numbers 5 and 6.
1332(2), traverse 8: crossed 0.5 mm of kamacite, 150 (im of the
third tip of the large schreibersite measured in traverses 5
through 7, and 0.5 mm of kamacite.
1332(2), traverse 9: crossed 0.4 mm of kamacite, 150 /xm of a
small schreibersite near the large schreibersite in traverses 5
through 8, and 0.3 mm of schreibersite.
1332(2), traverse 10: crossed 0.2 mm of kamacite, 30^im of a
grain boundary schreibersite, and 0.2 mm of kamacite.
1332(2), traverse 11: crossed 0.3 mm of massive schreibersite
close to traverse number 5, 0.1 mm of cohenite, a 20 ^im
wide schreibersite embedded in cohenite, 0.1 mm of kama-
cite, and 50 fim of cohenite.
The associations measured in Goose Lake are
similar in character to those described above for
other meteorites. The schreibersite precipitates in
this case, however, exist in an environment that is
more Ni-rich.
BALFOUR DOWNS
The Balfour Downs, Western Australia, mete-
orite is one of the highest Ni members of Group
I. The first prepared surface of this specimen was
perpendicular to the one shown in Figure 14,
along its bottom edge. This area of approximately
12 cm2 was free of large inclusions, having the
regular structure of the upper right-hand area in
Figure 14. On the basis of this casual observation,
Balfour Downs was assumed to be a low P medium
octahedrite. Actually, it contains localized massive
schreibersite enclosing significant quantities of sil-
icates. These schreibersites are normally enclosed
in cohenite and surrounded by swathing kamacite
areas that interrupt the regular Widmanstatten
32 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
TABLE 8. ?Goose Lake schreibersite-kamacite interface measurements
Specimen
No.
1332
1332
1332
1332
1332(2)
1332(2)
1332(2)
1332(2)
1332(2)
1332(2)
1332(2)
Traverse
No.
1
2
3
4
5
6
7
8
9
10
11
Structure traversed & schreibersftes measured
[y-a-Ph-y, 2.4 mm]
Schreibersite at y-border, 25 ym wide
[a-Ph-cohenite-Ph-cohenite-a, 1.6 mm]
Schreibersite embaying cohenite, 55 ym tra-
verse
Enter
Exit
Massive schreibersite enclosed in cohenite
(3.4x0.2 mm), 260 ym traverse
[Ph-a-Ph-cohenite-Ph, 3.1 mm]
Schreibersite near taenite, 40x70 ym
Schreibersite embaying cohenite, 30x50 ym
Massive schreibersite (6x2 mm), 300 ym
traverse
[y-a-Ph-a, 950 um]
Grain boundary schreibersite (60x250 ym),
60 ym traverse
[a-cohen1te-Ph-cohen i te-a-cohen1te-Ph-a,
5.3 mm]
Cohenite bordered massive schreibersite
(2x6 mm), 1.8 mm traverse
Enter
Middle
Exit
Large schreibersite with partial cohenite
border (1.4x0.6 mm), 100 um traverse
[a-cohenite-Ph-a, 2.2 mm]
Large schreibersite with partial cohenite
border (1.4x0.6 mm), 1.2 mm traverse
Enter
Middle
Exit
[Ph-a-Ph-a, 1.8 mm]
Large schreibersite of traverses 5 and 6
40 um traverse
200 ym traverse
Enter
Exit
[a-Ph-a, 1.2 mm]
Large schreibersite of traverses 5-7,
150 ym traverse
Enter
Exit
[a-Ph-a, 0.9 mm]
Small schreibersite near traverses 5-8
(250x400 ym), 150 ym
Enter
Exit
[a-Ph-a, 0.5 mm]
Grain boundary schreibersite (0.03x1.6 mm)
associated with plessite, 30 ym traverse
[Ph-cohenite-Ph-a-cohenite, 0.7 mm]
Massive schreibersite (same as traverse 5),
0.4 mm traverse
Start
Exit
Schreibersite (20x60 ym) embedded in
cohenite-a border
Weight
*N1Sch
52.0
35.0
34.0
25.0
46.5
36.5
18.5
41.0
18.7
18.2
18.5
31.3
29.3
27.3
27.3
31.0
27.0
27.0
29.1
29.1
27.2
27.2
44.0
18.3
18.3
37.3
Percent
a
6.0
4.1
6.0
4.3
5.1
3.8
3.6
3.9
3.3
3.5
3.7
3.8
3.1
3.6
5.8
4.5
%Pa
0.03
0.05
0.04
0.05
0.05
0.06
0.07
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.05
0.06
N1 Gradient
Length
(ym)
25
100
50
150
100
200
500
400
600
300
500
400
300
250
100
150
Wt.*N1
7.2
6.4
7.0
5.7
7.2
5.7
5.8
6.0
6.2
6.8
6.5
6.5
6.0
5.5
7.4
6.0
P Gradient
Length
(ym)
100
50
100
200
Flat
100
250
200
150
150
150
150
100
100
200
100
Wt.iSP
0.07
0.06
0.07
0.08
0.05
0.08
0.09
0.08
0.08
0.09
0.09
0.08
0.07
0.07
0.06
0.09
Atomic
*N1Sch
45.1
30.1
29.2
21.4
40.2
31.4
15.8
35.4
15.9
15.5
15.8
26.9
25.1
23.4
23.4
26.6
23.1
23.1
25.0
25.0
23.3
23.3
38.0
15.6
15.6
32.1
Percent
%M
a
5.7
3.9
5.7
4.1
4.9
3.6
3.4
3.7
3.1
3.3
3.5
3.6
3.0
3.4
5.5
4.3
%Pa
0.05
0.09
0.07
0.09
0.09
o.n
0.13
0.11
o.n
0.11
o.n
o.n
0.13
0.13
0.09
0.11
NUMBER 21 33
FIGURE 14. ?Etched surface of Balfour Downs meteorite, USNM 3202: left-hand side of the slice,
large schreibersites, several containing silicates; (outlined area indicates polished section that was
prepared for detailed study).
34 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
TABLE 9. ?Balfour Downs schreibersite-kamacite interface measurements
Specimen
No.
3202
3202
3202
3202
Traverse
No.
1
2
3
4
Structure traversed & schreibersites measured
[Ph-a-Ph-a-Ph-Y, 3.3mm]
Massive schreibersite,
Start
Exit
1550 ym traverse
Residual schreibersite 30x800 ym, asso-
ciated with taenite, crossed at two
locations
[Ph-a-Ph-a, 3.1 mm]
Massive schreibersite,
Start
Exit
Massive schreibersite,
Enter
Exit
[o-Ph-o-cohen1te-Ph, 2.3 n
Massive schreibersite,
Enter
Exit
Massive schreibersite,
Enter
End
[Ph-cohenite-a-cohenite-PI
Massive schreibersite.
Start
Exit
Massive schreibersite,
Enter
Stop
600 ym traverse
500 ym traverse
400 ym traverse
1350 ym traverse
,2.1 mm]
800 ym traverse
250 ym traverse
Weight
*N1Sch
20.5
21.0
41.0
41.0
21.3
21.5
21.5
21.5
21.0
21.0
20.0
20.0
19.5
19.5
20.5
21.0
Percent
*N1a
2.9
5.3
5.5
2.6
2.8
3.0
2.9
2.8
%Pa
0.05
0.04
0.03
0.05
0.05
0.05
0.05
0.05
Ni Gradient
Length
(ym)
1150
100
50
300
250
1500
250
150
Wt.%N1
6.7
00 00
4.8
4.8
7.0
5.3
4.8
P Gradient
Length
(ym)
200
100
300
200
300
200
150
Wt.fcP
0.09
0.09
0.09
0.09
0.11
0.10
0.08
Atomic
*N1Sch
17.5
17.9
35.4
35.4
18.2
18.4
18.4
18.4
17.9
17.9
17.1
17.1
16.7
16.7
17.5
17.9
Percent
a
2.8
5.1
5.2
2.5
2.7
2.9
2.7
2.7
%Pa
0.09
0.07
0.05
0.09
0.09
0.09
0.09
0.09
pattern (center left in Figure 14). Grain boundary
schreibersite, schreibersite associated with taenite-
plessite areas, and rhabdites in a variety of sizes
are present. Taenite-plessite areas are present in a
variety of forms and Neumann bands are com-
mon.
Section 3202 was prepared from the outlined
area in Figure 14. It was selected particularly for
the presence of massive schreibersite that was
partially free of cohenite borders. Figure 15 is a
photograph of a photomosaic of a portion of this
section. The paths of the four traverses described
below are indicated in Figure 16, a sketch based
on Figure 15.
3202, traverse 1: crossed 1.5 mm of massive schreibersite, 4.5
mm of kamacite, a 40 fim wide schreibersite surrounding a
taenite area, 0.1 mm of kamacite, recrossed a 30 fim width
of the same schreibersite previously crossed, 50 /urn of
kamacite, and 50 fim of taenite. Numerical data for this and
succeeding Balfour Downs traverses are given in Table 9.
3202, traverse 2: crossed 0.6 mm of massive schreibersite, 0.6
mm of kamacite, 0.5 mm of massive schreibersite, and 1.5
mm of kamacite.
3202, traverse 3: crossed 0.3 mm of kamacite, 0.4 mm of massive
schreibersite, 0.2 mm of kamacite, 80 jxm of cohenite, and
1.3 mm of massive schreibersite. The path of this traverse is
indicated in Figures 15 and 16 and the electron microprobe
profile is shown in Figure 17.
3202, traverse 4: crossed 0.8 mm of massive schreibersite, 0.1
mm of cohenite, 0.8 mm of kamacite, 0.1 of cohenite, and
0.2 mm of massive schreibersite.
Also indicated in Figure 16 are Ni concentrations
in various areas of the two massive schreibersites.
One is tempted to conclude that a slight Ni gra-
dient exists, with the lowest Ni values being adja-
cent to cohenite. The differences, however, are
small, and this should undoubtedly be looked at
more systematically in the future. The Ni and P
profile reproduced in Figure 17 illustrates several
interesting features. The Ni concentration in kam-
acite is low close to these large schreibersites, as
are the Ni interface values. There also appears to
be a Ni gradient in cohenite, with Ni increasing
away from the massive schreibersite.
NUMBER 21 35
FIGURE 15. ?Photograph of photomosaic of area of Balfour Downs meteorite examined, USNM
3202: large schreibersite inclusions in kamacite, partially bordered by cohenite. (Line indicates
position of traverse 3; see Figure 17.)
36 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
I mm
3202
FIGURE 16. ?Sketch based on Balfour Downs section illustrated in Figure 15 (arrrows indicate
location of traverses; numbers give schreibersite Ni concentrations in weight percent)
NUMBER 21 37
0.05
2.9
600
MICRONS
FIGURE 17. ?Ni and P profiles in Balfour Downs meteorite, USNM 3202, traverse 3 in Figure 16
(schreibersite inclusion at the right enclosed in cohenite).
Discussion
EQUILIBRIUM CONSIDERATIONS OF PHASE GROWTH
The total amount of P present in an iron mete-
orite has a marked effect on the structural devel-
opment process, and consequently on the final
structure observed in prepared sections. Studies
of Widmanstatten pattern growth (Wood, 1964;
Goldstein and Ogilvie, 1965b; Goldstein and Axon,
1973) have demonstrated that octahedrite meteor-
ites cooled from high temperatures and were in
equilibrium down to 700? C. Schreibersite growth
studies (Goldstein and Ogilvie, 1963; Doan and
Goldstein, 1969) led to similar conclusions concern-
ing equilibrium temperatures for iron meteorites
in general. In a discussion of the effects of P on
the development of the Widmanstatten pattern,
Goldstein and Doan (1972) estimate that equil-
brium in iron meteorites was maintained down to
650? C, and probably to 600? C. Recent work by
Randich (1975) suggests the schreibersite-kamacite
equilibrium is maintained down to 500? C for
diffusion distances as great as 1000 fim. Previous
work, therefore, suggests that many of the schrei-
bersite precipitates observed in iron meteorites
nucleated and grew under equilibrium conditions
during a significant part of their cooling history.
It is the purpose of this section to examine schrei-
bersite growth and related structural development
in the selected group of coarse-structured iron
meteorites in terms of the equilibrium Fe-Ni-P
diagram of Doan and Goldstein (1970). How far
can equilibrium considerations take us in explain-
ing observed structures?
Before proceeding, however, brief mention
must be made of the effects of S, C, and Co on
the Fe-Ni-P system. These elements are present in
iron meteorites in quantities that may be compa-
rable to or in excess of P. The role of S has been
discussed recently by Goldstein and Axon (1973)
and Wai (1974). Goldstein and Axon (1973) point
out that S may be important in lowering the
38 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
temperature of metal melts, thereby mobilizing
metal without melting silicates. This effect, how-
ever, would only extend down to temperatures in
the range of 1000? C. At lower temperatures,
material of Fe-FeS eutectic composition becomes
enclosed in solidifying metal, and the solubility of
S in the metallic phases becomes negligibly small.
Sulfur is removed from the metal by this process,
becoming isolated in complex sulfide nodules (El
Goresy, 1965).
The situation with respect to carbon is much
more complex. Graphite or graphite-troilite nod-
ules present in many meteorites represent high
temperature segregations of C as well as S. The
petrography of these complex associations has
been described by El Goresy (1965). Comparatively
large amounts of C, however, remain soluble in
the metal to lower temperatures, temperatures
within the schreibersite growth range. Brett (1967)
has discussed the occurrence of cohenite ((Fe,Ni)3C)
in meteorites and described its formation in terms
of phase equilibria using an extrapolated Fe-Ni-C
diagram. He proposed that cohenite formed
within a narrow range of bulk Ni values in the
temperature range of 650? to 610? C. Scott (1971a)
has discussed carbides in iron meteorites in great
detail from a petrographic point of view. He has
also reported two new carbides, haxonite
((Fe,Ni,Co)23C6) (Scott, 1971b), and a phase desig-
nated W-carbide ((Fe,Ni,Co)2.5C), both of which
contain considerably more Ni and Co in their
structure than is observed in cohenite. Scott
(1971a) suggests that cohenite may have formed at
somewhat lower temperatures than those sug-
gested by Brett (1967). Both Scott (1971a) and
Buchwald (1976) point out that cohenite is much
more widely distributed in meteorites than previ-
ously realized. Cohenite and schreibersite are fre-
quently observed in intimate association (Drake,
1970; Clarke, 1969; Buchwald, 1976), with cohenite
normally encapsulating large schreibersite inclu-
sions in the high C Group I meteorites. These
schreibersite-bordering cohenites normally enclose
occasional small later-formed schreibersites as has
been illustrated above (Figure 13). This morphol-
ogy suggests the possibility of concurrent schrei-
bersite-cohenite growth after major amounts of
schreibersite have formed. Carbon, therefore, is
seen to be a component of the system of direct
concern here. Experimental data on carbon's ef-
fect on the Fe-Ni-P system at low temperatures is
not available, and it has been assumed that it can
be ignored here except in cases where its physical
presence in the form of cohenite or other carbides
is important. This assumption may well require
reevaluation,when appropriate experimental data
becomes available.
Another simplifying assumption made in this
study is to ignore the presence of Co, an element
present in iron meteorites in the range of 0.3% to
0.7% (Moore et al., 1969). Co distributes itself
between the three phases of interest, following Fe
and seemingly having little effect on the equilibria
involved.
In the following sections, individual meteorites
will be discussed in terms of structural develop-
ment on equilibrium cooling through the low tem-
perature portion (995? to 550? C) of the Fe-Ni-P
system. The Doan and Goldstein (1970) isothermal
sections were recalculated on an atom percent
basis, and four of these sections plotted on a
triangular coordinate system are given in Figure
18.
Equilibrium in the system at hand may be de-
fined as (1) the phases present are homogeneous,
that is to say, free of measurable concentration
gradients, and (2) the compositions of individual
phases are related to the bulk composition by the
phase diagram. In the case at hand, this means
that (a) the bulk composition is the composition of
the single phase present when it lies within the
kamacite (a) or taenite (y) field of the diagram;
(b) the bulk composition is on a tie line joining the
compositions of the two phases present in the
kamacite + schreibersite (a + Ph), the kamacite +
taenite (a + y), and the taenite + schreibersite (y
+ Ph) fields of the diagram; and (c) the bulk
composition is related to the three phases present
by the compositions at the corners of the triangular
kamacite + taenite + schreibersite (a + y + Ph)
field of the diagram. The proportions of the
phases present in the two- and three-phase fields
may be calculated using lever rule principles. Cal-
culations of this type were carried out for each of
the meteorite compositions considered and are
presented in a series of tables to follow (Tables
10-15). Temperatures, the proportion of phases
present, and their Ni contents are given. These
data will be discussed for each meteorite in terms
of observed structures. Selected compositions rep-
resenting these meteorites are plotted on a series
of isothermal sections of the iron-rich corner of
NUMBER 21 39
OS?
40 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
TABLE 10. ?Equilibrium cooling of composi-
tions appropriate to the Coahuila meteorite
(percentage of phases present at indicated
temperatures with their Ni contents)
?c
995
975925
875850
750
700650
600
550
?C
995975
925875
850750
700
650600
550
0.5
%a
60
7593
99
99.7
1.1
%a
1250
82
9498
98
Atomic
*Y
100
100100
100100
40
257
1
Atomic 5
%y
100
100100
100
8850
18
5
i P, 5
SPh
0.3
5 P, 5
SPh
12
2
.3 Atomic
a
3.3
4.04.4
5.2
5.2
.3 Atomic 3
a
3.53.8
4.5
5.05.2
5.2
t N1
%N
5
55
55
6
912
14
. N1
%H
55
55
56
9
11
Y
.3
.3.3
.3.3
.5
.2
Y
.3.3
.3.3
.6.9
.1
2NiPh
14
*NiPh
9.013
13
the Fe-Ni-P system from 850? to 550? C in Figure
19.
COAHUILA. ?The Coahuila hexahedrite is a me-
teorite of both low Ni (5.6 weight %, 5.3 atomic
%) and low P (0.3 weight %, 0.5 atomic %). These
bulk composition values are well established in the
literature and are generally consistent with the
major features of the observed structure. The
presence of occasional large schreibersites as de-
scribed above suggests, however, that the P value
may be*somewhat low. For this reason the calcula-
tions given in Table 10 were made using two P
values, the literature value (0.5 atomic %) and one
approximately twice as high (1.1 atomic %). The
equilibrium phases present, their amounts in mole
percent, and their Ni contents in atomic percent
are listed in Table 10 for 10 temperatures between
995? to 550? C.
Due to Caohuila's low P (0.5 atomic %), the
predicted structural development sequence using
the Fe-Ni-P diagram is essentially the same as that
derived using the Fe-Ni diagram. At 995? C (Table
10), taenite of the bulk meteorite composition is
the only phase present, and this situation contin-
ues until the meteorite cools to below 850? C. As
the temperature drops an additional 200? C, tae-
nite transforms almost completely to massive single
crystal kamacite. The last of the taenite transforms
to kamacite below 600? C, in the temperature
range where schreibersite first nucleates. This se-
quence accounts for kamacite containing homoge-
neously nucleated rhabdite, the most prominent
feature of Caohuila. This low temperature of
initial schreibersite nucleation and the small
amount formed, however, make it difficult to
understand the presence of the occasional larger
schreibersites mentioned above.
If a somewhat higher P content is assumed,
significant differences are revealed. With an as-
sumed content of 1.1 atomic % P (Table 10),
transformation of taenite to kamacite starts more
than 50? C higher, extends over a slightly larger
temperature range, and is complete at a higher
temperature than in the previous case. Schreiber-
site precipitates, while more than 5% taenite re-
mains in the structure, and a total of 2% schreiber-
site is present at 600? C. In this case, one would
expect schreibersite to nucleate at taenite-kamacite
boundaries, but no direct evidence for this has
been developed. The actual equilibrium P value
may well lie between the two selected values,
perhaps 0.8 atomic %. This amount of P would
produce upon cooling a somewhat smaller quantity
of schreibersite, initially nucleating at a tempera-
ture below 650? C, and consuming any remaining
residual taenite. Both sufficient P and adequate
time at higher temperatures would be available to
permit growth of the isolated larger schreibersites
observed, and the bulk of the meteorite would
appear to have formed under conditions of lower
total P.
The 5.3 atomic % Ni and 0.8 atomic % P com-
positions are plotted on a series of isothermal
sections in Figure 19. At 850? C, Coahuila's com-
position is just inside the kamacite-taenite field of
the diagram, yielding a structure containing a
small amount of kamacite within a taenite matrix.
The field moves in relation to this composition
with decreasing temperature, until at 650? C only
a small amount of taenite remains in equilibrium
with a kamacite matrix. At 600? C, the composition
is within the kamacite-schreibersite field, perhaps
having passed through the corner of the kamacite-
taenite-schreibersite field. By 550? C, the composi-
tion lies well within the kamacite-schreibersite field
of the diagram, with approximately 98% kamacite
NUMBER 21 41
850?C
0
0
100% Fe
ABELLSBANK
? COAHUILA
? SANTA LUZIA
S-BALLINGER
? LEXINGTON COUNTY
- BAHJOI
AGOOSE LAKE-BALFOUR DOWNS
5.0
5.3
6.2
6.2
7.1
7.8
%P
3.6
0.8
2.7
1.4
1.8
2.7
10 15
ATOMIC %Ni
20
FIGURE 19. ? Meteorite bulk compositions plotted on series of isothermal sections of Fe-Ni-P
system.
42 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
containing 5.1 atomic % Ni and 0.5 atomic % P,
being in equilibrium with 2% schreibersite contain-
ing 13 atomic % Ni and 25 atomic % P.
The development sequence described above ac-
counts for the major features of Coahuila struc-
ture. Equilibrium cooling of this bulk composition
produced large single crystals of kamacite contain-
ing minor inclusions. The earliest-formed schrei-
bersite nucleated and grew on preexisting sulfide
inclusions where available, or perhaps at kamacite-
taenite interfaces. The great majority of the indi-
vidual schreibersite crystals, however, may be as-
sumed to have nucleated homogeneously within
kamacite. If this process had stopped at 550? C,
the meteorite would contain kamacite of 5.1 atomic
% Ni and 0.5 atomic % P, and schreibersite of 13
atomic % Ni. Phase composition measurements
from the literature and those reported above indi-
cate that diffusion controlled growth continued to
lower temperatures, modifying phase composi-
tions significantly, while producing only subtle
changes in the gross structural features. These
changes in phase compositions and subtle differ-
ences in structure will be discussed in some detail
later in this paper.
BALLINGER. ?The Ballinger meteorite is a low
Ni octahedrite probably containing more P than
the analytical value given in Table 1 (0.4 weight %
P, 0.7 atomic % P). The Ballinger photograph
(Figure 6) shows three areas containing large
schreibersite inclusions, perhaps suggesting that
the amount of P should be doubled. The calcula-
tions given in Table 11, therefore, were made
using 6.2 atomic % Ni (6.5 weight %) and both 0.7
atomic % and 1.4 atomic % P.
During equilibrium cooling in the low P case
(Table 11), taenite is the only phase present until
kamacite appears around 800? C. At 600? C, the
equilibrium structure contains 89% kamacite, 10%
taenite, and only 1% schreibersite. Upon cooling
to 500? C, all of the taenite would be expected to
transform, producing an equilibrium structure of
99% kamacite and 1% schreibersite. Both the pres-
ence of the massive schreibersites and residual
taenite in the actual structure argue against this
interpretation. The growth of such large schrei-
bersites requires more P, and the presence of
taenite demonstrates that equilibrium cooling was
not maintained down to 550? C.
The 1.4 atomic % P calculation for Ballinger
(Table 11 ) leads to a more reasonable interpreta-
TABLE 11. ?Equilibrium cooling of composi-
tions appropriate to the Ballinger meteorite
(percentage of phases present at indicated
temperatures with their Ni contents)
"C
995975
925875
850750
700650
600550
?C
995975
925875
850750
700650
600550
0
%a
1560
7889
99
1
%a
54
3062
7788
96
.7 Atomic
*Y
100100
100100
10085
4022
10
.4 Atomic
%y
100100
10095
9568
3620
8
% P, 6
%Ph
11
% P, 6
%Ph
12
23
44
.2 Atomic
*N1
a
3.74.0
4.95.2
6.0
.2 Atomic
XN1
a
3.94.0
4.04.5
5.05.2
5.7
* N1
XN1y
6.26.2
6.26.2
6.26.6
9.111
14
* N1
%N1y
6.26.2
6.26.3
6.37.2
9.111
14
*N1
1315
XN1
45
79
1315
Ph
Ph
.4.3
.1.0
tion of the structure. Small amounts of kamacite
form in taenite around 900? C, and by 850? C the
taenite matrix contains 4% kamacite and 1%
schreibersite, a situation that might be expected to
lead to the growth of several rather large schrei-
bersites on subsequent cooling rather than a num-
ber of small ones. By 750? C, one-third of the
meteorite is kamacite, undoubtedly the swathing
kamacite surrounding the large schreibersites that
are present. As the temperature falls to 650? C,
the meteorite transforms to 77% kamacite, and
the schreibersite doubles its Ni content to 9.0
atomic %, while increasing its amount by one-half.
As the temperature drops to 600? C, kamacite
increases in volume, schreibersite increases in both
amount and in Ni content, and taenite shrinks to
8%. The higher P content seems to be more
consistent with the observed structure and could
actually be a little low. The presence of taenite
and high Ni schreibersite in the actual structure
(see above) demonstrates that equilibrium cooling
did not persist down to 550? C, and that nonequi-
librium phase growth continued to lower temper-
atures. As in the case with Coahuila, however, the
gross features of the structure are accounted for
by equilibrium cooling of this particular composi-
tion (Figure 19).
NUMBER 21 43
SANTA LUZIA. ?The Santa Luzia meteorite
structure is dominated by schreibersite and schrei-
bersite-troilite inclusions surrounded by broad
areas of swathing kamacite (Figure 7). The litera-
ture values for its bulk composition, 6.3 atomic %
Ni and 1.6 atomic % P (6.6 weight % Ni and 0.9
weight % P, Table 1), are very close to those
assumed for the high P Ballinger composition
(Table 11), only a 0.1% increase in Ni and 0.2% in
P. This apparently minor change, however, has
important ramifications for the structural devel-
opment process in Santa Luzia. In Ballinger the
expected sequence of phases with falling tempera-
ture,
y ?> a + y ?? a + y + Ph,
is followed. In Santa Luzia this sequence is inter-
rupted around 850? C, where over a range of
temperature taenite is in equilibrium with schrei-
bersite, the sequence of phases being
TABLE 12. ?Equilibrium cooling of composi-
tions appropriate to the Santa Luzia meteorite
(percentage of phases present at indicated
temperatures with their Ni contents)
Ph Ph.
The growth of significant amounts of schreibersite
within taenite subsequent to the disappearance of
preexisting kamacite produced conditions that re-
sulted in the development of large volumes of
swathing kamacite and the unusual schreibersite
morphology observed in the meteorite.
The actual bulk P value for Santa Luzia may be
somewhat larger than the 1.6 atomic % value given
in Table 1. The calculations summarized in Table
12 indicate, however, that the same sequence of
phases would be expected whether 1.6 atomic %
or 2.7 atomic % P is assumed. In either case,
kamacite and taenite would have been in equilib-
rium at 925? C. It is reasonable to assume that this
kamacite nucleated within taenite at the preexist-
ing troilite-taenite grain boundaries, with kamacite
lamellae radiating away from the troilite. Upon
cooling to 875? C, the proportion of kamacite was
markedly reduced at both P levels. This small
amount of kamacite presumably remained local-
ized near the troilite nodules. The taenite inter-
faces with troilite and the kamacite-taenite inter-
faces acted as nucleation sites for schreibersite
upon further cooling, producing a skeletal struc-
ture that sets the pattern for subsequent schreiber-
site growth. Nearly half in one case, and more
than half in the other, of the schreibersite that
formed upon cooling the meteorite to 500? C
?c
995975
925
875850
750700
650600
550
?C
995975
925
875850
750
700650
600550
1
%a
10
3
2660
7687
95
2
%a
4350
35
4
27
6076
8891
.6 Atomic
%y
100100
90
9698
7137
218
.7 Atomic 3
*Y
5750
65
9093
66
3216
3
t p, 6
%Ph
12
33
35
5
i P, 6
SPh
67
7
88
9
9
.3 Atomic
%Ni
a
4.5
4.0
4.04.5
5.05.2
5.8
.2 Atomic 3
*N1a
5.35.4
4.9
4.0
4.0
4.55.0
5.25.4
i Ni
SN1Y
6.36.3
6.4
6.46.3
7.29.1
1114
i Ni
XN1
Y
6.87.1
7.0
6.46.3
7.2
9.111
14
%NiPh
4.44.5
5.37.1
9.013
15
*NiPh
4.44.5
5.3
7.19.0
1314
formed at the relatively high temperature of 850?
C. Upon further cooling Ni and P continued to
diffuse into the central schreibersite areas, P con-
tributing to additional growth and Ni contributing
to both growth and Ni enrichment. As a conse-
quence of this localized growth of schreibersite, a
large surrounding volume of low Ni metal was
produced, the swathing kamacite zone of the final
structure. This proposed growth sequence ex-
plains the morphology of these large sc(hreibersite
structures without the necessity of invoking precip-
itation from the liquid, a process that would re-
quire a bulk composition of more than 4.9 atomic
% P at high temperatures.
The broad swathing zone developed as the me-
teorite cooled from 850? C down into the 700? to
650? C range (Figure 7). This transformation takes
place easily at these temperatures as only a rela-
tively small amount of Ni movement is required
for both schreibersite growth and enrichment,
and for production of the taenite border at the
outer edge of the zone. This expanding taenite
border acts as a barrier to isolate the large central
schreibersites from the bulk material beyond the
swathing zone. As the temperature drops and
diffusion becomes increasingly restricted, condi-
tions are met for the nucleation of schreibersite at
44 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
this taenite border. By 550? C the taenite border
has been transformed and replaced with a contin-
uous grain boundary, approximately half of which
is occupied by grain boundary schreibersite. This
is amply supported by examination of Santa Luzia
specimens. The swathing zone boundary contains
a large amount of schreibersite and is, with a few
minor exceptions, free of residual taenite. The
swathing zone itself is also free of large schreiber-
sites and residual taenite, with the exception of
several small areas very close to the swathing zone
border. These appear to have been small areas of
residual taenite that became trapped within the
swathing zone at a very late stage and may have
been part of the taenite border system at higher
temperatures. Competition for P from neighbor-
ing schreibersites appears to be the reason that
these areas were not converted completely to
schreibersite.
Schreibersite equilibration with taenite in the
850? C temperature range results in a redistribu-
tion of P in the meteorite leading to the develop-
ment of areas of coarse Widmanstatten pattern
(Figure 7). At these high temperatures P diffuses
with ease, lowering its level even in the distant
bulk metal to approximately 1.0 atomic %. Subse-
quent to this, the swathing zone development
described above took place, resulting in the isola-
tion of that part of the meteorite from the areas
where the coarse Widmanstatten pattern formed.
The areas beyond the swathing zones developed
an effective bulk composition similar to that of the
low P Ballinger case described above. Widmanstat-
ten pattern nucleated in these areas at tempera-
tures below 750? C, and grain boundary schreiber-
site probably began nucleating at kamacite-taenite
interfaces around 600? C. The observed metallog-
raphy of these areas is consistent with this expla-
nation.
The bulk composition of Santa Luzia is plotted
on the diagrams in Figure 19. As in the previous
cases, it is obvious from the observations reported
in the results section that bulk equilibrium was not
maintained down to 550? C, and that phase modi-
fication persisted to considerably lower tempera-
tures.
LEXINGTON COUNTY AND BAHJOI. ?The bulk
compositions of the Lexington County and Bahjoi
meteorites appear to be poorly established (Table
1). They are structurally similar, carbon- and sul-
fur-rich members of Group I with intermediate
TABLE 13. ?Equilibrium cooling of composi-
tions appropriate to the Lexington County
and Bahjoi meteorites (percentages of phases
present at indicated temperatures with their
Ni contents)
?c
995
975925
875
850750
700650
600
550
?C
995975
925875
850750
700650
600550
0.
%a
4070
79
99
1.
%a
5
10
Tr
4260
78
94
5 Atomic
%y
100
100100
100100
100
6030
20
8 Atomic 3
%y
10095
9098
9796
5435
16
t P, 7.
%Ph
0.3
1
i p, 7.
0.32
34
45
5
6
1 Atomic
%M a
4.14.9
5.2
6.9
1 Atomic
%N1
a
5.5
5.3
(4.2)
4.55.0
5.2
6.5
S N1
%My
7.17.1
7.17.1
7.17.1
9.212
14
i Ni
%N1y
7.17.2
7.37.1
7.27.2
9.111
14
SUM
13
16
%M
5.5.
5.5.
7.9.
13
15
Ph
Ph
20
33
10
bulk Ni. The P values of Lexington County (0.5
atomic % P, 0.3 weight % P) are too low to be
reconciled with the amount of schreibersite ob-
served in prepared sections. The Lexington
County bulk Ni value is also probably somewhat
higher than 7.0 weight % Ni when the contribution
of large schreibersites is taken into consideration.
Much of the kamacite in this meteorite contains
7.0 weight % Ni, and some of it contains slightly
more Ni. Bahjoi also contains isolated large schrei-
bersites, requiring its effective bulk P to be greater
than 0.3 weight % P. For these reasons, the equilib-
rium phase calculations in Table 13 were made at
the intermediate Ni value of 7.1 atomic % (7.5
weight % Ni), and at 0.5 atomic % P and 1.8
atomic % P (1.0 weight % P). These calculations
suggest that the actual P value probably lies be-
tween the two, probably greater than 1 atomic % P.
Structure development in the cooling meteorite
does not start until below 750? C in the low P case
(Table 13), taenite being the only phase present at
higher temperatures. Schreibersite precipitation
does not begin until below 650? C, a temperature
at which more than 70% of the structure has
transformed to kamacite. At 600? C, only 0.3%
NUMBER 21 45
schreibersite is present. Under these conditions
schreibersite would have precipitated at kamacite-
taenite interfaces throughout a well-established
Widmanstatten pattern. P could not move through
the Widmanstatten pattern to precipitate as schrei-
bersite borders at isolated sulfide interfaces.
Schreibersite in these meteorites must have precip-
itated initially prior to extensive Widmanstatten
pattern development, requiring a greater bulk P
content.
The 1.8 atomic % P value used in Table 13 is
somewhat higher than necessary to account for
the observed structure. With this amount of P in
the system, kamacite would precipitate by 975? C
and schreibersite would be present by 925? C. If
the P content were reduced to 1.4 atomic %,
neither kamacite nor schreibersite would be pres-
ent in the system at 925? C or above. At 875? C,
however, schreibersite would be present growing
within taenite. Kamacite would not nucleate until
below 750? C. The important consideration here is
that within a reasonable range of P contents,
schreibersite nucleates within taenite, undoubtedly
at preexisting taenite interfaces with troilite or
troilite-silicate-graphite inclusions if they are avail-
able. Major quantities of schreibersite grow under
these conditions prior to kamacite nucleation and
the onset of Widmanstatten pattern development.
The 1.8 atomic % P value is used in Figure 19,
and it can be seen there that reasonable reduction
in the amount of P changes only the amounts of
the phases present. Nucleation temperatures for
both schreibersite and kamacite would be lowered
somewhat, and smaller amounts of P would pro-
duce smaller quantities of schreibersite. Neither
of these meteorites would appear to contain as
much as 6% schreibersite, but certainly they both
contain more than 1 %.
The structural development sequence in Lexing-
ton County and Bahjoi is similar to that of Santa
Luzia. With falling temperature a few massive
schreibersites grow within taenite, followed by
nucleation and growth of a swathing kamacite
zone. The Ni values in these massive schreibersites
(16 atomic % Ni for Bahjoi, 21 atomic % Ni for
Lexington County) require that Ni enrichment
continued to below 550? C. At some temperature
below 650? C, however, massive schreibersite
growth is stopped by precipitation of cohenite.
Cohenite contains no detectable P and, therefore,
isolates the massive schreibersite from a source of
P for continued growth. Ni gradients within the
cohenite borders (Figure 13) suggest that Ni may
continue to be supplied to the schreibersite
through the cohenite. Under these circumstances,
a distinct swathing zone morphology is not devel-
oped, but the surrounding kamacite does retain a
low Ni level and is comparatively free of later-
stage schreibersite precipitation.
Within the bulk metal surrounding these even-
tual cohenite-schreibersite inclusions, initial Wid-
manstatten pattern development started above
700? C, following its normal sequence. Bulk Ni is
low enough that a coarse-structured octahedrite
pattern formed. Both of these meteorites have
schreibersite morphologies and distributions simi-
lar to a meteorite like Ballinger, but they contain
more grain boundary taenite and residual taenite-
plessite areas, particularly Bahjoi.
GOOSE LAKE AND BALFOUR DOWNS. ?The Goose
Lake and Balfour Downs meteorites are two of
the highest Ni members of Group I (Table 1), and
they are meteorites that contain significant
amounts of C and S. The analytical P values from
the literature again appear to be too low to explain
the observed metallography. A planometric esti-
mate of 0.9 weight % P (1.6 atomic % P) for Goose
Lake was given by Doan and Goldstein (1969), and
this figure appears to be consistent with the ob-
served structure. Phase calculations are given in
Table 14 using a pair of compositions to represent
the two meteorites, 7.9 atomic % Ni (8.3 weight %
Ni), and 1.4 and 2.7 atomic % P (0.75 and 1.5
weight % P), bracketing what was undoubtedly
their effective bulk compositions.
When 1.4 atomic % P is assumed (Table 14),
schreibersite nucleates within taenite at a temper-
ature above 850? C. If 2.7 atomic % P is assumed,
schreibersite nucleates above 975? C in the pres-
ence of 20% kamacite. No structural evidence for
initial precipitation at kamacite-taenite interfaces
has been observed, supporting the suggestion that
the actual P level was in the neighborhood of 1.6
atomic % P. This means that schreibersite nu-
cleated around 900? C, undoubtedly at taenite-
troilite boundaries, were they available. The result
was growth of a large amount of schreibersite in
taenite, with nucleation of a kamacite swathing
zone following at a temperature somewhat above
700? C. As the temperature fell, a large supply of
Ni was required to keep these massive schreiber-
sites in equilibrium, a process that seems to have
46 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
TABLE 14. ?Equilibrium cooling of composi-
tions appropriate to the Goose Lake and Bal-
four Downs meteorites (percentage of phases
present at indicated temperatures with their
Ni contents)
?c
995975
925875
850750
700650
600550
?C
995975
925875
850750
700650
600550
1
%a
2547
7092
2
%a
2023
2547
7091
.4 Atomic
%y
100100
100100
9998
7250
264
.7 Atomic
*Y
8075
9695
9392
6744
21
% P, 7
XPh
12
33
44
% P, 7
%Ph
24
57
88
99
9
.9 Atomic
XN1
a
4.55.0
5.27.1
.8 Atomic
%N1a
6.26.3
4.55.0
5.27.0
% N1
XN1
Y
7.97.9
7.97.9
7.98.0
9.111
1418
% N1
%N1Y
8.28.3
7.87.9
7.97.9
9.111
14
*NiPh
6.67.0
7.19.0
1316
%N1Ph
6.05.7
6.06.2
6.67.1
9.013
16
continued down to below 550? C (17 to 24 atomic
% Ni in cohenite-enclosed massive schreibersite).
The size of the massive schreibersites requires that
the cohenite borders precipitated at temperatures
below 750? C. Within this same 700? to 550? C
temperature range, the bulk of the metal away
from the large schreibersites transformed to nor-
mal Widmanstatten pattern. Grain boundary
schreibersite, rhabdites, and taenite border schrei-
bersites were formed during this part of the se-
quence.
In Figure 19 the 2.7 atomic % P value is used. It
is obvious here that the same sequence of phases
is obtained if the P value is somewhat reduced.
The higher Ni is the significant structural devel-
opment factor in this case.
BELLSBANK. ?The Bellsbank meteorite is anom-
alous, unusually low in Ni and high in P. The
gross segregation of schreibersite in its structure
makes it unusually difficult to arrive at a satisfac-
tory bulk P content. For this reason the literature
values of 5.3 weight % Ni and 2 weight % P were
bracketed in Table 15 by assumed compositions of
5.3 weight % Ni and both 1 and 3 weight % P (5.0
atomic % Ni and 1.8 and 3.6 atomic % P, and 4.9
atomic % Ni and 5.3 atomic % P). The intermedi-
ate P level proved to be the most reasonable in
terms of explaining the observed structure and
will be considered first.
For the composition 5.0 atomic % Ni and 3.6
atomic % P (Table 15), kamacite is the only equilib-
rium phase present from 1100? C to below 975? C.
At a temperature somewhat below 975? C taenite
nucleates, and a small amount of it is in equilib-
rium with kamacite at 925? C. By 875? C, taenite
has grown to be nearly as abundant as kamacite
and the structure contains 6% schreibersite, un-
doubtedly having nucleated at preexisting kama-
cite-taenite grain boundaries as well as around
sulfide inclusions. At 700? C most of the taenite
has disappeared, and at 650? C, 90% kamacite is
in equilibrium with 10% schreibersite. The schrei-
bersite continues to increase in Ni as it cools,
reaching a value of 11 atomic % Ni at 600? C and
below, with a Ni content in the kamacite of 4.2
atomic percent. Both of these Ni values are in
general agreement with observed values, perhaps
a little on the high side.
If the 1.8 atomic % P value is assumed, kamacite
and taenite are the phases present in the equilib-
rium structure from 1100? C down to below 850?
C. Schreibersite first appears somewhat above 750?
C, and is present in the amount of 2% when that
temperature is reached. Nucleation under these
circumstances would be expected to distribute
schreibersite along kamacite-taenite borders, these
phases being present in approximately equal
amounts. Taenite disappears from the structure
below 700? C, and by 600? C, 3% schreibersite is in
equilibrium with kamacite. The final Ni contents
of both kamacite and schreibersite are somewhat
higher than in the 3.6 atomic % P case, the higher
P situation agreeing better with actual measure-
ments on the meteorite.
When 5.3 atomic % P is assumed, the bulk
composition lies within the kamacite + liquid field
of the phase diagram from 1100? C to somewhat
above 1010? C. At 995? C, 5% schreibersite is in
equilibrium with kamacite, and this schreibersite
remains constant in amount down to 925? C. Dur-
ing the next 50? drop in temperature one-third of
the structure transforms to taenite and the amount
of schreibersite more than doubles to 13%. The
taenite decreases in volume with further cooling
and disappears below 750? C. By 700? C the equilib-
rium phases are again kamacite and schreibersite,
NUMBER 21 47
TABLE 15. ?Equilibrium cooling of composi-
tions appropriate to the Bellsbank meteorite
(percentage of phases present at indicated
temperatures with their Ni contents)
?c
995975
925
875850
750700
650600
555
?C
995975
925875
850750
700650
600550
?C
995975
925875
850750
700650
600550
1
%a
1030
30
3555
6888
97
9695
3
%a
100100
8553
5063
8590
8888
5
U
9595
9552
4962
8383
8180
.8 Atomic
%y
9070
70
6545
309
.6 Atomic
%Y
1541
4228
5
.3 Atomic
%y
3536
22
X P, 5
%Ph
23
3
45
% P, 5
%Ph
68
910
1012
12
X P, 4
XPh
55
513
1516
1717
1920
.0 Atomic
%N1a
4.04.2
3.8
3.54.0
4.04.5
4.9
4.74.6
.0 Atomic
SN1
a
5.0
5.04.7
4.04.0
4.04.5
4.64.2
4.2
.9 Atomic
%Nia
4.84.8
4.84.0
4.04.0
4.44.4
3.83.8
% Ni
%N1Y
5.15.3
5.5
5.86.3
7.29.1
% N1
*N1
Y
6.66.4
6.37.2
9.1
% Ni
SN1
Y
6.46.3
7.2
?H1ph
5.37.1
8.9
1213
*NiPh
4.44.4
5.37.1
8.011
11
%Niph
6.46.0
5.24.4
4.45.3
7.07.5
1010
schreibersite being present in the amount of 17%.
With further cooling, schreibersite increases in
quantity and in Ni content, with a rather marked
decrease in kamacite Ni content. The final Ni
content of the schreibersite is possibly slightly
lower than the measured value, the quantity of
schreibersite seems too high, and the kamacite Ni
values too low. The 3.6 atomic % P figure, the
one plotted in Figure 19, seems to be the best
choice of the three. A somewhat higher P value,
perhaps 4 atomic %, would probably be even
better, however. This would mean that schreiber-
site would initially nucleate in the presence of less
taenite, and that final massive schreibersite and
kamacite would have slightly smaller Ni values.
The observed low temperature schreibersite mor-
phology suggests that an extensive Widmanstatten
pattern was not present when schreibersite nu-
cleated .
The phase growth calculations that have been
discussed above account for the major structural
features of the selected group of meteorites. Ap-
propriate compositions cooled under equilibrium
conditions to approximately 600? C would produce
the observed proportion of phases and account
for the general character of the structures. Coa-
huila is single crystal kamacite containing a few
isolated moderate-size schreibersite inclusions.
Ballinger has a coarse Widmanstatten pattern in-
terrupted by large schreibersites that may be sur-
rounded by cohenite. Santa Luzia contains massive
schreibersite areas surrounded by giant swathing
kamacite, separating areas of coarse Widmanstat-
ten pattern. The higher Ni members of the group
have a finer Widmanstatten pattern with massive
schreibersite morphology explainable in terms of
the amounts of P and C present. Bellsbank con-
tains massive schreibersite and low Ni kamacite
and has similarities to the giant swathing kamacite
areas in Santa Luzia.
It is interesting at this point to summarize equi-
librium data given above and compare it with
what would be expected in terms of Widmanstat-
ten pattern growth from the binary Fe-Ni system.
Here Widmanstatten pattern growth is construed
in its conventional usage of simply taenite trans-
forming to kamacite, ignoring the presence of P
either in solution or in schreibersite. The compar-
ison is made in Table 16, where the first two
columns give the compositions of the meteorites
as plotted in Figure 19, the compositions that best
fit the structural arguments given above. These
columns are followed by sets of data for the
ternary system and for the binary system (Figure
1). In each set the first column gives equilibrium
temperatures at which Widmanstatten pattern de-
velopment starts (y ?>a + y) if undercooling
effects are ignored, probably a reasonable assump-
tion when P is present. The second column gives
the expected temperatures at which taenite would
be completely transformed to kamacite (a + y ??
a) if equilibrium were maintained down to these
low temperatures. The third column gives the
expected temperature range for Widmanstatten
pattern growth (ATWPG). The last two columns
show the effect of P on increasing the tempera-
tures of kamacite nucleation (TaNuc) and taenite
disappearance (TyDis) over what would be ex-
pected from the binary system.
48 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES*
TABLE 16. ?Temperatures (? C) for Widmanstatten pattern development in the ternary
system as compared to the binary system
Meteorite
Bellsbank
Lexington County
& Bahjoi
Goose Lake
& Balfour Downs ? ?
Atomic
Ni
5.0
5.3
6.2
6.2
7.1
7.8
%
P
3.6
0.8
1.4
2.7
1.8
2.7
Fe-NI-P System
(-930?)"
860?
880?
840?
750?
730?
c+Y-a
* 690?
610?
580?
580?
565?
555?
ATWGP
240?
250?
300?
260?
185?
175?
Fe-N1
Y-a+Y <?
775?
765?
755?
755?
740?
725?
System
+Y+*
610?
580?
570?
570?
480?
ATWGP
165?
185?
185?
185?
260?
AT due
Sue
(155?)**
95?
125?
85?
10?
5?
to P
TYD1S
80?
30?
10?
10?
85?
* For this composition WPG starts with the reaction
** Nucleation of Y within a.
The presence of P in the coarsest structured
meteorites, Bellsbank through Santa Luzia, signifi-
cantly increases the temperature at which Wid-
manstatten pattern growth starts, increases the
temperature at which transformation of taenite
would be expected to be complete, and also signifi-
cantly increases the temperature range over which
growth takes place. These three factors all work
in the direction of producing larger kamacite crys-
tals than would be expected from the binary dia-
gram for the same cooling rate. In the coarse-
structured meteorites, Lexington County through
Belfour Downs, those with a well-developed Wid-
manstatten pattern, the amount of P has a less
dramatic effect. The important point here is that
kamacite nucleation takes place at significantly
lower temperatures than is the case for meteorites
with less Ni. The nucleation temperature is also
very close to that which would be expected from
the binary diagram. This leads to much more
restricted conditions for kamacite growth and the
retention of larger amounts of taenite and plessite
in the structure, thereby producing the finer and
more regular Widmanstatten pattern that is ob-
served in these meteorites.
Details of these various structures as given in
the results section, however, are not explained
satisfactorily by this discussion. Taenite and ples-
site are retained in significant amounts in most of
these meteorites, demonstrating that equilibrium
cooling did not extend to 550? C. Diffusion con-
trolled growth, modifying the equilibrium struc-
ture in significant ways continued to lower temper-
atures. It is this aspect of meteorite structure that
will be discussed below.
Low TEMPERATURE PHASE GROWTH AND THE
EQUILIBRIUM DIAGRAM
The preceding section has shown that the major
structural features of the selected meteorites may
be explained on the basis of nucleation and growth
of phases in a system responding to changing
equilibrium conditions with decreasing tempera-
ture. Bulk equilibrium may have pertained down
to 550? C or below for Bellsbank and Coahuila,
but the other six meteorites were not in complete
equilibrium at that temperature (Figure 19). Had
they been, their structures would consist only of
kamacite and schreibersite, and the taenite that
they contain would have transformed. All eight of
the meteorites also contain Ni and P concentration
gradients in kamacite adjacent to schreibersite (Ta-
bles 2-9), although the recent work of Randich
(1975) suggests that these gradients formed at
much lower temperatures. Some of the massive
and all of the smaller schreibersites have Ni con-
centrations that are greatly in excess of those
permitted for schreibersite in equilibrium with
kamacite at 550? C (Figures 18, 25). These obser-
vations are explained by the fact that as tempera-
ture drops the atomic mobility necessary to main-
tain bulk equilibrium is lost. Elemental diffusion
becomes slower and compositional changes take
place over much shorter distances, resulting in
composition gradients. Kinetic factors must now
be considered as well as equilibrium states. Upon
cooling under these restrictive.conditions, schrei-
bersite continues to grow in volume, may increase
in Ni content, and late-formed schreibersites nu-
cleate and grow. Taenite shrinks in volume and
increases in Ni, while kamacite decreases in Ni.
NUMBER 21 49
Phase interfaces are still in equilibrium in the
sense that interface compositions represent tie
lines within the Fe-Ni-P system (see below). Schrei-
bersite-kamacite interface measurements on mete-
orites, therefore, give equilibrium information rel-
evant to the Fe-Ni-P systems at temperatures well
below those that are available experimentally.
Combining this low temperature equilibrium data
with both an analysis of the kinetic record present
in the form of diffusion gradients and a detailed
knowledge of specimen petrography may be ex-
pected to provide new possibilities for the elucida-
tion of the structures at hand.
With these ideas in mind, it will be helpful first
to examine the Fe-Ni-P diagram asking what
changes would be expected with decreasing tem-
perature. Meteorite data (Reed 1965a, 1967, 1969,
and Tables 2-9 above) indicate major changes in
the diagram on cooling to low temperatures. The
top part of Figure 20 may be used as a starting
point for a discussion of this transition. It is a
schematic isothermal section of the ternary system
at a temperature slightly below 550? C. The scale
on the Fe-P axis has been interrupted so that the
lower part of the diagram could be enlarged for
clarity. The low P concentrations that are observed
in kamacite and taenite make it impossible to show
the three fields along the Fe-Ni axis and the three
schreibersite-containing fields using the same
scale.
Compositional data on schreibersite-kamacite
and schreibersite-taenite associations suggest the
following trends in the ternary diagram with de-
creasing temperature. The constant P content of
schreibersite requires that the schreibersite com-
position corner of the three-phase field (point D
in Figure 20) lie on the 25 atomic % P line regard-
less of temperature. Ni values for schreibersite
embedded in taenite borders, an association that
approximates the schreibersite corner of the three-
phase field, range from 41 to 45 atomic %. Point
D, therefore, must move at least that far to the
right with decreasing temperature. Higher Ni con-
tents in schreibersite have been measured (Reed,
1972), but these represent compositions in the
taenite-schreibersite field, while lower Ni contents
represent compositions in the kamacite-schreiber-
site field. Because of the steepness of the Ni
concentration gradients, no serious attempt has
been made to obtain taenite-schreibersite interface
compositions, but incidental measurements (Fig-
ures 9, 11) may be used to bracket the position of
point C. The Ni value in taenite associated with
schreibersite is less than the schreibersite Ni value,
perhaps by about 5 atomic %. This suggestion is
supported also by Reed's (1972) measurements on
the Octibbeha County meteorite and by similar
measurements by the authors on this meteorite
(60 atomic % Ni in schreibersite in contact with 55
atomic % Ni in taenite). Point C moves to the
right, but trails behind point D. The P values in
taenite at these interfaces are also not accurately
known, but sufficient data is available to say that
they are a fraction of the P value in kamacite
associate with the same schreibersite (see discus-
sion below and Figure 22). The Ni and P contents
for point B may be estimated from the kamacite-
schreibersite interface values of these taenite-
embedded schreibersites. They range from 5.7 to
6.2 atomic % Ni, with constant P values of 0.05
atomic %. In summary the diagram changes as
follows with decreasing temperature: point D
moves to the right to approximately 45 atomic %
Ni with point C moving to approximately 40 atomic
% Ni and to a P value that is much smaller than
0.05 atomic %. As a consequence the a + Ph and
a + y + Ph fields greatly expand in area mainly
at the expense of the y + Ph field. The three
small fields along the Ni axis also lose a large
proportion of their areas, but they still retain
considerable importance for the interpretation of
meteorite structures.
The preceding illustrates the direction and ex-
tent of change in the ternary system with decreas-
ing temperature, but it says nothing about what
that temperature range might be. Do meteorite
structures continue to develop to very low temper-
atures or is growth effectively concluded at tem-
peratures not far below 550? C? The bottom section
of Figure 20 suggests an approach that may be
useful here. We know that D' lies on the 25 atomic
% P line and can assume that it moves continuously
to the right with decreasing temperature. The
points on the Fe-Ni axis can be obtained from the
estimated portion of the Goldstein and Ogilvie
(1965) binary Fe-Ni diagram down to about 350?
C. If A', B', and C could be estimated as a
function of temperature, we would have much of
the information needed for extrapolated isother-
mal sections.
50 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
0
100 %Fe
20 30
ATOMIC %Ni
40
C'
20
ATOMIC % Ni
30 40
FIGURE 20. ?Fe-Ni-P system below 550?C: top, schematic isothermal section; bottom, parameters of
use in extrapolating to lower temperatures.
NUMBER 21 51
TABLE 17. ?Experimental values of P concentration in
kamacite and taenite in equilibrium with schreibersite
TABLE 18. ?Temperature versus P saturation concentra-
tion in kamacite and taenite taken from Figure 22
Fe-P System* Fe-Ni-P System**
T?C
1010
995975
925
875
850
750700
650600
550
1283
12681248
1198
1148
1123
1023973
923873
823
1/T ("KxlO"4)
7.79
7.89
8.018.35
8.71
8.90
9.7810.3
10.811.5
12.1
Atonic % Pa
4.8
4.1
2.9
1.71.3
0.990.59
0.45
Atomic % Pa
4.13.7
3.0
2.8
1.81.2
0.90.57
0.47
Atomic * PY
1.91.6
1.3
1.2
0.680.49
0.270.22
0.15
* Doan and Goldstein (1970), table IV.
** Doan and Goldstein (1970), table III.
A', B', and C represent corners of composition
fields, and these particular compositions are very
dilute solutions of P in Fe or in Fe-Ni. Data for
these points at a number of temperatures within
the experimental range are given by Doan and
Goldstein (1970), and their values converted to
atomic % P are given in Table 17. This type of
data is frequently used to extrapolate concentra-
tion-temperature relationships to lower tempera-
tures and is plotted accordingly in Figures 21 and
22. Atomic % P on a logarithmic scale is plotted
against 1/T. Figure 21 uses P saturated kamacite
values from the Fe-P system, and Figure 22 uses
both P saturated kamacite and taenite values from
the Fe-Ni-P system. Straight lines were drawn with
ease through the three sets of data, suggesting
that the extrapolations may give useful low tem-
perature composition estimates. The thermody-
namic rationale for this procedure is given in
"Appendix."
A particularly interesting observation based on
Figures 21 and 22 is that the kamacite line in the
plot of the Fe-Ni-P system data is identical to the
kamacite line in the plot of Fe-P system. There-
fore , both sets of data yield the same extrapolated
P-temperature relationship for kamacite. This
means that line A'B' is parallel to the Fe-Ni axis,
and that the saturation value of P in kamacite is
independent of Ni concentration. Should this ac-
tually prove to be the case, it would have interest-
ing consequences for the interpretation of mete-
orite structures. Kamacite-schreibersite interface
values would not only represent tie lines in an a +
Ph field of the ternary diagram, but the P value in
kamacite at a specific interface would represent
the temperature of the isothermal section to which
T?C
540527
508
488
473
465460
454
449
444
436
431424
417
410
403394
383
372
362348
333315
298
252
Atomic % Pa
0.400.35
0.30
0.25
0.22
0.200.19
0.180.17
0.16
0.15
0.140.13
0.12
0.11
0.10
0.0900.080
0.070
0.0600.050
0.040
0.030
0.020
0.010
Atomic X PY
0.14
0.12
0.10
0.080
0.070
0.0620.058
0.0540.052
0.0480.046
0.0410.038
0.0340.032
0.0280.025
0.022
0.019
0.0150.013
0.0090.007
0.005
0.002
Weight % Pa
Q.ZZ
0.19
0.17
0.14
0.12
0.110.11
0.100.09
0.090.08
0.080.07
0.070.06
0.060.05
0.04
0.04
0.030.03
0.020.02
0.01
0.006
that tie line belonged. The implication of this
would be that schreibersite growth effectively
stopped over a sequence of temperatures within a
given meteorite. Numerical values for P concentra-
tion variation with temperature for both kamacite
and taenite were read from Figure 22 and are
listed in Table 18.
Although a flat line A'B' has several interesting
implications, it is necessary to consider as an alter-
nate possibility that it actually slopes downward
from left to right. The experimental technique
employed by Doan and Goldstein (1970) was
stretched to its limit in obtaining the lower temper-
ature data, resulting in the possibility of significant
error for the problem at hand. There is also
ambiguity in their paper concerning one critical
value. The weight % P in kamacite value for the
Fe-P system at 550? C taken from their table IV is
0.25 ?0.03. The value used on the 550? C isother-
mal section (their figure 8) is 0.4 weight % P. This
difference probably represents confusion as to
which value is best rather than a simple misprint
in the diagram, a difficulty that can best be re-
solved by more careful experimental work. If this
0.4 weight % P value had been used in the extrap-
olation, the line in Figure 21 would have lain to
the right of the kamacite line in Figure 22. This
would mean a sloping line A'B' with P decreasing
as Ni increases. This situation could imply that all
schreibersite within a given meteorite grew down
to the same final temperature. Before attempting
52 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
.001
7 8 9 10 II
FIGURE 21. ?Extrapolation of P saturation values in kamacite in Fe-P system to lower temperatures.
12 13 14 15 16 17 18 19 20 21
I/T (?KxlO~4)
NUMBER 21 53
10 II 12 13 14 15 16 17 18 19 20 21
001
FIGURE 22. ?Extrapolation of P saturation values in kamacite and taenite in Fe-Ni-P system to
lower temperatures.
54 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
to resoive this and related problems in terms of
meteorite measurements, however, it will be help-
ful if we first look at the diffusion process as it
applies to schreibersite growth.
DIFFUSION-CONTROLLED SCHREIBERSITE GROWTH
The data presented above on Ni and P gradients
demonstrates that schreibersite grew by drawing
these elements from increasingly restricted vol-
umes of kamacite with decreasing temperature.
Petrographic considerations combined with an un-
derstanding of the Fe-Ni-P system suggest the
broad outlines of the nucleation and growth proc-
ess. With this background in mind, a set of con-
straints and assumptions can be developed that
allow the qualitative application of the nonisother-
mal diffusion-controlled growth theory for ternary
systems recently developed by Randich and Gold-
stein (1975) to be applied to schreibersite growth
in these meteorites (see also Randich, 1975).
Figure 23 may be used to relate schreibersite
growth both to the Fe-Ni-P diagram and to growth
equations. The top part of the diagram is a sche-
matic isothermal section at some temperature be-
low 550? C. The scale for P is broken so that very
low levels of P can be indicated. The tie line
represents schreibersite growing within kamacite.
Its ends give interface P and Ni compositions for
both kamacite (CP, CNi) and schreibersite (Cp,
C^i). The theory of Randich and Goldstein (1975)
requires that equilibrium be maintained to the
extent that Ni and P concentrations in the two
phases are related by a tie line in the ternary
system at the temperature that pertains. For dif-
fusion-controlled conditions, this tie line need not
be the equilibrium tie line (ETL), the one that
passes through the bulk composition of the system.
The lower part of Figure 23 indicates kinetic
factors that are important in the growth process.
A schreibersite (Ph) of half width ?p" equal to ?m is
growing within kamacite in the X direction. The P
and Ni values in schreibersite at the interface are
CP and CNi, and the equivalent values in kamacite
are CP and CNi . The P and Ni fluxes from kamacite
into the growing schreibersite are J? f+ and J$U(+ >
and the Ni flux from the interface into the schrei-
bersite is Sm.f ? The concentration gradients in P
and Ni are dCP/dX and aCNi/aX.
The diffusion controlled growth process may be
described by the mathematical treatment of Ran-
dich and Goldstein (1975). Diffusion in the Fe-Ni-
P system may be described using an extension of
Fick's first law. The P and Ni fluxes in terms of
concentration gradients in kamacite and schreiber-
site are as follows.
Ta = ?
JP
Ta ? _
pp dX PNi
NiNi dX
dX
ax
TPh = _r>Fe,PhNiNi
ax
,ph aCp
ax
Jp"h = o
(la)
(lb)
(lc)
(Id)
Separate sets of equations are needed for each
phase. The P flux in schreibersite is zero due to
the constant atomic proportion of this element in
schreibersite. The symbols D?pa and D?fN? are
diffusion coefficients, measures of the influence
of the concentration gradients of these two ele-
ments on their own flux, where as D?Nia and D^iPa
reflect cross effects usually referred to as the
ternary diffusional interaction. The latter coeffi-
cients have been found to be so small in this
system that the terms containing them may be
dropped from the above equations (Heyward and
Goldstein, 1973). These equations may then be
simplified by neglecting the diffusional interaction
terms and the P flux in schreibersite.
Ja ? Hx
ax
(2a)
(2b)
(2c)
Fick's second law is applied in order to consider
the time dependence of the fluxes. It is assumed
that diffusion coefficients are independent of com-
position.
at ax pp ax2 (3a)
NUMBER 21 55
ATOMIC %Ni
30
20
o
o
10
Ph
Ni
JP t+ aCp
x^ H
FIGURE 23. ?Relation of schreibersite growth both to Fe-Ni-P diagram and to growth equations:
top, schematic low temperature ternary with tie line indicating schreibersite growing in a + Ph
field; bottom, a + Ph interface compositions, composition gradients, and associated P and Ni
fluxes.
56 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
dt
at
ax
JNl
ax
NiNi
NiNi ax2
(3b)
(3c)
The resulting mass balance equations for P and
Ni at a schreibersite-kamacite interface are as fol-
lows.
(4a)dt
(4b)
The two mass balances are related to each other
as the rate of movement of the interface, d?/dt,
must be the same for both elements.
(5)dt dt dt
A schreibersite growth rate may then be expressed
in terms of interface Ni and P concentrations, the
flux of P in kamacite, and the fluxes of Ni in both
kamacite and schreibersite.
1
dt Cp ? Cp7(-JPV)
dt
(6a)
(6b)
It is beyond the scope of this study to attempt
the difficult numerical analysis required to solve
these growth equations. Randich (1975) has devel-
oped a model for this purpose and successfully
applied it to the growth of schreibersite in hexa-
hedrites. He compiled a computer program that
treats diffusion controlled phase growth in ternary
systems when the requisite portions of the phase
diagram are known. The model accomodates ter-
nary interactions, nonisothermal transformations
(variable cooling rates), and impingement (over-
lapping diffusion fields). It is based on a one-
dimensional space grid and is rigorous only for
lamellar schreibersite growing within kamacite.
The concepts that this model employs, however,
are valid for meteorites more complex than the
hexahedrites, and they will be used here in a
qualitative interpretation of the data.
Equations 6a and 6b relate P and Ni interface
concentrations to their gradients in kamacite and
schreibersite for the final stages of growth. Cool-
ing is assumed to be uninterrupted, with schreiber-
site growth continuing down to very low tempera-
tures. Measurements with the electron micro-
probe, however, do not reveal changes that take
place within less than 1 /urn of an interface. There-
fore, as a consequence of this limitation of the
measurement technique, growth apparently stops
when subsequent changes become unresolvable.
Randich's (1975) calculations indicate a lower limit
of 250? C for detectable change.
Equation 6a is a key to the understanding of
schreibersite growth under the conditions that
pertained in the meteorites under study. It dem-
onstrates that the P flux controlled the schreiber-
site growth rate. The quantity Cp in the equation
is the P content of schreibersite at the interface
(Figure 23), a constant at 25 atomic %. The quan-
tity Cp is the interface P content in kamacite. It is
a small number, 0.2 to 0.05 atomic %, in the
temperature range of formation of the observed
interface relationships. The quantity (CP ? CP),
therefore, may be considered to be independent
of temperature even though Cp is not. With this
simplifying assumption, the rate of growth of a
given interface, d?/dt, is seen to be dependent
only on the P flux, J?f+. The P flux, however,
will be dependent upon CP at any given tempera-
ture. The value of Cp combines with the P level in
the surrounding kamacite to establish the P gra-
dient, aCp/aX, the variable that determines the
flux (equation 2a).
The system responds to the constraint of the P
flux determining schreibersite growth rates by
adjusting individual interface Ni concentrations so
that the growth rate established by P (equation 6a)
is matched by the growth rate due to Ni (equation
6b). This permits individual schreibersites contain-
ing markedly different Ni levels to continue to
grow simultaneously within the cooling meteorite.
Kamacite and schreibersite interface Ni and P
concentrations must continue to be related by a tie
line relationship, a requirement that is met by tie
lines shifting from the equilibrium tie line. The
nature of these tie line shifts and their effects on
growth rates of individual schreibersites is illus-
trated in Figure 24.
Tie line shifts and their effects are illustrated
using hypothetical ternary diagrams in Figure 24.
The symbols for Ni and P are retained as a matter
of convenience, although scale problems preclude
NUMBER 21 57
1
1
1
^ 1
LJ 1
1
1
1
\ V
li HOJu3
<Ua.
EV
ese
a
ao
d -10 IN % DIW01V !N % OIW01V
2 "?C c
a
- "I3
C a,
?2 E
"rt OH
u'
3 +
?S ?
8.S!i
?* .y
" .S
o a a
58 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
use of actual values. The tie line indicated at the
higher temperature Ti, ETL(TX), is assumed to
represent bulk equilibrium in the a + Ph region
of the diagram. It passes through the bulk com-
position indicated by the small circle (5.5 atomic %
Ni and 6.0 atomic % P). Upon cooling to tempera-
ture T2 the a + Ph field expands along the a +
Ph/Ph boundary, and the a/a + Ph boundary
moves closer to the Fe-Ni axis. It is assumed that
in this lower temperature range mass transfer is
insufficient to achieve bulk equilibrium. Growth
becomes diffusion controlled and gradients de-
velop within the phases. Tie lines may therefore
shift from the equilibrium tie line so that the Ni
growth rate can match the P growth rate. Three
possible tie lines are indicated at T2, the equilib-
rium tie line for the same composition used above,
ETL(T2), and tie lines shifted to the right and to
the left of ETL(T2),
The interface relationships for these four tie
lines are illustrated at the right in Figure 24. The
upper diagram shows interface Ni and P composi-
tions for ETL(Tj). The bulk Ni value for the
system is indicated by Co1- The bulk P value is 0.5
atomic % higher and was omitted for clarity. Bulk
equilibrium has been achieved at temperature Tj
as is indicated by the absence of compositional
gradients in both kamacite and schreibersite.
Interface relationships for diffusion controlled
growth at temperature T2 are illustrated in the
lower right side of Figure 24. The P interface
values are assumed to be identical in the three
interface situations shown. The consequence is
similar P fluxes feeding schreibersite growth,
]P,(+, and the establishment of similar growth
rates, d?/dt, for the interfaces through equation
6a. Tie line shifts from ETL(T2) induced by con-
ditions of limited mass transfer produce a contin-
uously varying range of growth rates that permits
Ni flow to match that required by P.
The equilibrium tie line at the lower tempera-
ture, ETL(T2), passes through the bulk composi-
tion. Its interface Ni and P values are those that
would be observed if the system were at complete
equilibrium. The presence of compositional gra-
dients in both kamacite and schreibersite, how-
ever, show that complete equilibrium has not been
reached.
Shifting the tie line to the left results in an
increase in the growth rate due to Ni (equation
6b). The quantity 1/(CNI ? CNI) and the Ni flux in
kamacite, ]xU(+, become larger and the Ni flux in
schreibersite, J^,f~. becomes smaller. All three of
these changes increase the growth rate over that
of ETL(T2). Ni is supplied to the growing schrei-
bersite in a way that size is increased at the expense
of Ni enrichment.
Moving the tie line to the right of ETL(T2) has
the effect of slowing the growth rate due to Ni.
The quantity 1/(CNI ~ C^i) and the Ni flux in
kamacite, ]yiti+, become smaller, and the Ni flux
in schreibersite, J^.f ? becomes larger. All three
of these changes decrease the growth rate over
that for ETL(T2). Ni is supplied to the growing
schreibersite in a way that it becomes enriched in
Ni rather than growing in size. A theoretically
infinite range of growth rates may be obtained by
these tie line shifts. One of these possible tie lines
will give a unique growth rate that satisfies both
equations 6a and 6b.
COOLING RATE VARIATIONS
Cooling rates of meteorite parent bodies are
known to vary over several orders of magnitude
(Goldstein and Short, 1967b). These variations will
affect the growth of schreibersite, particularly in
the lower temperature ranges. Compositions and
associations we observe in meteorite specimens
will depend in part on specific cooling histories.
Measurements of schreibersite, therefore, would
be expected to be interpretable in terms of cooling
rates. Randich (1975) has recently demonstrated
the validity of this assumption by calculating cool-
ing rates for hexahedrites. In the course of his
work important observations were made that will
help in the interpretation of more complex mete-
orite structures. Several aspects of this work will
be reviewed here.
Calculations using the Randich and Goldstein
(1975) schreibersite growth model were made us-
ing a bulk composition of 2.1 weight % P, 4.1
weight % Ni, and 93.8 weight % Fe. Cooling rates
of 5 X 10~3, 5 x 10~4, and 5 X 10~50 C per second
were employed over a cooling interval of 900? C to
685? C. Decreasing the cooling rate produced
schreibersites of predicted greater widths and
higher Ni contents. P gradients were predicted in
kamacite and Ni gradients were predicted for both
kamacite and schreibersite. The calculated Ni in-
terface values were significantly lower than those
for the equilibrium tie line for this composition.
NUMBER 21 59
Tie lines had shifted to the left to increase the
growth rate for Ni. An experimental validation of
these calculations was carried out using this same
composition and the 5 x 10~4? C per second (43.2?
C per day) cooling rate. Calculated and observed
compositions agreed within the limits of electron
microprobe measurements.
Additional cooling rate calculations were made
by Randich (1975) in a study of schreibersite nu-
cleation temperatures. Cooling rates in the mete-
orite range of 1?, 10?, and 100? C per million years
were used with the Coahuila bulk composition of
5.49 weight % Ni and 0.49 weight % P. The
results were similar to those given above, the
slower cooling rates producing slightly larger
schreibersite containing slightly higher Ni values.
The calculated Ni values were within the range of
observed Ni values in hexahedrites, establishing
the validity of the model at cooling rates within
the meteoritic range. Ni gradients were predicted
for the schreibersites, contrary to observations on
meteorites. This is probably explained by the fact
that the ternary diffusion constant for Ni in schrei-
bersite is not known in this temperature range.
These calculations led to a conclusion that is
significant for our purpose. It was shown that if
nucleation occurs above 500? C the amount of
undercooling prior to nucleation in unimportant.
The kamacite-schreibersite assemblage at 500? C
will be in equilibrium, assuming a diffusion field
of 1000 [im or less. Randich (1975) also points out
that this situation leads to the conclusion that
Reed's (1965b) theory that lamellar schreibersite
and rhabdite nucleated simultaneously must be
correct. Had the plate schreibersite grown signifi-
cantly prior to nucleation of rhabdite, the rhabdite
would have been prevented from nucleating. Both
types are observed together over very short dis-
tances, requiring that nucleation temperatures
were very close. Under these circumstances, im-
pingment of P and cooling rate are the two factors
that control schreibersite growth for a given bulk
composition.
INTERFACE DATA AND SCHREIBERSITE
DISTRIBUTION
In a previous section we have seen that the
main petrographic features of coarse-structured
iron meteorites can be understood in terms of
equilibrium cooling of specific compositions in the
Fe-Ni-P system. This approach can be extended
to much lower temperatures with the aid of con-
cepts developed by Randich (1975) in his analysis
of schreibersite growth in hexahedrites. The appli-
cation of diffusion theory permits the interpreta-
tion of schreibersite-kamacite interface data in a
way that yields a more detailed understanding of
schreibersite distribution and composition than
has been possible previously.
Phases and compositions resulting from equilib-
rium cooling of the meteorite compositions of
interest have been summarized in Tables 10
through 15, and these same compositions have
been plotted on the iron-rich corner of isothermal
sections in Figure 19. At the top of Figure 25
these compositions have also been plotted on the
550?C isothermal section with their respective tie
lines. This is the lowest temperature for which an
experimentally based ternary section is available.
Table 19 lists the equilibrium percentages of kam-
acite and schreibersite with their Ni contents as
taken from Figure 25. This plot shows that had
complete equilibrium been reached at this temper-
ature for these compositions, taenite would have
disappeared and the observed structures would
consist of kamacite containing inclusions of schrei-
bersite.
The bottom section of Figure 25 contains an
estimated isothermal section at approximately 300?
C. The schreibersite corner of the three phase
field is plotted at 45 atomic % Ni, a value that is
probably close to the real value. The kamacite
corner of the three phase field is plotted at 5.5
atomic % Ni, and this value is considerably less
certain. The compositions used above are again
plotted with their tie lines. Although the diagram
may not be accurate, it does tell us something of
what would be expected if equilibrium cooling
extended to this very low temperature. Propor-
tions of phases and the expected compositions
taken from this diagram are also listed in Table
19.
The bulk compositions plotted in the lower
section of Figure 25 would also consist of kamacite
and schreibersite only, and these two phases would
be uniform in composition. The Bellsbank schrei-
bersite would contain much more Ni than was
observed for its massive schreibersite, and its kam-
acite would be considerably poorer in Ni than was
observed. The massive schreibersite in Santa Luzia
would be greatly enriched in Ni, its small schrei-
60 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
550? C
y +? Ph
too %FE 20 30ATOMIC % Ni 40
ABELLSBANK
? COAHUILA
? SANTA LUZIA
? BALLINGER
? LEXINGTON COUNTY
- BAHJOI
A GOOSE LAKE
- BALFOUR DOWNS
50
10 20 30
ATOMIC % Ni
40 50
FIGURE 25. ?Schreibersite compositions resulting from equilibrium cooling of meteorite composi-
tions: top, 550? C isothermal section with meteorite compositions and their respective equilibrium
tie lines; bottom, similar plot at approximately 300? C.
bersite would contain less Ni, and its kamacite
would be poorer in Ni. The Coahulia, Ballinger,
and Goose Lake-Balfour Downs compositions all
lie on the same tie line, but would consist of
different proportions of kamacite and schreiber-
site. The Lexington County-Bahjoi composition
would produce the most Ni rich schreibersite. The
structures suggested by this diagram, therefore,
are markedly different than those actually ob-
served in the meteorites under study. At what
NUMBER 21 61
TABLE 19. ?Equilibrium compositions at 550? C
and -300? C taken from Figure 25
Meteorite
Beilsbank
Coahuila
Santa Luzia
Lexington County &
Bahjoi
Goose Lake &
Balfour Downs
Meteorite
Bellsbank
Coahuila
Santa Luzia
Lexington County &
Bahjoi
Goose Lake &
Balfour Downs ? ? ? ?
550?C
% a % Ph % Nia  % Niph
87.0 13.0 4.1 11.9
99.3 0.7 5.2 13.3
91.3 8.7 5.4 14.0
96.3 3.7 5.8 15.0
94.8 5.2 6.6 16.2
91.2 8.8 6.9 17.0
-300?C
% a. % Ph % Nia  % Niph
86.4 13.6 2.5 20.8
97.2 2.8 4.3 38.0
90.2 9.8 3.6 30.5
94.5 5.5 4.3 38.0
93.3 6.7 4.7 41.2
90.0 10.0 4.3 38.0
temperatures do these observed differences be-
come introduced into the structures?
Taenite is retained in the form of the Widman-
statten pattern in the six octahedrites studied.
This shows that mass transfer required by the
taenite-kamacite transformation is insufficient at
somewhat above 550? C to achieve bulk equilibrium
in the cooling rate range pertaining. The presence
of the Widmanstatten pattern may also affect sub-
sequent P movement through the structure. Ran-
dich's (1975) calculations show that mass transfer
through 1000 /xm diffusion fields in kamacite is
sufficient for kamacite-schreibersite equilibrium to
be maintained down to 500? C in the same range
of cooling rates. Kamacite-taenite transformations
become restricted at higher temperatures than
kamacite-schreibersite transformations. There-
fore, it is reasonable to assume that as tempera-
tures drop below 600? C, localized compositional
changes become increasingly more important and
bulk compositional effects become increasingly less
important. As mass flow becomes more restricted,
individual structural units maintain interface equi-
librium and respond to compositional gradients in
increasingly small volumes of material. These
processes are responsible for the interface and
gradient data that have been reported above.
Tie lines representing interface data from the
Coahuila, Santa Luzia, and Goose Lake meteorites
(Tables 2, 5, and 8) are plotted on isothermal
sections in Figures 26. Average values were used
where more than one set of interface measure-
ments gave similar values. Any given tie line,
therefore, may represent more than one set of
measurements. The scale used made it impossible
to indicate the differing low P values in kamacite,
so all of these compositions are indicated by points
resting on the Fe-Ni axis. This treatment of the
data may well be an over simplification as various
tie lines within a given meteorite may actually
belong on different isothermal sections. A range
of temperatures may be involved, and this point
will be returned to in a later section. The three
meteorites selected are representative of the group
of eight meteorites studied.
Typical tie lines for the Coahuila meteorite are
given in the top section of Figure 26. They fan
out through a narrow composition range of 3.2 to
4.7 atomic % Ni in kamacite and 20 to 30 atomic
% Ni in schreibersite. Kamacite P values at the
interfaces range from 0.14 to 0.09 atomic %, a
significant point that will be dealt with later. The
structural associations related to compositions have
been discussed above, but it should be noted here
that the tie line on the left represents early-formed
structures (schreibersite bordering troilite-dau-
breelite inclusions and a large schreibersite), and
the three on the right are later-formed structures
(large rhabdites). The estimated bulk composition
of Coahuila is indicated on the diagram as a large
dot. The point apparently falls on the right-hand
tie line. All the measured schreibersite composi-
tions in Coahuila then appear to be to the left of
the ETL or on it, as was to be expected on the
basis of Randich's (1975) analysis. Reed (1965a)
has reported Coahuila schreibersite with 34 atomic
% Ni. It is not clear how this value would plot on
Figure 26 as the necessary Ni value in kamacite is
not available.
The relationship of the equilibrium tie line to
observed meteorite tie lines is actually somewhat
more complex then Randich (1975) has implied.
This can be concluded from consideration of Coa-
huila measurements, and it will become clearer
when measurements on the octahedrites are con-
sidered. At temperatures above the final growth
stages represented by the Coahuila tie lines in
Figure 26, mass transfer had become inhibited
except for very short-range interactions. Schrei-
bersite continues to grow as long as there is a P
62 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
COAHUILA
5.3%Ni, 08%P
SANTA LUZIA
6.2 %Ni, 2.7%P
1ST
ATOMIC % Ni
FIGURE 26. ?Representative tie lines from Coahuila, Santa Luzia, and Goose Lake meteorites
plotted on isothermal sections.
NUMBER 21 63
flow to the interface. Growth stops when P imping-
ment reduces the P flux to zero, or, for our
purpose, when we can no longer measure interface
relationship changes. Although a theoretical pos-
sibility, there is no evidence that any of the schrei-
bersites studied were in the process of dissolving.
As the P flux decreases with decreasing tempera-
ture, a given interface tie line would be expected
to shift to the right to conform to the change in
shape of the a + Ph field of the phase diagram. It
would also be expected to move farther to the
right in order to slow the Ni growth rate as the P
supply becomes more limited. The largest schrei-
bersites undoubtedly nucleated first and have
grown at comparatively high rates, drawing P
from large volumes of metal. They have the lowest
Ni concentrations. The smaller schreibersites that
grew at slower rates due to a more limited P
supply have higher Ni levels. The smallest schrei-
bersites would be expected to approach a Ni con-
tent given by the tie line whose Ni value in kama-
cite equals the Ni value of the kamacite in which
they are growing. Kamacite in the Coahuila mete-
orite has a higher Ni value than it would have had
if complete equilibrium had been reached. It is
therefore possible that microrhabdites in Coahuila
contain more Ni than would be expected from the
low temperature equilibrium tie line. Unfortu-
nately, the necessary measurements needed to
confirm this were beyond the capability of the
techniques available.
Tie lines for the Santa Luzia meteorite are
shown on a single temperature plot in the center
section of Figure 26. Kamacite Ni values range
from 2.2 to 6.2 atomic % with the corresponding
Ni values in schreibersite of 14 to 42 atomic %
covering a much broader range when compared
to Coahuila. Kamacite P values at the interfaces
range from 0.13 to 0.05 atomic %. The three tie
lines at the left represent the early formed massive
schreibersites, the three to their right are from
grain boundary schreibersite, and the one on the
extreme right is from a late-stage structure, a
small schreibersite partially embedded in taenite.
The Santa Luzia bulk composition lies in about
the center of the spread of tie lines, demonstrating
that interface tie lines depend upon local structure
and may fall on either side of the ETL.
Had complete equilibrium been achieved for
Santa Luzia at 550? C, kamacite would have con-
tained 5.4 atomic % Ni and schreibersite 14 atomic
% Ni. Equilibrium extending to the 300? C temper-
ature range would have resulted in kamacite of
3.6 atomic % Ni and schreibersite of 31 atomic %
Ni (Table 19, Figure 25). A very different situation
is indicated by the tie lines in Figure 26. These
represent interfaces from a range of schreibersite
morphologies from massive schreibersite sur-
rounded by large volumes of swathing kamacite to
taenite border schreibersite.
The explanation of these differences lies in the
fact that under conditions of diffusion-controlled
growth, P can be drawn from much greater dis-
tances than Ni within a given time period. Com-
plete equilibrium was not reached in the 600? to
500? C temperature range. Massive schreibersites
had nucleated at much higher temperatures and
grown to near their final size. This process had
reduced the level of P in the complete volume of
metal to near the equilibrium level. Even the
volumes of metal most remote from the large
schreibersites probably only contained about 0.6
atomic % P at 550? C (the equilibrium value is 0.4
atomic % P). Massive schreibersite growth may be
viewed as a process that equalizes P levels in
meteoritic metal in the 500? to 600? C temperature
range regardless of initial P concentrations. Differ-
ences in initial P level are reflected in the amount
of schreibersite produced.
While P diffusion in this temperature range
resulted in a meteorite with a modest P gradient,
the slower Ni diffusion resulted in an important
segregation of this element. Ni could not be moved
from a sufficiently large distance to maintain bulk
equilibrium in the times available. The massive
schreibersites had grown to near their final size,
but their Ni contents had to have been significantly
below the equilibrium value. Their iriterface tie
lines, therefore, must have been to the left of the
equilibrium tie line. The Ni gradient that probably
already existed within the swathing kamacite zone
became more pronounced. Beyond the swathing
zone, however, sufficient Ni was maintained to
yield the observed Wildmanstatten pattern. These
areas of the meteorite have a higher average Ni
content than the initial bulk Ni content of the
meteorite. As a matter of fact, some areas of
kamacite within the Widmanstatten pattern con-
tain more Ni than the accepted bulk Ni value.
The consequence of massive schreibersite
growth under these circumstances of composition
and cooling is to establish two distinctly different
64 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
structural areas within the meteorite, swathing
kamacite areas containing massive schreibersites
and areas of Widmanstatten pattern (Figure 7).
Both types of areas continue to change with de-
creasing temperature, but distances involved be-
come increasingly more restricted. The massive
schreibersites continue to grow, their growth rates
being determined by the flux of P from the swath-
ing kamacite. Interface Ni values shift to produce
a tie line with a matching growth rate. The shift is
undoubtedly to the right, resulting in a slower
growth rate due to Ni and Ni enrichment of the
schreibersite. The final Ni values of these schrei-
bersites approximates the equilibrium value at 550?
C.
The outer edge of the swathing zone develops
as a kamacite-taenite boundary that recedes from
the massive schreibersite with decreasing temper-
ature. The final stage in this progression is the
replacement of the boundary in part by grain
boundary schreibersite. P is supplied to the bound-
ary initially by the swathing kamacite, and the
taenite boundary supplies the initial Ni. Nuclea-
tion of this type results in concentrations that are
required by the three-phase field of the ternary
diagram. The initial schreibersite-kamacite tie line
will be the a + Ph/a + y + Ph boundary for the
given temperature. With decreasing temperature
the taenite border is replaced with a grain bound-
ary that contains grain boundary schreibersite over
about half of its distance. Nucleation may take
place over a range of temperatures as the taenite
recedes along the forming grain boundary, result-
ing in different initial Ni contents in the schreiber-
site and different initial growth rates. As growth
continues at the individual interfaces, Ni concen-
trations adjust to match the P controlled growth
rate. The general tendency will be for tie lines to
move to the right as the P supply dwindles.
Simultaneously with continued growth of the
massive schreibersites and development of the
exterior swathing zone grain boundary, Widman-
statten pattern development is continuing in the
remaining areas of the meteorite. The Ni level is
high enough in these areas so that a coarse Wid-
manstatten pattern forms, containing large kama-
cite lamellae and occasional plessite areas and
grain boundary taenite. Grain boundaries contain
occasional grain boundary schreibersite, and the
kamacite lamellae contain rhabdites and micro-
rhabdites. The rhabdites and grain boundary
schreibersites develop analogously to the similar
morphologies described above for Coahuila, react-
ing to specific local conditions to achieve appropri-
ate interface values. Nucleation of grain boundary
schreibersites under these circumstances may take
place at lower temperatures, undoubtedly leading
to smaller schreibersites with higher initial Ni
content that continues to increase upon further
cooling. Small rhabdites would also be expected to
reach high Ni values because they are growing in
a relatively Ni rich kamacite.
A final type of schreibersite morphology ob-
served in Santa Luzia is represented by the tie line
at the far right in Figure 26. It represents the
taenite border schreibersite illustrated in Figures
10 and 11. Schreibersite of this type appears to
nucleate within taenite at a taenite-kamacite inter-
face at a very late stage in the structural develop-
ment process. P is undoubtedly supplied mainly
from the kamacite, with most of the Ni coming
from the taenite. Associations of this type yield
the highest Ni schreibersites to be observed in
coarse octahedrites. The schreibersites may also
contain a slight Ni gradient, undoubtedly due to
the fact that they are associated with kamacite on
one side and taenite on the other. If this type of
schreibersite nucleates earlier or grows more rap-
idly due to a particularly high P gradient, it isolates
itself from the enclosing taenite by converting it to
kamacite.
Goose Lake meteorite tie lines are shown in the
bottom isothermal section in Figure 26. Differ-
ences from the Santa Luzia pattern result from
higher bulk concentrations of both carbon and
Ni. Only the upper portions of the two tie lines on
the left can be indicated. At some point in the
development of these schreibersites, cohenite pre-
cipitated in the adjacent metal, transforming the
interface to a cohenite-schreibersite interface and
preventing further schreibersite growth. The par-
tial range of Ni values in kamacite, therefore, is
3.2 to 5.7 atomic %, with Ni values in schreibersite
ranging from 16 to 45 atomic %. The value of 45
atomic % Ni is the highest observed for a schrei-
bersite in contact with kamacite. Kamacite P values
at the interfaces cover the same range as observed
in Santa Luzia, 0.13 to 0.05 atomic %. This range
in P values may indicate that these tie lines do not
all belong on the same isothermal section, a point
NUMBER 21 65
that will be discussed in a later section. The bulk
composition again lies in the center of the spread
of tie lines.
The general outline of the schreibersite growth
process may be summarized on the basis of this
discussion and that given in previous sections. In
the meteorites under study schreibersite petrogra-
phy has been strongly affected by sequential nu-
cleation. Initial schreibersite nucleation is a func-
tion of temperature and bulk composition. It took
place at greater than 850? C for all of the meteor-
ites containing massive schreibersite. The low P
Coahuila meteorite is the only exception. These
massive schreibersites grew rapidly under equilib-
rium or near equilibrium conditions, at least as
far as P is concerned, for most of their growth
period. Randich (1975) has shown that meteorite
cooling rates are sufficiently slow to allow this. It
is important to note that massive schreibersite has
achieved the major portion of its growth by the
time it has cooled to 600? C. On subsequent cool-
ing, Ni is enriched but size does not increase
appreciably. The massive schreibersite growth
process, therefore, has the effect of lowering the
P level in the bulk metal and isolating it, as
schreibersite enclosed in zones of swathing kama-
cite, from areas of developing Widmanstatten pat-
tern.
By 600? C the Widmanstatten pattern is well
established in these meteorites (Table 16). Kama-
cite lamellae have grown to near their final size,
and taenite bands have shrunk to near theirs.
Both taenite and kamacite will continue to increase
in Ni with cooling, but compositional changes will
take place over increasingly short distances. It is
in this environment, one that contains P in kama-
cite at the 0.5 to 0.8 atomic % level, that rhabdite
nucleation must take place. They nucleate homo-
geneously in kamacite within a narrow range of
temperatures and grow under conditions of equi-
librium combined with severe P impingment. It is
the limited amount of P and the competition for it
from closely spaced schreibersites that controls
their growth rates and consequently their eventual
size. The larger the volume of metal from which a
schreibersite draws P, the faster it grows and the
larger it becomes. Tie line shifts permit Ni levels
to vary depending upon growth rates. The larger
faster-grown rhabdites achieve lower Ni levels than
the smaller slower-grown schreibersites.
A third major type of schreibersite is grain
boundary schreibersite. Its petrography suggests
that it replaces taenite at late stages in structural
development. Heterogeneous nucleation probably
takes place at kamacite-taenite interfaces, resulting
in initial concentrations being controlled by the
three-phase a + y + Ph field of the ternary
diagram. P is supplied to the nucleation site mainly
from the bordering kamacite and Ni comes mainly
from the taenite. Equation 6 does not describe
initial growth rates but may well apply to kamacite-
schreibersite interfaces after they become isolated
from the influence of taenite. Nucleation temper-
atures probably depend upon specific local condi-
tions and may extend to considerably lower tem-
peratures than those appropriate for rhabdite for-
mation.
There are three types of associations that may
be included under the term grain boundary schrei-
bersite. They appear to represent a sequence of
stages in structural development. The first type is
the isolated grain boundary schreibersite. These
are elongated schreibersites that lie along kama-
cite-kamacite grain boundaries. They frequently
occupy grain boundary junctions and branch ac-
cordingly. Their morphology suggests that they
replaced residual taenite as it receded along the
grain boundary. Their Ni values decrease with
distance away from the nearest taenite ribbon or
taenite-plessite area, suggesting that the lowest Ni
grain boundary schreibersite nucleated first. An
alternate possibility is essentially simultaneous nu-
cleation with faster growth accounting for lower
Ni. These schreibersites are frequently somewhat
larger than the higher Ni ones closer to taenite.
There is no obvious reason, however, why P im-
pingment should be more critical at one site than
at another.
The second type of grain boundary schreibersite
is found in close association with taenite and tae-
nite-plessite areas. It appears to have formed sim-
ilarly to the isolated grain boundary schreibersites,
but it either nucleated later or grew more slowly.
It separated itself from taenite but was not able to
completely absorb it. The final stage in this se-
quence is the formation of taenite boundary
schreibersites. These are the small, highest Ni
schreibersites that appear to be embedded in tae-
nite at taenite-kamacite borders. These would
seem to be the final schreibersites to form.
66 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
INTERFACE DATA AND THE a/a + PH BOUNDARY
In the earlier discussion of the extrapolation of
experimental data to low temperatures the slope
of the a/a + Ph boundary was mentioned. The
extrapolation shows that the boundary moves to-
ward the Fe-Ni axis with decreasing temperature
and suggests that it is parallel to it (Figures 21,
22). This led to the simplifying assumption that P
saturation values in kamacite are independent of
Ni and Fe concentrations and may be equated to
specific temperatures (Table 18). This in turn
leads to a possible interpretation of the measured
P in kamacite values at kamacite-schreibersite in-
terfaces as being equivalent to a final growth
temperature, a temperature below which subse-
quent interface changes would be undetectable by
the technique employed.
Observed P interface values cover a range of
concentrations from 0.16 to 0.05 atomic %. A
factor of three is involved, indicating that small
differences in measured concentrations may have
more than trivial significance. The higher P values
are associated with massive schreibersite at inter-
faces with low Ni kamacite, and the lower P values
are associated with taenite border schreibersite at
interfaces with high Ni kamacite. Assuming that
these values represent solubility limits, we can
equate them to a temperature range of 445? to 350?
C (Table 18). Measurements on the Santa Luzia
meteorite alone covered most of this range, 0.14
to 0.05 atomic % P, equivalent to a temperature
difference of 430? to 350? C. Is it reasonable to
assume that large schreibersites stopped detectable
growth before the later formed schreibersites, or
does the a/a + Ph boundary really have a down-
ward slope with increasing Ni? Are there other
factors that may make a clear choice between these
two alternatives impossible on the basis of the
available data?
The P and Ni kamacite interface data is summa-
rized in Figure 27. The points represent individual
interface Ni and P concentrations in kamacite and
are plotted using the axes at the left and at the
bottom of the diagram. The symbols indicate the
meteorite from which the data were taken. There
is a general trend of decreasing P concentration in
kamacite with increasing Ni concentration. The
relationship of these values to the estimated a/a
+ y boundary of the Fe-Ni binary diagram of
Goldstein and Olgivie (1965) is also indicated. The
scale to the right of the diagram is the temperature
equivalent to the P concentration at the left, using
the data from Table 18. The dashed line is the a/
a + y boundary plotted in relation to temperature
and Ni concentration. It is obvious that interface
values for individual meteorites, as is the case for
the group as a whole, cover a range of both P and
Ni levels. A meteorite has interface values at sev-
eral P levels, and a range of Ni values may be
covered at any given P level. All of the interface
values are to the left of the a/a + y boundary, the
higher Ni values being closest to it.
There are at least three factors that must be
considered as possible contributors to the observed
distribution in Figure 27. These are (1) cooling
rates, (2) the possibility of a slopping a/a + Ph
boundary, and (3) the possibility of low tempera-
ture nucleation at a/y boundaries and subsequent
grain boundary diffusion playing a role in the
growth of grain boundary and taenite border
schreibersite. The available data is insufficient to
sort out these influences in a quantitative way, but
their expected effects can be outlined and sugges-
tive observations mentioned.
Cooling rates have only been reported for four
of the meteorites under consideration here. Values
of 2? C per million years for Lexington County
and Goose Lake and 3.5? C per million years for
Bahjoi were given by Goldstein and Short (1967b).
The recent measurements of Randich (1975) gave
a somewhat higher value of 5? C per million years
for Coahuila, and there is no reported value for
Bellsbank. Kamacite band widths for the other
three meteorites were taken from Buchwald
(1976), and cooling rates were estimated using the
Goldstein and Short (1967b) cooling rate curves.
Ballinger and Balfour Downs gave cooling rates
of 2? C per million years and Santa Luzia gave an
estimated 0.8? C per million years. These numbers
indicate that cooling rate values for this group of
meteorites cluster in a rather narrow range center-
ing around 2? C per million years.
The effect of cooling rate variations on schrei-
bersite-kamacite interface values has been dis-
cussed above. It is to be expected that slower
cooling rates would produce lower detectable in-
terface values of P in kamacite and lower levels of
P in kamacite generally. This would be equivalent
to detectable growth continuing to lower tempera-
tures. This type of variation may be present on a
minor scale in the data considered here. Coahuila
NUMBER 21 67
0.30
O25
0.22
0.20
0.19
0.18
0.17
0.16
0.15
0.14
a* o.i2
SJO.II
| 0.10
< 0.09
0.08
0.07
0.06
0.05-
0.04
0.03
0.02
A
?
?
?
O
A
D
a,BELLSBANK
b, COAHUILA
C, SANTA LUZIA
d, BALLIN6ER
e, LEXINGTON CO.
f, BAHJOI
Q, 600SE LAKE
h, BALFOUR DOWNS a + y
AA
(A CCE A A|?^ Jft.
*co
500
490
480
470
460
450
440
430
420
410
400
390
380
370
360
350
340
330
320
310
300
29O
280
I 23456789
ATOMIC %Nia
FIGURE 27. ?P in kamacite at kamacite-schreibersite interfaces plotted against Ni in kamacite
(temperature scale at right relates P saturation values to a/a + y boundary of Fe-Ni system).
68 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
does have the highest reported cooling rate of 5?
C per million years, and its interface P values and
P levels in kamacite do seem to be somewhat
higher than for some of the other meteorites
studied. Bellsbank also seems to have unusually
high levels of P in kamacite, and this could be the
consequence of a comparatively high cooling rate.
The points representing both Coahuila and Bells-
bank interfaces are in the upper part of Figure
27, perhaps meaning that detectable change ceased
at higher temperatures for these two meteorites
due to higher cooling rates.
An a/a + Ph boundary (AB in Figure 20) that
slopes downward from the Fe-P axis toward the
Fe-Ni axis would contribute to the type of data
distribution observed in Figure 27. At any given
temperature lower Ni schreibersites would have a
higher P interface value in kamacite than higher
Ni schreibersites. The range of the data, however,
seems to require much too steep a slope to be
reasonable in terms of the extrapolation given
above. An Fe-P axis intercept value of approxi-
mately 0.2 atomic % would be indicated. This
would mean either an unreasonably high P satu-
ration temperature in the neighborhood of 465? C
(Table 18), or that the actual curve bends sharply
to the right of the indicated extrapolation (Figure
21). It also seems unlikely that error in locating
the data points is a major factor. It was pointed
out above that reproducibility of P determinations
was ?0.005 weight % and for Ni was ?0.1 weight
%. This means that the P levels shown are distin-
guishable from their nearest neighbors with rea-
sonable certainty and that the Ni variations are
meaningful. Ni variations at a given P level range
from 0.8 to 3.0 atomic %. Variations for a specific
meteorite at a given P level are not so wide, but
they do range up to 1.5 atomic %. This range in
Ni values for a single meteorite at a given P level
suggests that factors other than simply a sloping
a/a + Ph boundary are involved. The boundary
may well have a gentle slope, but other factors
must also contribute to the observed distribution.
Tie lines representing three of the P levels in
Figure 27 are plotted on isothermal sections in
Figure 28. The symbols used in Figure 27 are
used at the schreibersite ends of the tie lines to
identify the meteorite represented. Tie lines ac-
tually belonging to a given isothermal section will
fan out across the field without overlap. When
overlap occurs it implies either error in the data
or plotting on the wrong isothermal section. We
saw in Figure 26 that all of the measured tie lines
for three of the meteorites can be plotted without
obvious cases of overlap. This could mean they all
belong to the same isothermal section, but it could
also be fortuitous. If interface data for all of the
meteorites could be plotted as tie lines on a single
intelligible drawing, several cases of overlap would
be observed. When tie lines are separated on the
basis of P level as in Figure 28, no serious cases of
overlap are encountered. The two tie lines at the
left on the 0.09 atomic % P section do overlap
slightly, but a small shift in Ni values could do
away with it. The surprising point is how little
overlap is seen when the data are taken directly
from the tables given earlier and plotted in this
way.
Had all of the tie lines in Figure 28 been plotted
on a single isothermal section, there would have
been several cases of overlap. The most serious
offender would be a Bellsbank tie line, the second
one from the left on the 0.09 atomic % P isother-
mal section. If it were plotted on the top section
of Figure 28, there would have been obvious
overlap with another Bellsbank tie line. All three
of the tie lines in this top isothermal section are
from the Bellsbank meteorite, and there is addi-
tional evidence to be discussed below that suggests
that two temperatures are involved.
Plotting tie lines as has been done in Figure 28
supports the idea that a temperature effect is real.
That is to say that late formed schreibersites con-
tinued significant growth to temperatures below
those of early formed schreibersites. Unfortu-
nately, these meteorites do not supply us with a
full range of tie lines at any one temperature.
More work will be required before the slope of
the a/a + Ph boundary within this low tempera-
ture range can be established with certainty.
A third possibly complicating process that could
be of importance here has been discussed by
Comerford (1969). He pointed out that grain
boundary diffusion might play an important role
in the development of grain boundary schreiber-
sites. He suggested that grain boundary diffusion
would supply Ni to schreibersites growing at low
temperatures more readily than lattice diffusion.
The discussion above has pointed out that P is the
controlling element rather than Ni, but his sugges-
tion would be equally valid for P.
The measurements on grain boundary schrei-
NUMBER 21 69
0.09 ATOMIC %P
395? C
4.4
0
100 % Fe
10 20 30
ATOMIC %Ni
40
FIGURE 28. ?Tie lines from various meteorites separated according to interface P saturation values
in kamacite for plotting on isothermal sections.
70 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
bersite in the Santa Luzia meteorite are represent-
ative of many such measurements reported above
(Figures 10, 11). Petrographic relationships sug-
gest that this type of schreibersite nucleated at
kamacite-taenite interfaces absorbing residual tae-
nite. Nucleation temperatures were probably low
compared to those for homogeneously nucleated
rhabdite, but the data does not allow a closer
temperature estimate. It would also appear that
nucleation occurred over a range of temperature,
nucleation of the lowest Ni schreibersite farthest
along the grain boundary away from taenite hav-
ing taken place first (Figure 11, traverse 11). The
Ni concentration of schreibersite nucleated at a
kamacite-taenite border is that of the schreibersite
corner of the three-phase field at the nucleation
temperature. The presence of the phase boundary
probably serves both as a site for nucleation and
as a source of P through the process of grain
boundary diffusion. As the schreibersite becomes
larger, however, the contribution of grain bound-
ary diffusion becomes increasingly less important.
The schreibersite grows rapidly, with its Ni level
increasing only slightly. Its interface with kamacite
represents a tie line that probably tends to shift to
the left with decreasing temperature, as P is drawn
from a relatively large diffusion field.
Along the grain boundary at a location closer to
the final residual taenite a second grain boundary
schreibersite nucleates at a lower temperature (Fig-
ure 11, traverse 12). Its initial Ni is higher, and
the P diffusion rate is lower. Grain boundary
diffusion again makes an important contribution
during initial growth, but it probably becomes
increasingly less important as the schreibersite
increases in size.
The observed taenite border schreibersite un-
doubtedly nucleated at a still lower temperature
(Figure 11, traverse 13). It formed at the taenite-
kamacite border and had a high initial Ni concen-
tration. Growth was insufficient to separate the
schreibersite completely from taenite. A small
schreibersite that has a Ni gradient due to the
fact that it is in contact with kamacite at one end
and taenite at the other resulted. This type of
schreibersite has both the highest observed Ni
concentrations and the lowest observed P interface
values in kamacite. A reasonable explanation of
these observations is that grain boundary diffusion
plays an increasingly important role both with
small schreibersite size and lower temperature of
growth.
Grain boundary diffusion may also help explain
the Bellsbank interface measurements summa-
rized in Figure 5. Four of the seven interfaces
shown have a P level in kamacite of 0.09 weight %
P (0.16 atomic %), and the other three are at 0.05
weight % P (0.09 atomic %). The schreibersite
interface illustrated at the top of the diagram is a
typical one for massive schreibersite. The schrei-
bersite illustrated at the lower left is a large rhab-
dite (Table 3, traverse 6) surrounded by clear
kamacite within an area of microrhabdites. There
were no grain boundaries or subgrain boundaries
observed. The schreibersite illustrated at the lower
right is another large elongated rhabdite (Table 3,
traverses 4 and 5) that was observed to be along a
definite subgrain boundary. Both of these rhab-
dites have extremely large P gradients, and their
P interface values differ by 0.04 weight % P (0.07
atomic %). The center schreibersite in the lower
section of the diagram is a large rhabdite (Table
3, traverses 2 and 3) along a subgrain boundary
with different P and Ni interface values in kama-
cite on its opposite sides. Tie lines representing
these interfaces are given in Figure 28. The three
high P tie lines are in the top isothermal section
and the fourth, representing the low P interfaces,
is at the left in the middle section. A possible
rationale for these observations is that Bellsbank
cooled more rapidly than the other meteorites
studied. As a result, growth apparently stopped at
higher temperatures than has been the case for
other rhabdite-containing meteorites. The pres-
ence of grain boundaries, however, permitted P
flow to continue to significantly lower tempera-
tures for the two schreibersites with the lower
interface levels. A faster type of diffusion permit-
ted detectable interface changes to extend to lower
temperatures. It would be interesting to find simi-
lar situations in other meteorites.
SCHREIBERSITE WITH COHENITE BORDERS
A prominent feature of the meteorites selected
for study is the presence of 100-to-200-/Lim-wide
cohenite borders separating massive schreibersite
from kamacite. It is a feature that can be recog-
nized with the unaided eye on many well-prepared
exhibit sections of Group I meteorites, but it has
NUMBER 21 71
been little noted in the literature until relatively
recently. Cohenite borders may partially or com-
pletely enclose schreibersite bordering troilite in-
clusions or massive schreibersite isolated within
kamacite. This type of association has been re-
ported above for the Ballinger (Figure 6), Lexing-
ton County (Figure 12), Bahjoi (Figure 13), Goose
Lake, and Balfour Downs (Figures 15, 16, 17)
meteorites. The Ni concentrations in these cohen-
ite enclosed schreibersites range from 16 to 21
atomic %, comparable to those observed for simi-
lar unenclosed schreibersites within the same me-
teorite or within similar meteorites. The cohenite
borders contain small amounts of Ni and no de-
tected P. Their Ni profiles show a gradient varying
from as much as 1.7 weight % Ni at the cohenite-
kamacite interface to as low as 0.8 weight % Ni at
the cohenite-schreibersite interface (Figures 13,
17).
Similar Ni gradients in cohenite borders have
been reported previously by Drake (1970) and
Scott (1971a). Drake (1970) noted that Ni diffuses
through cohenite, increasing the Ni content of
enclosed kamacite and taenite grains with decreas-
ing temperature, but he felt that cohenite shells
around schreibersite prevented equilibration of
schreibersite with the enclosing metal. Scott
(1971a) interpreted the presence of Ni gradients
in cohenite borders as implying equilibration of
Ni to low temperatures. The observed Ni concen-
trations in cohenite-enclosed schreibersite sup-
ports Scott's interpretation.
Massive schreibersite growth is apparently
blocked by enclosure in cohenite. The absence of
detectable P in, these cohenites indicates that P
solubility is very low. It seems reasonable to as-
sume, therefore, that cohenite borders isolate
schreibersite from a source of additional P. The
situation with Ni, however, is different. The co-
henite borders contain appreciable Ni, and the
measured Ni gradients suggest a flow of Ni to the
cohenite-schreibersite interfaces. Under these cir-
cumstances the size of the schreibersite indicates
the maximum temperature at which cohenite
could have nucleated. The temperature must have
been low enough to permit schreibersite to reach
its full size. At this maximum temperature, or at
some lower temperature, cohenite encases the
schreibersite. Growth is stopped but Ni enrich-
ment continues with decreasing temperature. The
minimum temperature at which cohenite could
have formed is at or below the temperature at
which the schreibersite achieved its final Ni con-
centration. According to the Fe-Ni-P diagram this
would be at 550? C or below. In this low tempera-
ture range schreibersite would be surrounded by
large volumes of kamacite. Is kamacite a reasona-
ble source for the large amount of carbon required
for cohenite growth? The work of Brett (1967)
and A. D. Romig, Jr. (1977, pers. comm.) on the
Fe-Ni-C system seems to require an alternate ex-
planation.
Consideration of the Goose Lake meteorite
structure in terms of both its cooling history within
the Fe-Ni-P system (Table 14) and Romig's Fe-Ni-
C diagram may be instructive at this point. Goose
Lake is a meteorite that has large amounts of
massive schreibersite. Surface areas totaling more
than 1000 cm2 were examined and found to con-
tain more than 20 areas of clustered massive schrei-
bersites. All of these schreibersites appeared to be
enclosed in cohenite, although poor condition of
surfaces left some ambiguity in one or two cases.
In any event, the ubiquitous nature of cohenite
bordering massive schreibersite in Goose Lake was
established. This suggested that Goose Lake might
be expected to yield a more straight-forward inter-
pretation of cohenite border growth than would
be possible using meteorites where the association
was more sporadic.
Equilibrium compositions of phases derived
from the Fe-Ni-P system for a range of decreasing
temperatures are given in the lower section of
Table 14 for the assumed Goose Lake meteorite
bulk composition. At 925? C schreibersite is in
equilibrium with 96% taenite. On cooling to 750?
C the schreibersite doubles in amount to 8% and
increases in Ni from 5.7 to 6.6 atomic %. Taenite
is slightly reduced in volume during this process,
but its Ni concentration remains essentially con-
stant at 7.9 atomic %. Cooling through this 175? C
range has resulted primarily in the localization of
P. Massive schreibersites have grown within taenite
and have achieved most of their eventual size.
In the 750? to 650? C temperature range major
structural changes take place. Taenite increases in
Ni from 7.9 to 11 atomic % and decreases in
volume by one-half. Schreibersite increases in Ni
from 6.6 to 9.0 atomic % and increases in volume
by approximately 10%. The shrinking taenite is
72 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
converted to kamacite of 5.0 atomic % Ni, which
makes up nearly half the structure at 650? C. With
further decreasing temperature, kamacite ex-
pands in volume, taenite shrinks, and schreibersite
holds its own. All three phases continue to increase
in Ni.
What happens if carbon is also present in the
system? Romig's Fe-Ni-C phase diagram gives car-
bon saturation values for taenite in equilibrium
with cohenite. These are given in Table 20 for
selected taenite compositions for the range of
temperatures covered. The taenite Ni values given
are those of taenite in equilibrium with schreiber-
site at the indicated temperature and the Goose
Lake bulk composition using the Fe-Ni-P system
(Table 14). Carbon has appreciable solubility in
taenite throughout this temperature range, but its
solubility in kamacite is negligible. This solubility
of carbon in taenite means that a meteorite such
as Goose Lake with a structure consisting of 92%
taenite at 750? C might well contain enough dis-
solved carbon to produce significant amounts of
cohenite on subsequent cooling. The important
factor is that the amount of taenite present is
drastically reduced with decreasing temperature.
This results in carbon concentration increasing in
the ever decreasing amount of taenite. Supersatu-
ration must result at some temperature if sufficient
carbon were present to start with.
Brett (1967) assumed that P could be neglected
in his discussion of cohenite formation in meteor-
ites. That may be true for some types of cohenite
associations, but it may well not hold for cohenite
bordering massive schreibersite. It is difficult to
understand these associations using the Fe-Ni-C
system alone. Brett (1967) shows that in this system
cohenite grows by the following reaction:
y = cohenite + a.
He also points out that when the Ni content is
high, kamacite and graphite form with falling
temperature. Cohenite is what we see, however,
not significant amounts of graphite in kamacite.
If we assume that carbon is present in taenite,
then we are actually dealing with the Fe-Ni-P-C
system. The more complex system permits the
postulation of the following sequence of events to
account for massive schreibersite with cohenite
borders. Massive schreibersite nucleates at high
temperature and grows upon cooling to near its
TABLE 20. ?Carbon saturation values for taenite in
equilibrium with cohenite from the Fe-Ni-C dia-
gram of A. D. Romig, Jr. (taenite Ni values are
equilibrium values using the Goose Lake bulk com-
position, Table 14).
?c
730
650
600
500
Weight % C
0.75
0.45
0.36
0.18
Atomic % Ni
n
14
?v25
final size, perhaps in the 650? to 700? C tempera-
ture range, assuming the Goose Lake bulk com-
position. These schreibersites are in contact with
taenite that has become saturated in carbon. With
further cooling both taenite and schreibersite re-
quire more Ni, and taenite requires a repository
for its carbon. Under these circumstances it is
reasonable to assume the following nucleation re-
action taking place at taenite-schreibersite inter-
faces:
OKQat) + Ph = y + cohenite + Ph.
A cohenite border at the preexisting taenite-schrei-
bersite interface results. Cohenite becomes a re-
pository for carbon and the Ni that previously
resided there is absorbed by taenite and/or schrei-
bersite. As the structure develops, a dynamic equi-
librium involving two interfaces is established; car-
bon saturated taenite-cohenite and cohenite-
schreibersite. Cohenite grows as long as carbon
saturated taenite is present. Taenite shrinks in
volume, cohenite fills in behind the receding tae-
nite, and schreibersite becomes enriched in Ni. At
some point kamacite precipitates at the cohenite-
taenite interface and then kamacite becomes the
source of Ni for transport to schreibersite. The
cohenite border is established and subsequent
changes involve diffusion of Ni and Fe only.
The postulated reaction sequence would permit
cohenite to grow over a broader temperature
range than was suggested by Brett (1967). The
maximum temperature can be estimated from the
size of the cohenite-enclosed schreibersite. What
is the highest temperature that could have pro-
duced schreibersite of the observed size starting
with a given bulk composition? In a meteorite like
Goose Lake the temperature might be as high as
750? C. The minimum temperature of nucleation
NUMBER 21 73
would be established by the disappearance of suf-
ficient taenite to supply the required carbon super-
saturation. It seems unlikely for the meteorites
that have been studied here that this would be
below 600? C. This postulated growth sequence
could extend Brett's (1967) temperature range of
610? to 650? C upward, but it is unlikely that it
would be lowered appreciably. If Brett's (1967)
estimated rates of cohenite decomposition with
temperature are correct, nucleation within the
lower part of the possible temperature range
would favor preservation of cohenite. Confirma-
tion of this proposed sequence of events will have
to await the results of detailed experimentation.
Summary and Conclusions
The role that schreibersite growth played in the
structural development process in coarse-struc-
tured iron meteorites has been examined. The
availability to the writer of many large meteorite
surfaces and an extensive collection of metallo-
graphic sections made it possible to undertake a
comprehensive survey of schreibersite petrogra-
phy. This study was the basis for the selection of
samples for detailed electron microprobe analysis.
Samples containing representative structures from
eight chemical Groups I and IIAB meteorites were
selected. All of these meteorites contain schreiber-
site, ranging in amount from easily detectable to
abundant, and they contain additional P in solu-
tion in kamacite and taenite. Their detailed metal-
lography indicates that they are comparatively free
of secondary effects such as shock and reheating
that might mask or distort relationships of interest.
The range of bulk compositions covered is one
where small changes in P-Ni concentration rela-
tionships may be more important to the structural
development process than would be the case at
higher Ni levels.
Lengthy electron microprobe traverses were
made across structures representative of the ob-
served range of schreibersite associations, with the
exception of microrhabdites. Particular emphasis
was placed on schreibersite-kamacite interface
compositions. Ni and P interface concentrations
in kamacite and Ni interface values in schreibersite
were determined, along with associated Ni and P
concentration gradients in kamacite and Ni gra-
dients in schreibersite. An analysis of these data
has led to a more comprehensive description of
the structural development process in these indi-
vidual meteorites than has been available previ-
ously.
Massive schreibersite, one of the four major
types of schreibersite encountered, may be ac-
counted for by equilibrium considerations. The
Fe-Ni-P equilibrium diagram and estimated bulk
compositions for individual meteorites were used
to explain the basic structures present. Subsolidus
nucleation and growth with slow cooling from
temperatures at least as high as 850? C, and prob-
ably much higher, explain the phase relationships
that one sees in meteorite specimens. No need
was found to invoke eutectic liquids to account for
any of the observed massive schreibersite mor-
phologies. After nucleation, schreibersite and
kamacite grew in volume and increased in Ni
content while taenite receded and increased in Ni
content under equilibrium or near equilibrium
conditions into the 600? C temperature range. The
retention of taenite in the octahedrites establishes
that bulk equilibrium did not extend as low as 550?
C. Schreibersite undoubtedly continued in equilib-
rium with its enclosing kamacite to lower temper-
atures. The growth of massive schreibersite from
high temperatures into the 600? C temperature
range served both to localize P by reducing its
level throughout the bulk metal and to produce
the large swathing kamacite zones that are ob-
served.
The second type of schreibersite to form is
homogeneously nucleated rhabdite. It nucleated
in kamacite in the 600? C temperature range,
either as a consequence of low initial P level or
after local P supersaturation developed following
massive schreibersite growth. Growth of these
schreibersites was also under equilibrium condi-
tions to below 500? C. They drew their source of P
from limited diffusion fields when compared to
the massive schreibersites.
The third type of schreibersite is grain boundary
and taenite border schreibersite. It formed at
kamacite-taenite interfaces, absorbing residual tae-
nite. Nucleation took place successively along grain
boundaries over a range of temperatures starting
as high as 500? C or perhaps slightly higher. With
decreasing temperature taenite receded and be-
came richer in Ni and the schreibersite that nu-
cleated at these interfaces had higher Ni. The
74 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
highest Ni schreibersites are the small ones that
nucleated at low temperatures and are observed
still in contact with taenite. Grain boundary diffu-
sion probably became an increasingly important
factor in the growth of these schreibersites with
decreasing temperature.
The fourth type of schreibersite is microrhab-
dite. These schreibersites nucleated homogene-
ously in supersaturated kamacite at temperatures
in the 400? C range or below.
P diffusion controlled the growth rate of schrei-
bersite. The Ni flux to a growing interface had to
produce a growth rate equal to that established by
the P flux. This was accomplished by tie line shifts
that permitted a broad range of Ni growth rates
and accounts for the observed range of Ni concen-
trations in schreibersite. Interface measurements
on meteoritic schreibersites, therefore, yield a
range of tie lines in the low temperature Fe-Ni-P
system. Equilibrium conditions pertained at
growth interfaces to temperatures far below those
available experimentally. Kinetic factors, however,
restricted mass transfer to increasingly small vol-
umes of material with decreasing temperature.
Schreibersite with cohenite borders is a common
occurrence in many meteorites. It is suggested
that it formed as a reaction in the Fe-Ni-P-C
system in the 750? to 600? C temperature range.
Carbon saturated taenite reacted with schreibersite
to form cohenite. Cohenite then grew in equilib-
rium with both taenite and schreibersite.
Berzelius' prescient comment of 1832 on the
role of P in the development of the Widmanstatten
pattern has been confirmed and expanded upon.
Appendix
In the discussion of the low-temperature ternary
diagram, experimental values for P saturation con-
centrations in kamacite and taenite were extrapo-
lated to lower temperatures (Table 17, Figures 21,
22). This is a commonly used procedure when
dealing with dilute solutions and is a variation on
a method used for the determination of partial
molal properties of substances. It is applicable to
solid state transformations in dilute metallic sys-
tems and may be justified by the following ther-
modynamic reasoning adapted from Swalin
(1972:168-171).
At a given temperature (Tx) the end points of a
kamacite-schreibersite tie line in the kamacite-
schreibersite field of the equilibrium diagram rep-
resent equilibrium concentrations of the two
phases. The relationship between P-saturated
kamacite (ap(Sat)) and schreibersite (Ph) may be
expressed as follows:
ttp(sat) ^ Ph + a AG = 0.
Since the reaction represents equilibrium the
change in the Gibbs free energy (AG) is zero.
Therefore, the chemical potentials (fi) of P in the
two phases are equal,
f^P M'P(sat)
as are their activities (a),
?Ph _ ?<*
aP ? aP(sat).
Since schreibersite is a pure substance with respect
toP
requiring that
-?Ph _
aP ?
? 1 ?
If the saturation concentration of P in kamacite is
sufficiently small, P may be assumed to follow
Henry's law and the activity coefficient of P in
kamacite, y?, may be evaluated.
aP(sat) ? 7P AP(sat)
1
In order to derive a relationship for the solvus
line (kamacite saturated with P) as a function of
temperature (T), we must consider a situation
where AG for the reaction is not zero. This may
be done by assuming a P concentration in kama-
cite, Xg, such that Xp" is within the kamacite field
of the equilibrium diagram
XaP -P(sat)-
Kamacite of this concentration would react with
schreibersite to form P-saturated kamacite,
aP + Ph ?* ?P(Sat) AGX.
The following equation may be written relating
the Gibbs free energy ( AG2) for the above reaction,
the chemical potentials of the reacting species,
and the activities of these species,
AGX = n% - At?Pph = RT In -^ ?
for a pure substance,
aPh = 1
for a dilute solution following Henry's law
a? = yAX?
and from equation given above
AP-m) =
?(sat) ^-PCsat)
Substituting these values of aPh and aP in the
expression for AGX given above we obtain
fi$ - n?Pph = RT In Xg - RT In Xp\sat) - RT In (1).
We know that the enthalpy of a component i, H,,
is related to the temperature and chemical poten-
tial by
= Hi.
75
76 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
Substituting, we obtain
- Hph
d(RT In Xg/T) a(RT In
3(1/1)
Xp and (1) are constants, dropping out these two
terms,
HPa - HPh = - Rd(lnXHsat))a(VT)
3(1/'T) R = - 3(ln
In Xp(Sat) = -AHFR
Therefore, when Xp\sat) is plotted on a log scale
against 1/T, a straight line with slope ? AHP/R
should result. Straight lines with negative slopes
are observed in our case, and it is reasonable to
assume on the basis of experience in other systems
that AHPa is independent of temperature over a
reasonable temperature range. It would seem then
that an extrapolation over a reasonable tempera-
ture range is justified. A similar treatment would
apply to P-saturated taenite data.
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