de c,
y of
titut
Revised 23 February 2010
Accepted 23 February 2010
Available online 12 March 2010
Keywords:
used to classify asteroids, identify meteorite parent bodies, and understand the structure of the asteroid
widely-applied tool used in the search for these parent asteroids,
due to the strong 1 lm and/or 2 lm absorption bands present in
the dominant chondritic minerals olivine and pyroxene (Adams,
1974, 1975; Burns et al., 1972; Cloutis, 1985; Cloutis and Gaffey,
1991). This approach requires a direct comparison of spectral band
parameters (e.g., Band I and II centers, band area ratios) between
meteorite and asteroids. However, if robust relationships between
complete petrologic range of the equilibrated ordinary chondrites
(types 4?6) (Van Schmus and Wood, 1967). This is the ?rst study
in which spectral calibrations have been derived using actual mea-
sured ordinary chondrite modal abundances. Prior to this study,
spectral calibrations were based on simple mixtures of ma?c sili-
cates (Cloutis et al., 1986) or normative abundances of ordinary
chondrites (Burbineet al., 2003), due to thedif?cultyassociatedwith
quantifying modal abundances of ?ne-grained samples. Normative
mineralogies are calculated from bulk chemistry using a standard
CIPW (Cross, Iddings, Pirsson, and Washington) algorithm (Cross
et al., 1902), andareusedmostoften todetermineabundancesof?ne
* Corresponding author at: Department of Geography?Geology, Illinois State
University, Normal, IL 61761, USA.
Icarus 208 (2010) 789?797
Contents lists availab
ru
.e lE-mail address: tldunn@ilstu.edu (T.L. Dunn).Over the past few decades, an ongoing debate has centered on
the identities of the ordinary chondrite parent bodies, the most
common meteorites seen to fall to Earth. While earlier workers
posited that ordinary chondrite-like bodies should be common in
the asteroid belt, it is now generally recognized that only three
parent bodies are required to account for the chemically-distinct
H, L and LL chondrites. Each chondrite group is chemically homog-
enous (Dodd, 1981), oxygen isotopes cluster within a narrow range
(Clayton et al., 1991), and radiometric ages indicate that many H
and L chondrites were ejected from the same parent body (Keil
et al., 1994). Visible/near-infrared spectra have been the most
dances and compositions) could be established, these comparisons
could be made using the same criteria that are used in meteoritics.
Such a comparison has the advantage that the H, L and LL chemical
groups were originally de?ned on the basis of bulk chemistry, par-
ticularly total iron abundance and ma?c silicate compositions, and
not on spectral parameters. The H, L and LL chondrite groups exhi-
bit distinct compositional hiatuses, particularly with respect to
ma?c silicate compositions, although some authors have ques-
tioned whether L and LL chondrites are transitional (Rubin, 1990).
In this paper, we revisit spectral calibrations for asteroids using
a data set of 48 measured ordinary chondrite modal abundances
and corresponding silicate mineral analyses, which represent theAsteroids
Spectroscopy
Meteorites
1. Introduction0019-1035/$ - see front matter  2010 Elsevier Inc. A
doi:10.1016/j.icarus.2010.02.016belt. Using a suite of 48 equilibrated (types 4?6) ordinary (H, L, and LL) chondrites containing orthopy-
roxene, clinopyroxene, and olivine, new relationships between spectra and mineralogy have been estab-
lished. Contrary to previous suggestions, no meaningful correlation is observed between band parameters
and cpx/(opx + cpx) ratios. We derive new calibrations for determining mineral abundances (ol/(ol + px))
and ma?c silicate compositions (Fa in olivine, Fs in pyroxene) from VIS/NIR spectra. These calibrations
con?rm that band area ratio (BAR) is controlled by mineral abundances, while Band I center is controlled
by ma?c silicate compositions. Spectrally-derived mineralogical parameters correctly classify H, L and LL
chondrites in 80% of cases, suggesting that these are robust relationships that can be applied to S(IV)
asteroids with ordinary chondrites mineralogies. Comparison of asteroids and meteorites using these
new mineralogical parameters has the advantage that H, L and LL chemical groups were originally
de?ned on the basis of ma?c silicate compositions.
 2010 Elsevier Inc. All rights reserved.
these spectral parameters and mineralogical parameters (abun-Article history:
Received 12 November 2009
Mineral compositions and abundances derived from visible/near-infrared (VIS/NIR or VNIR) spectra areA coordinated spectral, mineralogical, an
chondrites
Tasha L. Dunn a,d,*, Timothy J. McCoy b, J.M. Sunshin
aDepartment of Earth and Planetary Sciences, Planetary Geosciences Institute, Universit
bDepartment of Mineral Sciences, National Museum of Natural History, Smithsonian Ins
cDepartment of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
dDepartment of Geography?Geology, Illinois State University, Normal, IL 61761, USA
a r t i c l e i n f o a b s t r a c t
Ica
journal homepage: wwwll rights reserved.compositional study of ordinary
Harry Y. McSween Jr. a
Tennessee, Knoxville, TN 37902, USA
ion, Washington, DC 20013-7012, USA
le at ScienceDirect
s
sevier .com/locate / icarus
mineralogical interpretations of asteroid spectra.
systematic differences between the two data sets.
us 23. Analytical methods2. Background
The primary diagnostic feature in olivine is a composite absorp-
tion feature at 1 lm, which consists of three distinct absorption
bands. The composite 1 lm band, which is attributed to electronic
transitions of Fe2+ occupying both the M1 and M2 crystallographic
sites (Burns, 1970), moves to longer wavelengths as FeO content
increases (King and Ridle, 1987; Sunshine and Pieters, 1998).
Pyroxenes have two absorption bands at 1 lm and 2 lm that
are associated with crystal ?eld transitions in Fe2+, which preferen-
tially occupy the M2 site (Clark, 1957; Burns, 1970). Low-calcium
pyroxenes, which are conventionally de?ned as having less than
11 mol% CaSiO3 (wollastonite or Wo) (Adams, 1974), show a
well-de?ned relationship between absorption band positions and
composition, as both Band I and Band II positions increase with
increasing ferrous iron content (Adams, 1974; Burns et al., 1972;
Cloutis, 1985). There is also a correlation between composition
and band positions in high-calcium pyroxene, although the rela-
tionship is complicated by the presence of calcium in addition to
iron.
In spectra containing both olivine and pyroxene absorptions,
the combined absorption features near 1 lm (Band I) and near
2 lm (Band II) are also sensitive to the relative proportions of oliv-
ine and pyroxene. The ratio of the areas of these two bands (Band
II/Band I) is commonly used to estimate olivine and pyroxene
abundances in meteorites and asteroids (Cloutis et al., 1986). The
linear relationship between this band area ratio (BAR) and the ratio
of pyroxene to olivine + pyroxene (px/(ol + px)) was ?rst recog-
nized Cloutis et al. (1986), who expressed this relationship as
BAR ? 0:024 ?px=?ol? px??  1:25: ?1?
While Cloutis et al. (1986) utilized this equation to derive BARs
from mixtures of known mineral proportions, Gastineau-Lyons
et al. (2002) used this relationship to derive mineral abundances
from BARs of asteroid spectra, recasting the Cloutis et al. (1986)
calibration as
px=?ol? px? ? 0:417 BAR ? 0:052: ?2?
However, because the Cloutis et al. (1986) regression was based
on simple mixtures of olivine and orthopyroxene, the presence of
more than one pyroxene (or other additional phases) would com-
plicate spectral interpretations of asteroids made using this cali-
bration (Gaffey et al., 1993; Sunshine et al., 2004).
In an attempt to determine the mineralogy of the S-type aster-
oids, Burbine et al. (2003) used normative abundances of the ordin-
ary chondrites, which contain olivine, orthopyroxene and
clinopyroxene, to derive a relationship between BAR and ol/
(ol + px). Burbine et al. (2003) expressed their equation as
ol=?ol? px? ? 0:228 BAR ? 0:768: ?3?
The Burbine et al. (2003) calibration yields ordinary chondrite
ol/(ol + px) ratios that fall within the same general range of ol/
(ol + px) ratios measured from normative mineral abundances of
ordinary chondrites (McSween et al., 1991). However, there aregrained terrestrial samples, such as volcanic rocks. Our new calibra-
tions based on measured abundances should yield more accurate
790 T.L. Dunn et al. / IcarA total of 48 ordinary chondrite samples representing each of
the ordinary chondrites groups (H, L, and LL) and petrologic catego-
ries 4?6 (Van Schmus and Wood, 1967) were selected for analysis.To ensure that samples represented a single petrographic type, vis-
ibly polymict samples were excluded from the study, and only
unbrecciated falls with minimal terrestrial weathering were se-
lected for analysis. Re?ectance spectra of ordinary chondrites were
acquired using a bidirectional spectrometer at Brown University?s
Keck/NASA Re?ectance Experiment Laboratory (RELAB) (Pieters
and Hirio, 2004). Spectra of 37 chondrite falls in this study
(samples with RELAB IDs TB-TJM-XXX; Table 1) were collected
by Burbine et al. (2003) from powders originally prepared for bulk
chemical analysis by Jarosewich as part of the Smithsonian Institu-
tion?s Analyzed Meteorite Powder Collection. Small chips of the
remaining 11 chondrites (samples TH-HYM-XXX; Table 1) were ac-
quired from the Natural History Museum in London and from the
Smithsonian Institution. Consistent with sample preparation de-
scribed in Burbine et al. (2003), samples were ground with mortar
and pestle into a ?ne powder (<150 lm), and the metal fraction
of the samples was then magnetically removed from the powder
in preparation for spectral analysis. Only the silicate portion of
each sample was analyzed. Although removing metal from the
sample may alter the slope of the spectrum, it should not affect sil-
icate spectral features (i.e. BAR, Band I or Band II center) or spec-
trally-derived silicate mineral abundances.
For the 11 chondrites obtained for this study (TH-HYM-XXX;
Table 1), availability of material was limited, and the mass of mate-
rial used to prepare these samples was signi?cantly smaller than
the multi-gram masses used by Jarosewich (1990, 2006). It is pos-
sible that these powders may not be as representative as those pre-
pared by Jarosewich (1990, 2006), but they were necessary to
ensure that the H, L, and LL ordinary chondrite groups were equally
represented. Unrepresentative sample powders may be expected
to yield modal abundances that are higher or lower than actual val-
ues, thereby skewing trends based on modal data. For example,
measured abundances of low-Ca pyroxene in two L5 chondrites
with limited material (Ausson and Blackwell) are 1?2 wt.% higher
than abundances in the remaining L5 chondrites. This results in ol/
(ol + px) ratios that are lower than those of the remaining L5 chon-
drites. However, most samples obtained for this study do not ap-
pear to show anomalous modal abundances, and it is unlikely
that a few potentially unrepresentative samples would alter con-
clusions based on 48 samples.
Spectra for all 48 samples were collected over a range of 0.32?
2.55 lm at a sampling interval of 0.01 lm. An incident angle of 30
and an emission angle of 0 were used for spectral measurements.
Band area ratios and band centers were determined by ?rst divid-
ing out a straight-line continuum using points on either side of
Band I and Band II (with 2.5 lm as the furthest data point). The
area of each band was measured as the area between the absorp-
tion band and a tangent line drawn between two peaks on either
side of the absorption, and the band area ratio (BAR) was calcu-
lated by dividing the area of Band II by the area of Band I. The aver-
age error associated with BAR is 0.01 lm. Band center was
determined by ?tting a second order polynomial to the bottom
of the continuum removed feature, and the minimum point of
the polynomial was used as the band center. The uncertainty of
band centers is typically between 0.01 and 0.03 lm, with an aver-
age error of 0.02 lm. Classi?cations, grain sizes, and spectral
parameters (BCI, BCII, and BAR) are listed in Table 1.
XRD data were collected using an INEL curved position-sensitive
detector (PSD) at the Natural History Museum in London, England.
Mineral abundances were determined using a whole-pattern XRD
?tting procedure, ?rst introduced by Cressey and Scho?eld (1996)
and further developed by Batchelder and Cressey (1998). Experi-
mental con?gurations and a detailed description of the XRD ?tting
08 (2010) 789?797procedure are reported in Dunn et al. (2010a). Olivine and low-
Ca pyroxene compositions in 38 of the ordinary chondrites in this
study were determined using a Cameca SX-50 electron microprobe
Gra
<15
<15
<15
<15
<15
<15
<15
<15
<15
<15
<15
Karatu LL6 TB-TJM-077 <75
Saint-S?verin LL6 TB-TJM-145 <15
<15
<15
<15
<15
us 2Attarra L4 TB-TJM-065
Bald Mountain L4 TB-TJM-102
Rio Negro L4 TB-TJM-081
Rupota L4 TB-TJM-121Table 1
Meteorites measured in this study and their spectral parameters.
Meteorite Type RELAB IDa
Bernares (a) LL4 MT-HYM-083
Greenwell Springs LL4 TB-TJM-075
Hamlet LL4 MT-HYM-075
Witsand Farm LL4 MT-HYM-076
Aldsworth LL5 MT-HYM-077
Alta?ameem LL5 MT-HYM-078
Olivenza LL5 MT-HYM-085
Paragould LL5 MT-HYM-079
Tuxtuac LL5 MT-HYM-080
Bandong LL6 TB-TJM-067
Cherokee Springs LL6 TB-TJM-075
T.L. Dunn et al. / Icarat the University of Tennessee. Silicate mineral analyses and a
detailed discussion of experimental con?gurations are provided in
Dunn et al. (2010b).
4. Results
4.1. Olivine and pyroxene abundances
XRD-measured abundances of olivine, orthopyroxene, clinopy-
roxene, and pigeonite (reported as ol/(ol + px)) are presented in Ta-
ble 2, along with ol/(ol + px) ratios calculated from CIPW norms
(McSween et al., 1991) using the bulk chemical analyses of
Jarosewich (1990, 2006). The average error associated with XRD-
measured ol/(ol + px) ratios is 0.03, which is based on the uncer-
tainty in determination of olivine and pyroxene abundances
(?2 wt.%) (Dunn et al., 2010a). The average uncertainty for norma-
tive ratios is cited as 0.01 by Burbine et al. (2003). XRD-measured
weight ratios of ol/(ol + px) differ slightly from those calculated
using normative abundances due to systematic differences
Ausson L5 MT-HYM-084 <15
Blackwell L5 MT-HYM-081 <15
Cilimus L5 MT-HYM-082 <15
Guibga L5 TB-TJM-134 <15
Mabwe-Khoywa L5 TB-TJM-107 <15
Malakal L5 TB-TJM-109 <15
Messina L5 TB-TJM-099 <15
Apt L6 TB-TJM-064 <15
Aumale L6 TB-TJM-101 <15
Karkh L6 TB-TJM-137 <15
Kunashak L6 TB-TJM-139 <15
Kyushu L6 TB-TJM-140 <15
New Concord L6 TB-TJM-130 <15
Farmville H4 TB-TJM-128 <15
Forest Vale H4 TB-TJM-093 <75
Kabo H4 TB-TJM-136 <15
Marilia H4 TB-TJM-078 <15
S?o Jose do Rio Preto H4 TB-TJM-082 <15
Allegan H5 TB-TJM-104 nm
Ehole H5 TB-TJM-074 <15
Itapicuru-Mirim H5 TB-TJM-097 <15
Lost City H5 TB-TJM-129 <15
Primbram H5 TB-TJM-143 <15
Schenectady H5 TB-TJM-083 <15
Uberaba H5 TB-TJM-085 <15
Andura H6 TB-TJM-088 <75
Bustura H6 TB-TJM-069 <15
Canon City H6 TB-TJM-131 <15
Chiang Khan H6 TB-TJM-132 <15
Guarena H6 TB-TJM-094 <15
Ipiranga H6 TB-TJM-135 <15
a Spectra are available on the RELAB database at http://www.planetary.brown.edu/relin size Band I (lm) Band II (lm) BAR
0 lm 1.008 1.984 0.522
0 lm 0.991 1.959 0.512
0 lm 0.980 1.974 0.561
0 lm 1.004 2.001 0.457
0 lm 0.977 1.960 0.561
0 lm 0.986 1.952 0.435
0 lm 1.012 1.969 0.387
0 lm 0.985 1.968 0.409
0 lm 1.035 1.950 0.310
0 lm 1.001 1.987 0.274
0 lm 0.989 1.940 0.406
lm 1.006 1.992 0.311
0 lm 1.003 1.881 0.269
0 lm 0.957 1.927 0.726
0 lm 0.929 2.001 0.960
0 lm 0.953 1.932 0.813
0 lm 0.955 1.951 0.582
08 (2010) 789?797 791between normative and XRD-measured abundances, particularly
in olivine and high-Ca pyroxene. Normative olivine abundances
are higher than XRD-derived abundances of olivine by an average
of 3 wt.%, while normative high-Ca pyroxene abundances are lower
by an average of 2.7 wt.%.
The disparity between silicate abundances is a result of the way
in which CIPW normative phases are calculated. Because norma-
tive abundances are calculated as a limited set of ideal minerals,
minerals that are present in a sample may be incorrectly calculated
during the CIPW norm procedure if that phase is not a possible
normative mineral. Ordinary chondrites contain three pyroxenes:
enstatite, diopside, and pigeonite (Wo5?20). Enstatite and diopside
are normative phases, but pigeonite is not. As a result, the oxides
associated with pigeonite (primarily FeO and MgO) are incorrectly
allocated in the CIPW norm calculation. A comparison of normative
mineralogies (McSween et al., 1991), electron microprobe-mea-
sured chondrite abundances (Gastineau-Lyons et al., 2002), and
our XRD-measured abundances suggests that these oxides are
incorrectly allocated to olivine, resulting in overestimated norma-
tive olivine abundances and lower than expected high-Ca pyroxene
0 lm 0.930 1.918 1.031
0 lm 0.955 1.953 0.526
0 lm 0.950 1.925 0.515
0 lm 0.962 1.943 0.633
0 lm 0.954 1.944 0.744
0 lm 0.954 1.977 0.709
0 lm 0.959 1.960 0.595
0 lm 0.963 1.940 0.511
0 lm 0.960 1.946 0.614
0 lm 0.964 1.930 0.371
0 lm 0.972 1.942 0.518
0 lm 0.970 1.933 0.544
0 lm 0.960 1.928 0.606
0 lm 0.939 1.928 0.789
lm 0.937 1.934 0.800
0 lm 0.939 1.934 0.915
0 lm 0.934 1.945 0.851
0 lm 0.942 1.901 0.992
0.930 1.897 1.068
0 lm 0.940 1.945 0.999
0 lm 0.944 1.922 0.878
0 lm 0.940 1.936 0.857
0 lm 0.940 1.906 0.857
0 lm 0.937 1.913 0.786
0 lm 0.945 1.943 0.881
lm 0.927 1.929 0.889
0 lm 0.936 1.918 0.795
0 lm 0.953 1.932 0.752
0 lm 0.946 1.920 0.846
0 lm 0.938 1.916 0.748
0 lm 0.939 1.921 0.835
abdata/.
us 2Table 2
ol/(ol + px) ratios derived from XRD, normative, and spectral data.
Meteorite Type ol/(ol + px)
(XRD)
ol/(ol + px)
(norm)a
ol/ol + px
(spectra)b
Bernares (a) LL4 0.58 ? 0.60
Greenwell Springs LL4 0.64 0.66 0.60
Hamlet LL4 0.62 ? 0.59
Witsand Farm LL4 0.65 ? 0.62
Aldsworth LL5 0.65 ? 0.59
Alat?ameen LL5 0.62 ? 0.62
Olivenza LL5 0.64 0.67 0.63
Paragould LL5 0.65 ? 0.63
Tuxtuac LL5 0.63 ? 0.65
Bandong LL6 0.66 0.73 0.66
Cherokee Springs LL6 0.67 0.68 0.63
Karatu LL6 0.69 0.72 0.65
Saint-S?verin LL6 0.65 0.70 0.66
792 T.L. Dunn et al. / Icar(Dunn et al., 2010a). Due to the absence of pigeonite from the set of
ideal normative minerals, ratios of ol/(ol + px) calculated using
normative abundances (Burbine et al., 2003) are not representative
of ordinary chondrite mineralogies and do not provide the most
accurate calibration for spectral abundances.
Because the CIPW norm is not well-suited for ordinary chon-
drites, we derived a new equation for determining ol/(ol + px) ra-
tios from spectra using XRD-measured modal abundances. Linear
regression of ol/(ol + px) modal ratios as a function of BARs is pre-
sented in Fig. 1a. A least-squares ?t of the data yields
ol=?ol? px? ? 0:242 BAR ? 0:728; ?4?
with an R2 value of 0.73, which is higher than that of Burbine et al.
(2003). The authors would like to note that although the published
R2 in Burbine et al. (1993) is 0.93, the correct value is 0.61 (Burbine,
personal communication). ol/(ol + px) ratios derived from re?ec-
tance spectra using Eq. (4) are presented in Table 2. The root mean
Attarra L4 0.56 0.62 0.55
Bald Mountain L4 0.55 0.56 0.50
Rio Negro L4 0.55 0.59 0.53
Rupota L4 0.57 0.58 0.59
Ausson L5 0.55 0.64 0.48
Blackwell L5 0.56 ? 0.60
Cilimus L5 0.58 ? 0.60
Guibga L5 0.58 0.60 0.57
Mabwe-Khoywa L5 0.58 0.60 0.55
Malakal L5 0.57 0.60 0.56
Messina L5 0.57 0.60 0.58
Apt L6 0.56 0.67 0.60
Aumale L6 0.59 0.60 0.58
Karkh L6 0.59 0.65 0.64
Kunashak L6 0.60 0.63 0.60
Kyushu L6 0.61 0.61 0.60
New Concord L6 0.60 0.64 0.58
Farmville H4 0.46 0.51 0.54
Forest Vale H4 0.48 0.57 0.53
Kabo H4 0.46 0.51 0.51
Marilia H4 0.46 0.49 0.52
S?o Jose do Rio Preto H4 0.46 0.54 0.49
Allegan H5 0.47 0.48 0.47
Ehole H5 0.51 0.55 0.49
Itapicuru-Mirim H5 0.53 0.50 0.52
Lost City H5 0.49 0.54 0.52
Primbram H5 0.49 0.51 0.52
Schenectady H4 0.54 0.59 0.54
Uberaba H5 0.51 0.56 0.51
Andura H6 0.53 0.60 0.51
Bustura H6 0.51 0.53 0.54
Canon City H6 0.54 0.53 0.55
Chiang Khan H6 0.53 0.53 0.52
Guarena H6 0.60 0.54 0.55
Ipiranga H6 0.52 0.53 0.53
a From McSween et al. (1991); ? indicates that a norm has not been calculated.
b Errors are 0.03 for XRD-measured ratios, 0.01 for normative ratios, and 0.03 for
spectral ratios.a
b
08 (2010) 789?797square error of the spectrally derived ol/(ol + px) ratios is 0.03.
Fig. 1b compares spectrally-derived and XRD-measured ol/(ol + px)
ratios.
4.2. Low- and high-Ca pyroxene abundances
Gaffey et al. (2002) suggested that ma?c silicate abundances
can be constrained even further by considering the effect of high-
Ca pyroxene on the Band II position, which they suggested is an al-
most linear function of the relative abundance of the two pyrox-
enes. This assertion requires that the cpx/(opx + cpx) ratio is
well-correlated with Band II center in the range of values found
in ordinary chondrites. Gaffey et al. (2002) derived cpx/(opx + cpx)
ratios from spectra, but did not provide a calibration for this calcu-
lation. Using XRD-measured pyroxene abundances and Band II
centers, we have derived an equation for establishing the ratio of
high-Ca pyroxene to total pyroxene. A least-squares ?t of the data
yields
cpx=?opx? cpx? ? 0:533 Band II center 0:795; ?5?
with a very low R2 value of 0.15. The remarkably low R2 value indi-
cates this correlation is poorly constrained. This relationship ap-
pears to be even less valid for normative cpx/(cpx + opx) ratios,
which yield an R2 value of only 0.09. XRD-measured cpx/(opx + cpx)
ratios, normative ratios, and ratios derived from re?ectance spectra
are presented in Table 3. These results indicate that there is no sig-
ni?cant correlation between Band II position and relative pyroxene
abundance. We conclude that Band II should not used to derive
Fig. 1. (a) Linear regression (indicated by bolded black lines) of band area ratios vs.
XRD-measured ol/(ol + px) ratios. The solid grey lines are from Cloutis et al. (1986).
Error bars are not shown for clarity. (b) XRD-measured ol/(ol + px) ratios vs.
spectrally-derived ol/(ol + px) ratios for the ordinary chondrites in this study. The
solid diagonal line represents a 1:1 measured to derived ratios. Uncertainties are
stated in Table 2.
Bandong LL6 0.29 0.23 0.26
us 2Cherokee Springs LL6 0.26 0.22 0.24
Karatu LL6 0.32 0.24 0.27
Saint-S?verin LL6 0.27 0.22 0.21Table 3
XRD-measured, normative, and spectral cpx/(opx + cpx) ratios.
Meteorite Type cpx/(opx + cpx)
XRD
cpx/(opx + cpx)
norma
cpx/(opx + cpx)
spectra
Bernares (a) LL4 0.29 ? 0.26
Greenwell Springs LL4 0.23 0.21 0.25
Hamlet LL4 0.24 ? 0.26
Witsand Farm LL4 0.25 ? 0.27
Aldsworth LL5 0.24 ? 0.25
Alat?ameen LL5 0.25 ? 0.25
Olivenza LL5 0.24 0.18 0.25
Paragould LL5 0.25 ? 0.25
Tuxtuac LL5 0.26 ? 0.24
T.L. Dunn et al. / Icarrelative abundances of high-calcium pyroxene to total pyroxene
from re?ectance spectra.
5. Discussion
Since the ?rst comprehensive study of ordinary chondrite spec-
tra was completed (Gaffey, 1976), the ongoing search for the
parental asteroids of the ordinary chondrite has centered on the
S(IV) subgroup, one of seven subgroups of the S-type asteroids
(Gaffey et al., 1993). The S-type asteroids represent a suite of mix-
tures ranging in composition from pure olivine to pure pyroxene
(with potential meteorite analogues including ureilites [S(II)],
lodranites [S(III) and S(V)], and mesosiderites [S(VII)] (Gaffey
et al., 1993). The S(IV) subgroup is thought to contain objects with
mineralogies similar to the ordinary chondrites (Gaffey et al.,
1993). The S(IV) Asteroid 6 Hebe has been hypothesized to be
the parent body of the H chondrites (Gaffey and Gilbert, 1998),
Attarra L4 0.34 0.25 0.23
Bald Mountain L4 0.25 0.15 0.27
Rio Negro L4 0.25 0.15 0.23
Rupota L4 0.25 0.16 0.24
Ausson L5 0.22 0.17 0.23
Blackwell L5 0.27 ? 0.25
Cilimus L5 0.21 ? 0.23
Guibga L5 0.27 0.16 0.24
Mabwe-Khoywa L5 0.20 0.16 0.24
Malakal L5 0.26 0.18 0.26
Messina L5 0.26 0.15 0.25
Apt L6 0.27 0.20 0.24
Aumale L6 0.27 0.17 0.24
Karkh L6 0.26 0.15 0.23
Kunashak L6 0.26 0.16 0.24
Kyushu L6 0.29 0.17 0.24
New Concord L6 0.29 0.19 0.23
Farmville H4 0.21 0.12 0.23
Forest Vale H4 0.22 0.15 0.24
Kabo H4 0.20 0.12 0.24
Marilia H4 0.19 0.11 0.24
S?o Jose do Rio Preto H4 0.24 0.13 0.22
Allegan H5 0.21 0.13 0.22
Ehole H5 0.21 0.15 0.24
Itapicuru-Mirim H5 0.21 0.12 0.23
Lost City H5 0.23 0.14 0.24
Primbram H5 0.23 0.14 0.22
Schenectady H4 0.20 0.14 0.22
Uberaba H5 0.25 0.14 0.24
Andura H6 0.16 0.22 0.23
Bustura H6 0.18 0.17 0.23
Canon City H6 0.20 0.15 0.23
Chiang Khan H6 0.20 0.13 0.23
Guarena H6 0.18 0.13 0.23
Ipiranga H6 0.20 0.15 0.23
Errors are the same as those in Table 2.
a Normative cpx/(opx + cpx) ratios are from abundances in McSween et al.
(1991); ? indicates a sample for which norms have not been calculated.and the Flora Family has been suggested as possible source of
the L chondrites (Nesvorny? et al., 2002). In addition, a large portion
of studied Near Earth Asteroids (NEAs) have re?ectance spectra
similar to ordinary chondrites (Thomas and Binzel, 2009). Like
Fig. 2. Current classi?cation for S(IV)-type asteroids modi?ed from Gaffey et al.
(1993) and Gaffey and Gilbert (1998). The curve represents a simple mixing line
between olivine and low-Ca pyroxene (Gaffey et al., 1993). Spectral characteristics
of 48 ordinary chondrites measured in this study are plotted on the diagram. The H,
L, and LL chondrites appear to form linear trends, shown as bolded horizontal lines,
in which each group de?nes a relatively restricted range of Band I center and a
wider range of band area ratio that appears to mirror the criterion (e.g., FeO in ma?c
silicates) originally used to distinguish the chemical groups of ordinary chondrites.
08 (2010) 789?797 793the S(IV) asteroids, ordinary chondrites scatter along a mixing line
between olivine and low-Ca pyroxene when plotted in band area
ratio vs. Band I center space (Fig. 2). This representation suggests
that ordinary chondrites form a nearly continuous sequence be-
tween olivine-rich LL chondrites and relatively pyroxene-rich H
chondrites (Gaffey et al., 1993).
Examination of the ordinary chondrite spectral data from this
study (Fig. 2) suggests that there is an alternative interpretation;
the H, L, and LL chondrites appear to form linear trends, in which
each group de?nes a relatively restricted range of Band I center
and a wider range of band area ratio. We suggest that this distinc-
tion mirrors the criterion (e.g., FeO in ma?c silicates) originally
used to distinguish the chemical groups of ordinary chondrites. Be-
cause BAR is a measure of olivine and pyroxene abundances, it
should be proportional to ol/(ol + px) ratios, a relationship which
was con?rmed earlier in the paper. Given this proportionality,
two meteorites with identical BARs should have, within error,
identical ol/(ol + px) ratios. If they have identical ol/(ol + px) ratios,
Band I center should be controlled almost entirely by the abun-
dance of FeO in olivine and pyroxene. We can test this hypothesis
using the 38 chondrite samples for which both modal abundances
and silicate mineral compositions were analyzed.
The relationship between measured FeO in ma?c minerals and
Band I center is shown in Figs. 3a and 4a. Fig. 3a demonstrates that
a correlation exists between fayalite (Fa) in olivine and Band I cen-
ter, which is best described by a second order polynomial ?t:
Fa ? 1284:9 ?BIC?2 ? 2656:5 ?BIC?  1342:3; ?6?
with a R2 of 0.92. Two L chondrites (Ausson and Bald Mountain)
were not included in this correlation due to their anomalous Band
I centers. As shown in Fig. 3b, Fa values derived from this equation
correlate well with measured Fa, with slight positive offsets for
us 2a
b
794 T.L. Dunn et al. / Icarmost derived H chondrite compositions and very slight negative off-
sets for L and LL chondrites. A similar conclusion is reached for fer-
rosilite (Fs) in pyroxene (Fig. 4a), which correlates with Band I
center by a second order polynomial ?t:
Fs ? 879:1 ?BIC?2 ? 1824:9 ?BIC?  921:7; ?7?
with a slightly lower R2 of 0.91. Again, L chondrites Ausson and Bald
Mountain were not included in this correlation. Like derived Fa val-
ues, derived Fs values (Fig. 4b) show slight positive offsets for H
chondrites and slight negative offsets for L and LL chondrites. How-
ever, because both Fa and Fs in ma?c silicates correlate well with
Band I center, there also appears to be a well-established correlation
between Fa and Fs.
Along with the previously established linear relationship be-
tween BAR and ol/(ol + px), the correlation between ma?c silicate
compositions (Fa, Fs) and Band I center allows us to interpret
S(IV) asteroid spectra not simply in the context of BAR vs. Band I
center, but in the context of FeO vs. ol/(ol + px) ratios ? the same
criteria used to classify ordinary chondrites. Figs. 5a and 6a show
mol% Fa and mol% Fs, respectively, plotted as a function of ol/
(ol + px). Dashed boxes represent the range of XRD-measured ol/
(ol + px) (from this study) and the range of measured Fa and Fs
contents in ordinary chondrites (Brearley and Jones (1998) and ref-
Fig. 3. For the 38 chondrite samples for which both modal abundances and mineral
compositions were analyzed, (a) mol% Fa in olivine plotted as a function of Band I
center demonstrates that a correlation exists between fayalite (Fa) in olivine and
Band I center, and (b) measured vs. derived Fa values correlate reasonably well with
measured Fa, with slight positive offsets for derived H chondrite compositions and
very slight negative offsets for L and LL chondrites. The correlation between Fa and
Band I center allows us to interpret asteroid spectra not simply in the context of
BAR vs. Band I center, but in the context of FeO vs. ol/(ol + px) ratios.a
b
08 (2010) 789?797erences therein). In each plot, the H, L, and LL chondrites are clearly
separated in ma?c silicate composition and exhibit only minimal
overlap in ol/(ol + px). In Figs. 5b and 6b, we plot the spectrally-de-
rived ol/(ol + px) vs. derived Fa or Fs compositions (calculated
using Eqs. (6) and (7), respectively). Dashed boxes are the same
as those in Figs. 5a and 6a. The solid grey boxes represent the least
root mean square of the errors on these spectrally-derived values
(0.03 for ol/(ol + px), 1.3 mol% for Fa, and 1.4 mol% for Fs). Our cal-
ibrations and their associated errors are presented in Table 4.
If we classi?ed the meteorites in this study using only spec-
trally-derived mineral abundances and silicate compositions, we
would correctly classify 86% of H chondrites (12/14), 77% of L chon-
drites (10/14) and 100% of LL chondrites (10/10). If the solid boxes,
which account for the RMS error, are used as parameters for clas-
si?cation instead, all but two the L chondrites outliers could be cor-
rectly classi?ed. However, if these calibrations were applied to
additional samples, the overlap between L and LL chondrites im-
plied by our derived data, and suggested by previous authors based
on mineral chemistries (e.g., Rubin, 1990), may make some L and
LL chondrites dif?cult to classify.
The correct classi?cation of 83% of the ordinary chondrites in
this study indicates that similar success would be expected when
attempting to determine the mineralogies of S(IV) asteroids. This
statement is true, but it is not without caveats. First, these calibra-
tions are only useful for samples with chondrite-like mineralogies
and chemistries. When applying these calibrations to asteroid
Fig. 4. As with Fa in olivine, (a) mol% Fs in orthopyroxene plotted as a function of
Band I center demonstrates that a correlation exist between Fs in pyroxene and
Band I center, and (b) derived Fs values correlate well with measured values, with
slight offsets in the Hs and LLs. Because of the correlations between Fa and Band I
and Fs and Band I, there also appears to be a good correlation between Fa and Fs.
us 2a
T.L. Dunn et al. / Icarspectra, one must ?rst be con?dent that the asteroid in question is
an S(IV) type (and a likely chondrite parent body). The S(IV) types
can be distinguished from other S-type asteroids on the correla-
tions between spectra slope, band depth, and albedo (Gaffey
et al., 1993). In addition, probability models for determining aster-
oid class and source region (Thomas and Binzel, 2009) have re-
cently been developed that may improve the classi?cation of
asteroids, particularly in a large sample group. Secondly, it is also
important to consider the possible effects of space weathering pro-
cesses (Clark et al., 2003). Though asteroid spectra become redder
as a result of space weathering, laboratory studies have shown that
spectral parameters (i.e. band depth) remain unchanged (Marchie
et al., 2005; Sasaki et al., 2001); therefore, interpreted mineralogies
and compositions should not be in?uenced by space weathering
(Vernazza et al., 2008).
Moving beyond simple classi?cation, our calibrations may also
allow us to determine the geologic histories of some S-type aster-
oids, speci?cally whether they are unaltered (ordinary chondrite)
or have experienced partial melting (primitive achondrite). One
parameter used to decipher this history is spectrally-derived FeO
content, where FeO-rich pyroxene is interpreted as an indicator
b
Fig. 5. (a) Mol% Fa plotted as a function of ol/(ol + px) in the 38 chondrite samples
for which both modal abundances and mineral compositions were analyzed, and (b)
spectrally-derived Fa compositions vs. derived ol/(ol + px). Dashed boxes represent
the range of XRD-measured ol/(ol + px) (from this study) and the range of measured
Fa contents in ordinary chondrites (Brearley and Jones (1998) and references
therein). The solid boxes include the least square root mean of the errors on these
spectrally-derived values (0.03 for ol/(ol + px) and 1.3 mol% for Fa). Based on
derived Fa content and ol/(ol + px), we can correctly classify 86% of H chondrites,
71% of L chondrites, and 100% of LL chondrites.a
08 (2010) 789?797 795of partial melting and chondrite-like FeO compositions are sugges-
tive of a primitive asteroid. For this parameter to be useful, derived
Fs compositions must be accurate. However, calibrations used to
determine Fs values in asteroids (Gaffey et al., 2002; Gaffey,
2007) have been shown to yield higher than measured Fs compo-
sitions when applied to mixtures of ordinary chondrite-like com-
positions (McCoy et al., 2007), and their limitations when applied
to spectra of ordinary chondrite-like compositions has been
acknowledged (Gaffey, 2009). Because our calibrations were de-
rived entirely from ordinary chondrites, they should provide more
accurate Fs compositions and may provide some insight into the
partial melting debate. One example is the S-type asteroid Itokawa,
which has identi?ed as a possible partial melt based on FeO con-
tent (Fs43?5) (Abell et al., 2007) and as an LL-chondrite parent body
based on spectral parameters (Abe et al., 2006). Using spectral
parameters measured by Thomas and Binzel (2009), we derived
an Fs composition of Fs25, which falls into the range of ordinary
chondrite FeO values and suggests that partial melting did not
occur. When derived ol/ol + px ratios are plotted along with Fs
compositions, Itokawa plots in the range de?ned by the LL
chondrites.
b
Fig. 6. (a) Mol% Fs plotted as a function of ol/(ol + px) in the 38 chondrite samples
for which both modal abundances and mineral compositions were analyzed, and (b)
spectrally-derived Fs compositions vs. derived ol/(ol + px). Dashed boxes represent
the range of XRD-measured ol/(ol + px) (from this study) and the range of measured
Fs contents in ordinary chondrites (Brearley and Jones (1998) and references
therein). The solid boxes include the least square root mean of the errors on these
spectrally-derived values (0.03 for ol/(ol + px) and 1.4 mol% for Fs.) Based on
derived Fs content and ol/(ol + px), we can correctly classify 86% of H chondrites,
71% of L chondrites, and 100% of LL chondrites.
tral features (e.g., Band I and II centers, band area ratios) in both
these tools, we can move beyond band parameters to directly dis-
us 2cussing asteroid and meteorite properties using a common lan-
guage of mineral abundance and composition.
Acknowledgments
We would like to thank Gordon Cressey, at the Natural History
Museum in London, for his assistance in the collection and inter-
pretation of XRD modal data, and Takahiro Hiroi, at Brown Univer-
sity?s Keck/NASA Re?ectance Experiment Laboratory (RELAB), for
collecting VNIR spectra of several samples examined in this study.
Thanks also to the Smithsonian Institution and the Natural History
Museum for providing powder samples for modal and spectral
analysis. We would also like to thank Tom Burbine and Vishnu
Reddy for their helpful reviews, which greatly improved this man-
uscript. This work was supported by NASA through cosmochemis-
try Grant NNG06GG36G to H.Y.M. and planetary geology and
geophysics Grant NNX06AH69G to J.M.S.
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