Microchemical Journal 121 (2015) 157–162
Contents lists available at ScienceDirect
Microchemical Journal
j ourna l homepage: www.e lsev ie r .com/ locate /mic rocIdentification of pyroxene minerals used as black pigments in painted
human bones excavated in Northern Patagonia by Raman spectroscopy
and XRDEugenia P. Tomasini a, Cristian M. Favier Dubois b, Nicole C. Little c, Silvia A. Centeno d,⁎, Marta S. Maier a
a UMYMFOR-CONICET and Departamento de Química Orgánica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pabellón 2, Ciudad Universitaria (C1428EGA), Ciudad
Autónoma de Buenos Aires, Argentina
b INCUAPA-CONICET and Departamento de Arqueología, Facultad de Ciencias Sociales, UNCPBA, Av. del Valle 5737 (B7400JWI), Olavarría, Argentina
c Smithsonian Institution, Museum Conservation Institute, Museum Support Center, 4210 Silver Hill Road, Suitland, MD 20746, USA
d The Metropolitan Museum of Art, Department of Scientific Research, 1000 Fifth Avenue, New York, NY 10028, USA⁎ Corresponding author. Tel.: +1 212 650 2114; fax: +
E-mail addresses: eugeniatomasini@gmail.com (E.P. To
(C.M. Favier Dubois), silvia.centeno@metmuseum.org (S.A
maier@qo.fcen.uba.ar (M.S. Maier).
http://dx.doi.org/10.1016/j.microc.2015.03.003
0026-265X/© 2015 Elsevier B.V. All rights reserved.a b s t r a c ta r t i c l e i n f oArticle history:
Received 4 March 2015
Received in revised form 9 March 2015
Accepted 9 March 2015
Available online 17 March 2015
Keywords:
Raman microspectroscopy
Micro-X-ray diffraction
Pyroxenes
Painted bones
Iron oxidesThe skeletal remains of seven individuals were excavated in a secondary burial context in the site of Cima de los
Huesos, in the San Matías Gulf (Río Negro, Argentina). AMS dating of two samples for this site to 1173 ± 45 and
1225 ± 47 years BP make it one of the earliest burials of its kind uncovered so far in the Patagonian region.
Among the findings, the skeleton of a male painted with parallel lines alternating red and black colors was
uncovered. SEM-EDS elemental analysis of microsamples removed from the red and the black pigments showed
the presence of Mn and Fe as the main components, respectively. Raman microspectroscopy combined with
micro-X-ray diffraction analysis showed that the red pigment contains hematite and that the black pigment is
composed of members of the pyroxene mineral group, ferrosilite (FeSiO3) and enstatite (MgSiO3) along with
kanoite (MnMgSi2O6). This is, to our knowledge, the first report on the use of pyroxenes as black pigments to
decorate human remains or archeological artifacts in South America. No organic compounds that could
have been used as binders for the paints were detected by FTIR-ATR. Contamination due to quartz and
aluminosilicates, mainly microcline and albite, from the burial environment did not allow determining whether
claymineralswere used in the paints as binders and/or extenders. Themultitechnique approach usedwas crucial
to overcome the limitations of the individual techniques to firmly identify Mn-containing black pigments.
© 2015 Elsevier B.V. All rights reserved.1. Introduction
The occupation by hunter–gatherer groups of the San Matías Gulf
coast, in the Argentinean province of Río Negro (Patagonia), dates back
to about 6000 years ago. During the period, primary burials in which
all the bones of skeletons were placed in an anatomical articulated
position were the most common. However, since about 1200 years
ago, another mortuary practice called secondary burial emerged. In
this case, the skeletons were unearthed by members of the social
group and reburied in a different place, arranging the bones in particu-
larways. During the excavation of the Cima de los Huesos site, in the Bajo
de la Quinta locality (BQ-CH) (Fig. 1a), the human remains of at least
seven individuals, two sub-adults (with indeterminate sex) and five
adults (two males, two females and one with indeterminate sex),
were discovered. In this burial, long bones were placed parallel,1 212 396 5060.
masini), cfavier3@gmail.com
. Centeno),oriented predominantly East–West (Fig. 1b). Among these remains,
the bones of an adult male, including all the long bones, the ribs, tarsal
bones, pelvis, and skull, were found to be carefully painted. In the long
bones, alternating red and black lines approximately 3 mm wide are
observed, while more irregular lines are found in the ribs (Fig. 2).
The BQ-CH site lies in a low terrain covered by sand dunes. A tempo-
rary freshwater pool that developed in the area must have attracted the
human populations which inhabited the semiarid coast of the Northern
Patagonia in the past. This is evidenced by the fact that several human
burials, along with diverse archeological artifacts have been uncovered
by archeologists in the area [1]. The human bones examined and
analyzed in this study correspond to a unique secondary burial, located
on a Pleistocene marine terrace, about 40 m above the sea level and
about 1000 m from the sea. Two samples (a skull fragment and a
tooth) corresponding to different individuals from the site were dated
by AMS to 1173 ± 45 and 1225 ± 47 years BP [2], respectively. There-
fore, this is one of the earliest burials of its kind so far uncovered in
the Patagonian region.
Bones painted with red and black pigments were previously discov-
ered in San Blas, Province of Buenos Aires [3], although no chronological
Fig. 1. a) Location of the Bajo de la Quinta locality, in the Northern Patagonian Atlantic
coast. b) Photograph of the ground excavation of BQ-CH, where it can be observed the
arrangement East–West of the long bones and the fragmented state of the material (the
red arrow indicates the North). (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
Fig. 2. a) Ribs painted with alternating red and black lines. b) Photomicrograph of a rib
surface taken with a stereomicroscope. (For interpretation of the references to color in
this figure legend, the reader is referred to the web version of this article.)
158 E.P. Tomasini et al. / Microchemical Journal 121 (2015) 157–162information is available for this finding. In secondary burials of the
Pampa and Patagonia regions, the most frequent practice consisted in
sprinkling pigments or applying a uniform paint film over the bones.
Therefore, the discovery of alternating colored lines carefully adorning
all the bones of a single individual in the Cima de los Huesos burial is
unusual, and suggests that the deceased had a very special status
among the individuals of the group.
Most prehistoric paintings found in rock art sites, decorated
archeological ceramics and, as in this case, bone remains in ritual
contexts, contain red and black pigments as the main colorants [4–7].
Physicochemical analyses have shown that, in their great majority, the
red paints in these artifacts are based on Fe oxides/hydroxides, and
that the black pigments are either carbon-based, such as charcoal, or
Mn oxides/hydroxides [4–15]. Raman spectroscopy is awell established
and powerful technique for the non-invasive identification of materials
in cultural heritage objects. However the variety of oxidation states and
polymorphic forms ofMn oxides andhydroxides presents a challenge to
their firm identification [16–19]. These and other difficulties involved in
the identification of Mn-based black pigments by Raman spectroscopy
have been previously discussed [9,12,15,19]. Nevertheless, the combi-
nation of vibrational spectroscopic techniques and X-ray diffraction
has proved to be a powerful methodology to unambiguously identify
Mn-based black pigments [12,15,17]. In this work, Raman spectroscopy,X-ray diffraction, FTIR spectroscopy, and SEM-EDS elemental analysis
were used to identify the components of the red and black paints.
Our study aims to shed light onto burial practices of ancient hunter–
gatherer groups living in the Argentinean Patagonia.
2. Experimental
2.1. Painted bone samples
The skeletal remains examined in the present study are in a
fragmentary state. Microscopic paint samples were removed from rib
fragments by scrapping them with a stainless steel scalpel, and also
some bone fragments were non-invasively analyzed in situ, without
removing any samples.
2.2. Instrumentation
The samples were observed under a Leica MZ6 stereomicroscope
and photographed with a Canon S50 camera. A Carl Zeiss Axio Imager
equipped with Z2m sources in visible and ultraviolet light polarized
normal modes were used. Images were taken with an AxioCam HRc
camera using the Axio Vision software for acquisition and processing.
Information on the surface morphology of the samples and elemen-
tal composition were obtained using a field environmental scanning
electron microscope (FE-SEM) Zeiss:Supra 40 coupled with an energy-
Fig. 3. Raman spectra representative of those acquired in a sample of the red paint: in red
pigment particles (spectrum a) and in white particles in the same sample (spectrum b),
respectively.
159E.P. Tomasini et al. / Microchemical Journal 121 (2015) 157–162dispersive X-ray spectrometer (SEM-EDS) INCA X Sight (Oxford
Instruments).
Raman microspectroscopy analyses were carried out with a
Renishaw System 1000 configured with a Leica DM LM microscope,
notch filters, and a thermoelectrically cooled charged-coupled device
(CCD) detector. A 785 nm laser line was used for excitation and was
focused on different areas of the sample fragments using the 50×
objective lens of themicroscope attached to the spectrometer, achieving
a lateral resolution of ~2–3 μm. In order to avoid changes of the sample
materials due to overheating, neutral density filters were used to set
the laser power at the sample to values between 0.2 and 1.0 mW.
A 1200 lines/mm grating was used and integration times were set
between 10 and 300 s. The wavenumber stability and the accuracy
were checked by recording the Raman spectrum of a silicon wafer
(520 cm−1).
Micro-X-ray diffraction (XRD) patterns were obtained using a
Rigaku D/Max Rapid Diffractometer. Microscopic sample scrapings
weremounted on glass fibers using Elmer's® glue and the instrumental
parameters usedwere as follows: Cu Kα (λ=1.542 Å) radiation (50 kV
accelerating voltage and 40mA current), chi (χ) fixed at 45°, omega (ω)
fixed at 0°, and phi (φ) spun from 60° to 120° at 1°/s, 0.8mmcollimator,
10 min exposure time.
FTIR-ATR spectra were obtained using a Nicolet iS50 FTIR spectrom-
eter with a diamond single-bounce ATR crystal. For each reference
sample, 64 scans were recorded in the 4000–400 cm−1 spectral range
in the reflectance mode with a 4 cm−1 resolution. Spectral data were
collected with the Omnic v9.2 (Thermo Electron Corp.) software with-
out post-run processing. The spectrum of air was used as background.
3. Results and discussion
The painted bones analyzed in the present study are shown in Fig. 2a
and photomicrographs of the bone areas with red and black paints are
presented in Fig. 2b. Table 1 shows the results of the SEM-EDS analysis
carried out of the surface of red and black paints in bone fragments
and in an unpainted bone area. Mn was found to be present in the
black paint only, while Fe was identified both in the red and black
paints, in relatively larger amounts in the former. All samples showed
Ca and P from the bone hydroxyapatite along with relatively large
amounts of Al and Si, together with minor amounts of K, Mg, and Na,
that indicate the presence of silicates.
Raman analysis performed on sample scrapings of the red paint
gave peaks characteristic of hematite (α-Fe2O3) at ca. 223, 291, 408
and 610 cm−1 [7] (Fig. 3 spectrum a), consistently with the results
of the elemental analysis. In many of the spots analyzed, bands at
ca. 960 cm−1 that are characteristic of the P–O symmetrical stretching
were observed, along with features due to carbonate present in
hydroxyapatite at ca. 281, 712, and 1086 cm−1 (Fig. 3, spectrum b) [20].
The main bands observed in the Raman spectra acquired on sample
scrapings of the black paint are consistent with the presence ofTable 1
Results from the SEM-EDS elemental analysis of the red and black paints expressed as % of
the total atomic content.
Element Unpainted
bone
Black
paint
Red
paint
Fe 0.2 1.8 6.9
Mn – 0.8 –
Ca 5.5 5.6 6.2
P 3.3 2.8 3.3
Mg 0.5 0.8 0.6
Al 1.0 2.3 1.9
Si 2.4 6.6 5.8
Na 0.3 0.4 0.4
K 0.1 0.3 0.3
C 29.3 16.9 17.5
O 57.5 61.8 59.1pyroxenes, with characteristic bands below 700 cm−1 and above
1000 cm−1 (Fig. 4) [21]. Minerals of the pyroxenes group (MSiO3)
are one of the most abundant rock-forming minerals on Earth. The
spectrum presented in Fig. 4a corresponds to a member of the
enstatite–ferrosilite (En–Fs) series [21–23]. Ferrosilite (FeSiO3) forms
a solid solution with enstatite (MgSiO3) and the positions of the
Raman bands for this solution depend on the Fe content, which partially
replaces the sites occupied by the Mg atoms, whose orthorhombic
structure includes it in the orthopyroxenes group [21,22]. Raman spec-
tra recorded in other particles of the black paint revealed the presence
of kanoite (MnMgSi2O6) (Fig. 4 b), a clinopyroxene with a monoclinic
structure [24,25]. Although kanoite is a mineral found in different
geographical areas, scant spectroscopic information has been published
about it [26,27]. The Raman spectra of pyroxenes share general features
according to Wang et al. [21]. However, differences in symmetries be-
tween orthopyroxenes and clinopyroxenes make the number, position,
and relative intensities of the peaks in each spectral range different,
allowing to identify them. For example, in the range between 800 and
600 cm−1, orthopyroxenes usually present a strong doublet, and for
clinopyroxenes, such as kanoite, an asymmetric single peak near
670 cm−1 is expected instead. In Table 2, the assignments of the various
Raman spectral ranges for En–Fs and for kanoite are presented. XRDpat-
terns obtained for different microsamples of the black paint matchFig. 4. Raman spectra representative of those recorded in different particles in the black
paint: black particles (a and b) and white particle (c), respectively.
Table 2
Tentative assignments of the Raman bands observed for the pyroxenes En–Fs and kanoite
based in Wang et al. [22] (*) Fig. 4a, (+) Fig. 4b.
ν (cm−1)
En–Fs⁎
ν (cm−1)
kanoite+
Assignments
231 231 M–O strech/bend
335 325 M–O strech/bend
389 393
531 531 O–Si–O bend
570 563
657sh 666 Si–O–Si stretch
677
942 930 Si–O strech
1004 1010
160 E.P. Tomasini et al. / Microchemical Journal 121 (2015) 157–162reference patterns of ferrosilite and estantite (Fig. 5a), and of kanoite
(Fig. 5 b), respectively, however no reference patterns for the solid solu-
tions En–Fs were available in our libraries.
It is important to note that some bands in the Raman spectra
acquired for the black paintmatch those previously reported formanga-
nese oxides such as Mn2O3 [18]. These comprise features in the 300–Fig. 5.Micro-XRD patterns representative of those acquired in samples removed from the black
Peaks that do not fit to the patterns of the minerals indicated cannot be assigned with certaint350, 500–550, and 650–700 cm−1 ranges [16–19]. However, bands at
ca. 1000–1050, 800–600, and 450–300 cm−1 have not been reported
for manganese oxides, so our results are strongly suggesting the pres-
ence of pyroxenes as discussed above. No Mn oxides were identified
by XRD analysis, however these compounds can also be amorphous
[28].
Raman spectra recorded from white particles present in the black
paint samples showed the presence of quartz and the feldspars micro-
cline (KAlSi3O8) and albite (NaAlSi3O8) (Fig. 4, spectrum c) [29]. The
presence of aluminosilicates is also inferred from the results of the
SEM-EDS elemental analysis for all the samples, including the unpainted
bone (Table 1). These compounds are most likely due to contamination
from the burial environment.
FTIR-ATRmeasurements performed in-situ onpainted andunpainted
areas of the bone fragments showed bands due to hydroxyapatite, with
PO4−3 characteristic frequencies at ca. 1087, 962, 599, 557 and 468 cm−1
[30,31], and CO3−2 modes at ca. 1418, 873, and 713 cm−1 (Fig. 6, spectra
a, b and c). These results are consistentwith those of the Raman analysis
discussed above. Similar featureswere observed in the spectra acquired
in a sample removed from the inner zone of the bone (Fig. 6, spectrum
d), strongly suggesting that the presence of CO3−2 observed in thepaint in the bones, showing the presence of ferrosilite and enstatite (a) and of kanoite (b).
y.
Fig. 6. FTIR-ATR spectra acquired in situ in the black paint (a), in the red paint (b), in a
bone surface area with no paint visible (c), and in a spot in the interior of the bone (d),
respectively.
161E.P. Tomasini et al. / Microchemical Journal 121 (2015) 157–162painted areas by FTIR is most likely due to the replacement of PO4−3 in
bone [31,32], and not to the use of calcite mixed in the paint or applied
in a preparatory layer.
In the FTIR-ATR spectra recorded on painted and unpainted areas on
the outer surface of the bone, the most intense band occurs near
1007 cm−1 together with two very weak doublets around 780–800
and 420–460 cm−1, respectively, which are assigned to feldspars [33],
and that were also observed by Raman. These bands are not observed
in the spectra recorded in the samples removed from the inner zone
of the bone.
All the samples analyzed by FTIR-ATR showed a weak band around
1636 cm−1 and a group of weaker features in the 1520–1560 cm−1
range. The first band could be assigned to the C_O stretching (Amide
I) and the second to the N–H deformation (Amide II) originating in
residues of collagen from the bone [31]. The FTIR-ATR spectra show no
bands due to the red or black pigments in the paints. It is possible that
the bands due to feldspars and hydroxyapatite overlap the bands
expected for ferrosilite [34], in the ranges around 735 and 1010 cm−1,
and at ca. 950 and 875 cm−1. No bands that could be assigned to organic
compounds used as binders in the paints were detected in the FTIR
spectra either.4. Conclusions
This is, to our knowledge, the first report on the use of black
pigments containing the pyroxene minerals enstatite, ferrosilite, and
kanoite in South American painted bones, most likely because in most
studies carried out previously only elemental analyses were performed.
Therefore, thiswork highlights the importance of the use of complemen-
tary microanalytical techniques to properly identify Mn-containing
minerals in archeological black paints. The Raman spectra obtained
can be used as references to non-invasively identify the compounds in
objects of cultural significance.
Even though the possible use of clays has been previously reported
as binders and/or extenders for paints in some archeological objects,
the results presented above show the challenges presented when the
artifacts are contaminated by the soil in the burial environment.
As the correct identification of minerals allows researchers to locate
the potential sources and learn about the artistic techniques used in
their application, our study provides new information on this particular
burial practice of hunter–gatherer groups in the Argentinean Patagonia
about 1200 years ago.Acknowledgments
The authors are indebted to the Consejo Nacional de Investigaciones
Científicas y Técnicas (CONICET), Agencia Nacional de Promoción
Científica y Tecnológica (ANPCyT), and the University of Buenos Aires
(008BA), Argentina, for financial support. E.P.T. thanks CONICET
(11220130100288CO) and Fundación Bunge y Born for a Postdoctoral
Fellowship; C.M.F.D., E.P.T. and M.S.M. are research members of
CONICET.
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