Near-infrared spectra of clinopyroxenes:
Effects of calcium content and crystal structure
Rachel L. Klima
Johns Hopkins University, Applied Physics Laboratory
Laurel, MD 20723
M. Darby Dyar
Department of Astronomy, Mount Holyoke College
South Hadley, MA 01075, USA
Carlé M. Pieters
Department of Geological Sciences, Brown University
Providence, Rhode Island 02912, USA
Submitted to Meteoritics and Planetary Science on
May 3, 2010
Number of Figures: 14 (3 color?)
Number of Tables: 1
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Abstract
Pyroxenes are among the most common minerals in the solar system and are ideally
suited for remote geochemical analysis because of the sensitivity of their distinctive spectra to
mineral composition. Spectral features arising from pyroxene have long been recognized in
remote telescopic and orbital data. Fe2+ is responsible for the dominant pyroxene absorption
bands in the visible and near-infrared. However, other cations such as Ca2+have drastic effects
on a pyroxene spectrum by changing the lattice parameters of its crystal structure and thus the
crystal field splitting energies of the Fe cations. To define spectral systematics resulting from the
major pyroxene cations (Ca2+, Mg2+, and Fe2+), we focus here on a suite of pyroxenes
synthesized with only Ca2+, Mg2+, and Fe2+ in the two octahedral cation sites, allowing us to
specifically examine the effect of Ca2+ on pyroxene absorption bands. The Modified Gaussian
Model is used as a tool to deconvolve pyroxene spectra into their component bands that can then
be linked directly to crystal field absorptions. In orthopyroxenes and low-Ca clinopyroxenes,
Ca2+-content has a strong and predictable effect on the positions of the crystal field bands. When
the lattice reaches a threshold around Wo30, however, the crystal field environment stagnates and
the M2 crystal field bands cease to change significantly as more Ca2+ is added. At Wo50, when
almost all of the M2 sites are filled by Ca2+, band positions do not change drastically, though the
presence and strengths of the 1 and 2 m bands are affected by even trace amounts of Fe2+ in the
M2 site. It is thus apparent that next nearest neighbors and the distortions they impose on the
pyroxene lattice affect the electronic states around the Fe2+ cations and control absorption band
properties.
1. Introduction
Characterizing the specific pyroxene mineralogy of planets and asteroids allows us to
address questions about the magmatic evolution and cooling history of those bodies. Pyroxene
spectra are in most cases easily distinguished from other minerals, and band positions are wellknown to vary in response to total iron and calcium content. Laboratory studies of natural
pyroxenes have provided a background for near-infrared spectral analysis (e.g., Adams, 1974;
Hazen et al., 1978; Rossman, 1980; Cloutis and Gaffey, 1991; Burns, 1993; Sunshine and
Pieters, 1993). However, our understanding of the crystal chemical constraints on the diversity
of pyroxene spectra is inherently limited because most pyroxenes that have been measured and
well-characterized in the literature are necessarily terrestrial and often contain exsolution,
inversion, zoning, and/or significant amounts of non-quadrilateral components. In this work, we
build on our analyses of synthetic Ca-free orthopyroxenes (Klima et al., 2007) by investigating
synthetic clinopyroxenes with compositions spanning most of the Ca-Fe-Mg pyroxene
quadrilateral. The presence of Ca2 in the M2 sites means that distortions are imposed on the
next-nearest neighbor Fe2+ (and Mg2+) cations in the M1 site. These distortions in turn change
the crystal field splitting energies, as observed in Ca-bearing clinopyroxenes (e.g., Adams, 1974;
Hazen et al., 1978; Cloutis and Gaffey, 1991). Furthermore, Ca-bearing clinopyroxenes are not
restricted to one space group, but rather generally take the form of P21/c pyroxene (e.g.,
pigeonite) at low Ca2+content, and C2/c (e.g., augite) at higher Ca2+ content. We here explore
the effects of structure and bulk composition on near-infrared spectra of the synthetic pyroxenes
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to refine the amount of crystal chemical and compositional information that can be derived from
the spectra.
2. Background
2.1. Clinopyroxene Structure and Composition
Clinopyroxenes encompass a broad range of compositions within the pyroxene mineral
group. We here focus strictly on the Mg2+-Fe2+-Ca2+ quadrilateral clinopyroxenes, without
additional cations such as Al3+, Cr3+, Mn2+, Na+, etc. All clinopyroxenes can be broadly grouped
as monoclinic, but depending on the bulk composition, temperature, and pressure of formation
they may belong to either the P21/c or C2/c space groups (Fig. 1). Augite
((Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6) and the Ca2+-saturated end-members diopside (CaMgSi2O6)
and hedenbergite (CaFeSi2O6) belong to the C2/c space group. Pigeonite
((Mg,Fe2+,Ca)(Mg,Fe2+)Si2O6), which ranges in composition from about Wo5-Wo15, exhibits
C2/c symmetry at high temperatures (high pigeonite), but on cooling generally transforms to the
P21/c space group (low pigeonite). In this work, we will use the term pigeonite to describe
pyroxenes with a P21/c structure. The term augite will be used to refer to pyroxenes exhibiting a
C2/c structure. Because of the differences in structure, there is only full solid solution between
augites and high pigeonites, and natural pyroxenes are often exsolved or inverted at some scale.
All pyroxenes are composed of infinite chains of edge-sharing M1 octahedra in the c
direction, flanked by 6- or 8-coordinated M2 sites. These are linked to parallel infinite chains of
corner-sharing octahedra. The P21/c and C2/c space groups of interest in this study differ largely
because of the size of cations in the M2 site, which causes kinking and rotation of the Si-O
chains that are linked to them (Fig. 2). In augites, the edges of the tetrahedra that share corners
with other tetrahedra line up nearly straight, but in pigeonites the angles between linked Si
tetrahedral are kinked (Cameron and Papike, 1980). The alternating chain structure of
pigeonites results in site geometries for M1 and M2 that are somewhat similar to those observed
in orthopyroxenes (Fig. 1). Because crystal field splitting is directly a result of the ligand field
surrounding the Fe2+ cations (or the oxygen arrangement), pigeonites and orthopyroxenes should
exhibit spectra that are more similar than augites and orthopyroxenes.
2.2. Spectroscopy of Clinopyroxenes
Spin-allowed crystal field transitions take place when transition metal cations reside in a
ligand field (in the case of pyroxenes, this is an octahedral framework of closely-packed oxygen
atoms). For pyroxenes, the ligands surrounding the M1 and M2 cation sites produce an
asymmetric electrostatic field, splitting the energy levels of d-orbital electrons of transition
metals within these sites (Fig. 3). An incident photon may be absorbed at certain wavelengths,
exciting an electron from a lower to a higher energy state (Burns, 1993). If the resultant
electronic transition is spin-allowed, it produces a prominent absorption feature such as the
familiar 1 and 2 m Fe2+ pyroxene bands (Adams, 1974; Cloutis and Gaffey, 1991), which result
from transitions made possible by splitting within the eg and t2g orbitals around the M1 and M2
Fe cations. The relative intensities of such absorptions increase with the asymmetry of the
octahedral site (Burns, 1993). Thus, the asymmetric M2 site in pyroxenes (cf. Fig. 2) results in
the relatively strong 1 and 2 m absorptions, while the more symmetric M1 site produces much
weaker absorptions near 1 and 1.2 m. The energy of the crystal field splitting and the resulting
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band positions are a function of the ligand field, which changes in response to the structure and
composition of the pyroxene.
Previous studies of 1 and 2 m crystal field band positions suggest that no distinct
transitions are crossed, and that there is a smooth variation in pyroxene spectra across all
structures and compositions (Adams, 1974; Cloutis and Gaffey, 1991). In clinopyroxenes,
however, structural changes from the more orthopyroxene-like P21/c pigeonite structure to the
C2/c augite structure should, in principle, result in changes in the ligand field that are manifested
as spectral variations (Fig. 1).
Pyroxenes that are near Ca2+-saturated exhibit drastically different spectra based on how
much Fe2+ is present in the M2 site, and have been classified into two spectral groups (Adams,
1975; Cloutis and Gaffey, 1991). Type A pyroxenes exhibit two bands near 1 m, and no 2 m
band, while type B pyroxenes exhibit spectra that are generally similar to Ca2+-undersaturated
pyroxenes, with strong 1 and 2 m bands. The type A pyroxenes, which exhibit only bands
assigned to Fe2+ in the M1 site, are saturated with Ca2+ or other cations other than Fe2+ in the M2
site. In a comprehensive study of terrestrial high Ca2+ (>Wo45) pyroxenes, Schade et al (2004)
noted that most of the naturally-occurring type A pyroxenes contain Mn or Fe3+ in addition to the
quadrilateral cations.
3. Methods
3.1. Pyroxene Synthesis
The pyroxenes used in this study were synthesized by Don Lindsley and colleagues
between 1972 and 2007 using the procedures detailed in Turnock et al. (1973). All samples were
prepared from reagent grade chemicals and the oxygen fugacity was buffered at the iron-wüstite
curve. Specific synthesis methods varied by composition, and were chosen to prevent nucleation
of pyroxenoids and produce a single, homogeneous pyroxene. Pyroxene compositions were
validated by Lindsley and colleagues using X-ray diffraction.
The synthesis procedures produce pyroxenes that are fine powders, with individual grains
approximately 15-25 m in diameter. Many of the powders formed clumps of grains, which were
crushed manually and sieved to <45 m and >45 m grain size fractions for further analysis.
The <45 m size fraction was used for Vis/NIR and Mössbauer spectroscopy, and, when
available, the >45 m size fraction was used to prepare grain mounts for electron microprobe
analysis. Between 12-41 mg of each pyroxene was mixed with sugar, ground under acetone and
mounted for collection of Mössbauer spectra.
3.2. Compositional and fO2 Analysis
Pyroxene composition and homogeneity were investigated using the CAMECA SX-100
electron probe microanalyzer (EPMA) at Brown University. Electron backscatter was used to
identify samples with unreacted starting oxides or excess mineral phases. No starting oxides
were detected in any samples, though one sample contained trace amounts of fayalite. A number
of mid-quadrilateral through high-Ca2+ samples contain a small amount of interstitial material
that was found to be high in iron and sometimes calcium (Table 1). The bulk composition of this
phase is indistinguishable from that of a pyroxene, but based on its texture, it is likely to be a
quench glass. Minor (generally ≤1%) glass was reported for several of the samples previously
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described by Turnock et al. (1973). Though the glass is negligible in most cases, it has a modest
effect on the spectra of most of the Ca2+-rich pyroxenes due to their generally high albedos (none
was detected for either hedenbergite sample).
The compositions of the pyroxenes as measured by EPMA are presented on a pyroxene
quadrilateral in Fig. 4 and listed in Table 1. The light-grey region on the quadrilateral indicates a
region in which pyroxenes were synthesized but the resulting crystals contained two phases of
pyroxene. These samples have been excluded from this work but are described and modeled in
Klima (2008). The corners of the quadrilateral are the pyroxene end-members diopside (Di),
hedenbergite (Hd), Enstatite (En) and Ferrosillite (Fs). Wollastonite (Ca2Si2O6, or Wo) would lie
at the top of the ternary plot, but compositions with Ca>2 pfu per Si2O6 are not true pyroxenes,
but pyroxenoids. Some of the probed compositions are several percent less FeO-rich than the
intended compositions, due to loss of FeO to the capsule or to a vapor during synthesis or to the
interstitial glass. The samples that were furthest from their intended compositions, and therefore
most likely to have their spectra affected by the glass contamination, are highlighted in Table 1.
Note that a number of the pyroxenes fall within the region of the pyroxene quadrilateral often
referred to as the ‘forbidden zone’ as pyroxenes of that composition are metastable with respect
to silica, olivine and augite at ambient atmospheric pressures and temperatures (Lindsley, 1983).
However, the pyroxenes in that region of the quadrilateral were synthesized at up to 22.5 kbar in
order to be well within the pyroxene stability field during the experiments.
Mössbauer spectra were analyzed to ensure that samples were free of Fe3+ and to assess
the proportion of Fe2+ in the M1 and M2 sites. Iron-bearing glass, however, cannot be easily
differentiated from pyroxene using Mössbauer spectra due to similarities in doublet parameters
(Dyar, 1985). Mössbauer spectra were collected at Mount Holyoke College using a WEB Co.
model W100 spectrometer with a 45 mCi 57Co in Rh source. Spectra were calibrated against an
α-Fe foil of 6 m thickness and 99% purity. Data were modeled using an in-house program from
the University of Ghent, in Belgium. The Dist3e program models spectra using quadrupole
splitting or hyperfine field distributions for which the subspectra are constituted by Lorentzianshaped lines; it uses velocity approximations rather than solving the full Hamiltonian. Errors on
isomer shift and quadrupole splitting are ±0.02 mm/s, and errors on peak areas are ±<1%
absolute. The spectra and raw data for all of the Mössbauer spectra can be found on the Mount
Holyoke College Mars Mineral Spectroscopy Database
(http://www.mtholyoke.edu/courses/mdyar/marsmins/). Fits to these data will be reported in a
work in preparation.
None of the pyroxenes was found to contain Fe3+ or additional iron-bearing crystalline
phases (magnetite, wüstite, olivine, etc). Based on these Mossbauer data and related petrological
literature on pyroxene order/disorder (e.g. Wang et al., 2005), it is apparent that ordering of Fe
and Mg between the two sites is a fundamental issue that needs to be considered, but it is so
complicated that it is beyond the scope of this paper. Site populations for the M1 and M2 sites
and the relationship to reflectance spectra are the subject of ongoing and future work.
3.3. Spectral Methods
Visible and near-infrared spectra were collected using the Bidirectional Reflectance
Spectrometer (BDR) at the NASA/Keck Reflectance Experiment Lab (RELAB) at Brown
University. Spectra of the <45 m grain size fraction were measured relative to a halon
reference standard at 5 nm intervals over the wavelength range of 0.3-2.6 m and corrected for
the small features of halon. An incidence angle of 30° and an emission angle of 0° were selected
5
for bidirectional measurements. High resolution (0.5 nm interval) spectra were also collected in
the visible between 0.4-0.6 m.
Spin-allowed crystal field bands were modeled using the Modified Gaussian Model
(MGM). The MGM deconvolves a spectrum into a continuum and a series of modified Gaussian
curves that can be attributed to specific absorptions (Sunshine et al., 1990; Sunshine et al., 1999).
It also allows individual pyroxene phases to be deconvolved from a composite spectrum
(Sunshine and Pieters, 1993). The MGM fits spectra using the natural log of reflectance instead
of reflectance to better approximate absorbance.
For this study, pyroxene spectra were modeled with a straight line continuum plus 2-3
broad bands in the visible to account for the Fe2+-O charge transfer band edge, and, in some
cases, a band near 3 m to model out water adsorbed on the sample. In theory, two spin-allowed
crystal field bands should occur for each of the pyroxene cation sites. However, the higher
energy splitting of the M1 and M2 sites occurs at very similar wavelengths for most pyroxene
compositions. Because the MGM provides a mathematical deconvolution of a spectrum, the
model always improves when more Gaussian curves are allowed. However, this improvement
does not mean that the fit is more accurate. It is important to reach a balance between using the
minimum number of curves necessary to fit a spectrum and the theoretical origins of those bands.
If bands are going to be fit without restricting the model, it is particularly important to use the
minimum number of absorptions needed to describe a spectrum.
We have thus chosen to use one band in the 1 m region to represent a combination M1 +
M2 of absorption, unless two are absolutely necessary (as is the case for the Wo49-50 pyroxenes).
Additional spin-allowed crystal field bands were added near 1.2 and 2 m. In Ca2+-saturated
pyroxenes containing high-iron glass, an additional broad absorption feature was needed near 1.8
m to allow a proper fit of the 2 m band. This wavelength is consistent with the longer
wavelength absorption band of iron-bearing silicate glasses (Bell et al., 1976). An additional
broad glass band should occur above 1 m, but because of extensive overlap with the 1.2 m
band the model solutions did not clearly indicate that an additional band was necessary. We
have omitted this band for the reasons discussed above.
Initial analyses were conducted using only the pyroxenes that were close to the intended
composition, as they are the least likely to be affected by the accessory glass. The pyroxenes
with a larger proportion of glass (Table 1) were found to follow the trends observed for the more
completely-reacted samples, and have therefore been included in this study.
4. Results
4.1. Near-Infrared Spectra (0.3-2.6 m at 0.5 m sampling)
4.1.1. Quadrilateral Transects – Constant Mg Number
To highlight the variability in pyroxene spectra as a function of Ca2+ and structure, four
transects of roughly constant Fe2+-Mg2+ ratio but varying Ca2+ are shown in Fig. 5. The spectra
of synthetic Ca-Free orthopyroxenes from Klima et al. (2007) that lie along each of these
transects have also been included in each panel for comparison. The spectra in these figures have
been offset by multiples of 0.1 to allow easy comparison of features. These transects represent
pyroxenes with roughly constant Mg number (Mg0, Mg25, Mg50, and Mg80) with Mg number (Mg
#) defined as nMg/(nFe2+ + nMg) × 100.
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Shown in Fig. 5A are the spectra of pyroxenes in the Mg-free transect, ranging from
hedenbergite through ferrosillite. The spectrum of pyroxene 083, a type A hedenbergite, is
dominated by two M1 bands near 1 and 1.2 m. Progressing down the Mg-free limb of the
pyroxene quadrilateral from pyroxene 082 through 087, the 1 and 2 m bands strengthen but
their positions do not change much, suggesting that the geometry of the M1 coordination
polyhedra remains relatively constant. This is in fact what is observed in single-crystal X-ray
refinements (SREF) of the atom positions (Fig. 6). The M1 site is completely filled with Fe2+
throughout this transect because Ca2+ cannot enter the M1 site, so visual differences in the band
strength of the 1.2 m band are most likely due to albedo differences among the samples. For
example, all the absorption bands in pyroxene 088 are weaker than in pyroxene 087. The
absolute strengths of all absorption bands are depressed with very high Fe contents, although the
relative strengths remain roughly constant until any band becomes optically saturated (this
occurs more at lower Fe contents for the M2 bands because of the distortion of the site).
The 1 m band occurs at slightly shorter wavelengths as the amount of Ca2+ decreases
along the full transect. The position of the 2 m band in the spectra of pyroxenes 082 through
087 does not change significantly, despite the variability in Ca2+. In contrast, the 2 m bands of
pyroxenes 088, reported to be a P21/c structure (Dowty and Lindsley, 1973), and 061, an
orthopyroxene, occur at shorter wavelengths as expected for their lower Ca2+ contents because
the shape of M2 is different in the P21/c and Pbca space group pyroxenes. This change is also
clearly seen in the SREF data in Fig. 6.
The Mg25 transect, shown in Fig. 5B, offers the most complete cut of single phase
pyroxenes through the quadrilateral. Starting at the most Ca2+-rich sample and moving down the
transect, the strengths of the 1 and 2 m absorption bands initially increase sharply until
pyroxene 055 (Wo26) is reached, after which the strengths of the 2 m bands are approximately
equal. Because Mg2+ is also present in this transect, the strengths of the 1.2 m bands are not
only dependent on the bulk composition of the pyroxenes, but also on the partitioning of Fe2+
and Mg2+ between the M1 and M2 sites. The intensity of the 1.2 m band is strongest for
pyroxenes 066 and 070, in which the majority of M2 sites are filled with Ca2+, and decreases
gradually from Wo26 through Wo0.
As in the Mg2+-free system, the positions of the 1 m bands move to shorter wavelengths
with decreasing Ca2+ content. However, the 2 m band again occurs near 2.3 m for all samples
of Wo20 or greater. The shifting of the 2 m band is not visually obvious until the transition into
low-Ca clinopyroxenes and orthopyroxenes. The 2 m band center for pyroxene 053 occurs at a
similar wavelength to the P21/c pyroxene 088. Based on its composition, it is reasonable to
conclude that pyroxene 053 may also exhibit a P21/c crystal structure.
Shown in Fig. 5C is the Mg50 transect. At Mg50, the first evidence of the midquadrilateral miscibility gap becomes apparent. Although these pyroxenes were synthesized at
temperatures and pressures designed to produce a single phase pyroxene, it is extremely difficult
to fully quench this metastable phase. Pyroxene 009, shown in light blue in Fig. 5, exhibits an
abnormally broad 2 m band, generally indicative of zoning or fine-scale exsolution. No
exsolution or partial inversion to orthopyroxene was evident at the scale of the electron
microprobe, though the sample was found to be compositionally zoned. A number of
clinopyroxenes were synthesized along the Mg80 transect; however all those with <Wo25 were
found to contain patches of high and low-Ca pyroxene when examined by EMPA. Thus, the
Mg80 transect, shown in Fig. 5D, is limited to only higher Ca pyroxenes. As observed in the
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more iron-rich pyroxenes, the positions of the 1 m bands in the Mg50 and Mg80 transect move to
shorter wavelengths with decreasing Ca2+ content, and the 2 m bands of all pyroxenes above
Wo20 occur near 2.3 m.
4.1.2. Quadrilateral Horizontal Transects – Constant Ca
We examined five transects of roughly constant Ca2+ content to determine whether the
effect of Fe2+ can be distinguished from the effects of Ca2+. Four of the transects are within the
augite structural region of the quadrilateral, and one is within the pigeonite structural zone.
Shown in Fig. 7A are near-infrared spectra of the pyroxenes with compositions near Wo50.
Despite these pyroxenes all being nominally Ca2+-saturated, only one sample (083) is a ‘type A’
pyroxene. The remainder of the samples would be considered transitional between types A and
B (or type AB). Though the small amount of Fe2+ in the M2 sites of these pyroxenes was
essentially invisible to Mossbauer spectroscopy, it still has a pronounced effect on the reflectance
spectra. In stark contrast to the spectra of lower-Ca clinopyroxenes and orthopyroxenes, the
absorption feature near 1 m is clearly dominated by the M1 absorption bands. In particular, the
M1 component of the 1 m band, normally nested within and dwarfed by the M2 1 m band, is
clearly present in all spectra.
The double M1 bands are most clearly visible in the ‘type A’ pyroxene 083. Though the
spectrum suggests two bands of approximately equal intensity, MGM models consistently result
in a stronger 1.2 m M1 band and a weaker 1 m M1 band (Klima et al., 2008). A similar result
was found when Schade et al. (2004) performed MGM fits on their suite of natural high-Ca
pyroxenes. The M2 bands are weak in comparison to the M1 bands, yet relatively strong
considering the very small amount of Fe2+ present in the M2 site. For example, in pyroxene 083,
a slight band is present near 2 m, and MGM models require a weak M2 band in the 1 m region
as well (Klima et al., 2008). EPMA and Mössbauer measurements of pyroxene 083 predict that
the M2 site is completely saturated by Ca2+ atoms, and should thus have no M2 crystal field
bands. Nevertheless, the presence of a band near 2.3 m suggests that a trace of Fe must be
present in at least some M2 sites. This observation underscores the effect of the distortion of the
M2 site from regular octahedral symmetry, because the intensity of crystal field absorption bands
is a function of both the iron content in a site and the distortion of the site.
C2/c (augite) structure pyroxenes with less than 50% Ca exhibit more typical pyroxene
spectral shapes. When Ca content is held approximately constant, the difference in 1 and 2 m
band position between pyroxenes of differing Fe content is minimal. As shown in Figs. 7B-D,
the shift of the 1 m band in augites is only on the order of 10-20 nm for any of the transects.
The 2 m band is also relatively unaffected by changes in Fe2+ content, with the maximum
difference between positions being roughly 40 nm. These observations are consistent with the
long-held notion that there are few structural changes across the diopside-hendebergite solid
solution (Rutstein and Yund, 1969); note the similarities of the M1 and M2 polyhedra between
diopside and hedenbergite in Fig. 1. In contrast, the positions of the 1 and 2 m bands do shift
significantly as a function of Fe2+ within the pigeonite zone, as illustrated in Fig. 7E.
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5. Discussion
5.1. Spin-Allowed Crystal Field Bands in the Near-Infrared
Absorption band parameters derived from MGM fits for each of the pyroxenes are listed
in Table 1. Examples of the general fit types are shown in Fig. 8. In each panel, the bands
attributed to each cation site are indicated. The measured spectrum is shown as a thick grey line
and the modeled fit is shown as a thin black line. The residual as a function of wavelength is
shown at the top of each panel. Shown in Fig. 8A is a typical low-Ca clinopyroxene that falls
within the pigeonite structural field. Only three major absorption bands were needed to model
this spectrum, so the band near 1 m represents a combination of the strong M2 and weaker M1
absorptions. Augites with <Wo45 were modeled similarly, as shown in Fig. 8B. In both cases,
there is a residual error near 1 m, likely due to the missing M1 band. Fits were attempted using
an additional M1 band near 1 m, but the resulting deconvolutions were not physically realistic
(i.e., bands became positive) or were directly dependent on the input parameters. For pyroxenes
with close to Wo50, the additional M1 band was critical to obtain a modeled fit. Examples are
shown in Fig. 8 for a type AB (Fig. 8C) and type A (Fig. 8D) pyroxene.
5.1.1. Spectral Variability as a Function of Ca Content
Shown in Fig. 9 are a series of scatter plots of the MGM-derived 1 and 2 m band
positions along each of the pyroxene transects of constant Mg-Fe ratio. The 1 m band moves
regularly to longer wavelengths with increasing Ca2+ content regardless of crystal structure.
These trends are well described by linear fits (R2 values over 0.96). From Wo0 through roughly
Wo<15, the 2 m band moves substantially to longer wavelengths with increasing Ca2+ content.
However, beyond Wo20, the 2 m band reaches roughly 2.3 m and then remains near that value
through Wo50. The 2 m band moves to longer wavelengths but then plateaus around 2.3 m
when the pyroxenes reach about Wo30. This plateau in 2 m band position occurs from near
Wo20 through Wo50 in all transects, regardless of iron-magnesium ratio. Though the 2 m band
variations as a function of Wo can be fit with a linear regression (R2 ≤ 80), they are better
described by a second order polynomial (Fig. 9).
Because the crystal field absorption band positions depend on the crystal field splitting,
which in turn depends on the ligand field environment and crystal structure, the plateau in the
position of the 2.3 m band suggests that there is indeed a spectral transition corresponding to
the structural change from P21/c pigeonite to C2/c augite (Fig. 1). The almost-stationary position
of the 2 band in C2/c clinopyroxenes implies that the ligand field around the M2 sites does not
change significantly even as more Ca2+ is added to an augite (Fig. 6).
5.1.2. Spectral Variability as a Function of Iron Content
Shown in Fig. 10 is a scatter plot of the positions of the MGM-derived 1 and 2 m bands
as a function of Fe2+ for the transects of approximately constant Ca content. For the highest Ca
pyroxenes, there is virtually no correlation between Fe2+ content and the position of the 2 m
band, probably because Ca2+ is controlling the distortion of the M2 polyhedron and it is not
changing across this transect. The 1 m band moves to slightly longer wavelengths with
increasing Fe2+, but there is a large amount of scatter in band position. The general trends
9
become slightly tighter for augites with lower Ca content. The relationship of Fe2+ to the
positions of the 1 and 2 m bands become better described by linear fits as the amount of Ca
decreases. The 1 m band moves to slightly longer wavelengths, while the 2 m band moves to
slightly shorter wavelengths with the addition of more Fe2+. For all augites the difference in 1
m band positions is roughly 10-20 nm maximum, and of the 2 m band is around 20-40 nm.
Given the poor correlation and the small shift in position, it is doubtful that the Fs content of
augites (especially higher Ca augites) could be reliably assessed from their reflectance spectra.
In contrast, the 1 and 2 m bands of pigeonites trend consistently towards longer
wavelengths as a function of Fe2+ content. There is over 35 nm difference between the 1 m
band positions of a pigeonite of roughly Fs40 and one of around Fs90, and roughly 200 nm
between the 2 m band positions of the same pigeonites. This spectral behavior is more similar
to that of orthopyroxenes than of other clinopyroxenes, probably due to the low total Ca in the
structure.
5.1.3. Overall Spectral Trends
Shown in Fig. 11 are scatter plots of the 1 and 2 m band positions as a function of Fe2+
and Ca content for the full suite of clinopyroxenes. If all are considered together, irrespective
of structure, the relationship between band position and Ca2+ content is clearly much stronger
than that with Fe2+. All 1 m bands move to longer wavelengths with increasing Ca2+, and, as
observed in the Mg transects, the 2 m bands increase through roughly Wo20 and the reach a
plateau around 2.3 m. When considered as a whole, the 1 and 2 m bands appear to move to
shorter wavelengths with increasing Fe2+ content. However, this effect is overprinted (and
obscured) by the variations caused by Ca2+. Pyroxenes with Fs contents above 50% necessarily
contain less Ca2+ than those with lower Fs contents.
When structural group is considered, the trends become quite different. Pigeonites have
been colored black in Fig. 11, and augites are shown as grey. Though all 1 m bands move to
longer wavelengths with Ca2+ content, the scatter in the pigeonites is greater, as illustrated in Fig.
11A. This is additional evidence that the Fe2+ content has a stronger effect on pigeonites than
augites. Indeed, in Fig. 11C, the pigeonites define a line that increases very regularly with Fe2+
content. The two structural groups are even better separated when the 2 m band is considered.
Pigeonites move regularly to longer wavelengths with both increasing Fe2+ and Ca2+ content,
followed by a gap from 2.20-2.25 m where no 2 m bands are centered. The correlations
between augites, which all plot above this gap, and Fe2+ and Ca2+ content are weak at best.
The relative changes in the 1 and 2 m band position can be clearly seen in a plot of 1
and 2 m band centers, as used by Adams (1974) and Cloutis and Gaffey (1991). As illustrated
in Fig. 12, the variation in 1 and 2 m bands for orthopyroxenes and pigeonites follows a very
regular trend. However, the augite data cluster in a cloud rather than along any type of regular
line. If the opx-pigeonite trend is extended through the augite field, it defines a lower limit to the
augite cluster.
In previous studies, band 1/band 2 plots have been constructed using absolute band
minima or band centers, without spectral deconvolution and usually without information on the
crystal structure or space group (Adams, 1974; Hazen et al., 1978; Cloutis and Gaffey, 1991).
For example, Hazen et al. (1978) show smoothly-contoured quadrilaterals for the position of the
1 and 2 m pyroxene absorption maxima vs. composition. However, because of the midquadrilateral miscibility gap between pigeonites and augites, a large number of natural pyroxenes
2+
10
used in previous studies are extensively zoned or exsolved. If only a single 1 and 2 m band
position are measured for multiphase pyroxenes, it represents a combination of all phases and
results in an apparently continuous trend in pyroxene band 1/band 2 positions that varies as a
function of Fe2+ (for low Ca2+-pyroxenes) and then Ca2+ (Adams and McCord, 1972; Adams,
1974; Cloutis and Gaffey, 1991).
However, our synthetic samples with better coverage of the Fe-rich compositions and a
broader range of representation of space groups allow the finer details of changes with crystal
structure to be discerned. As seen in Fig. 12, when only single phase pyroxenes are considered a
band 1/band 2 plot separates pyroxenes into their structural groups (orthopyroxene, pigeonite,
augite), suggesting that the previous studies were overly simplistic.
Previous authors have used contoured quadrilaterals as a way to display the variability of
1 and 2 m bands as a function of composition (Hazen et al., 1978; Cloutis and Gaffey, 1991;
Denevi et al., 2007). Shown in Fig. 13 are pyroxene quadrilaterals that have been contoured with
the positions of the 1 and 2 m bands. Because of the spectral transitions induced by structural
changes, each structural group must be treated separately. Because we do not have single-phase
pyroxenes within the light grey region, we have avoided extrapolating our pigeonite trends to
lower Mg contents. The slopes of the contours in the pigeonite fields are also somewhat
qualitative, as most of our pigeonite data clusters around Wo10. The 1 m band is contoured
using 10 nm intervals; however, the 95% confidence range is generally on the order of 10 nm for
each of the modeled fits. No clear compositional trends could be contoured for the 2 m band in
augites, as the entire wavelength spread for augites is only about 40 nm.
The quadrilaterals in Fig. 13 are qualitatively similar to those proposed by Cloutis and
Gaffey (1991) and Denevi et al. (2007) for natural pyroxenes, if only the regions where previous
authors had data (or single-phase pyroxene data) are considered. If the natural pyroxenes
measured by these authors were plotted on the diagrams constructed from the synthetic pyroxene
data, they would also be within the error ranges of the contours. Unique to this data set are the
large number of Fe2+-rich samples. In previous studies, contours had to be extrapolated through
these compositions, so the gap in band parameter space between the pigeonites and augites was
not clearly defined.
A question that arises is why is the 2 m band seemingly insensitive to variations in Ca
content for augites, yet the 1 m band continues to shift regularly to longer wavelengths with
increasing Ca2+ content? The answer must lie in steric constraints on the crystal structures. The
many single crystal refinements for pyroxenes in the Ca-Mg-Fe system suggest some possible
explanations. Consider again the Ca-Fe series from Fs65Wo35 to Fs85Wo15 on the right side of the
quadrilateral, which was studied by Ohashi et al. (1975). The polyhedral volume and bond
distances of the M1 site do not change appreciably along this join except for a slight distortion of
the octahedron at the Fs-rich end. So it is unlikely that changes in the M1 octahedron are
causing the wavelength shift of the 1 m band.
The M2 site is solely responsible for the 2 m absorption, and dominates the 1 m
absorption for all but the most Ca-saturated pyroxenes. Crystal field splitting is sensitive to the
interactions between the 3d electronic states in the Fe cation and the surrounding oxygens. The
two eg orbitals both have the majority of their electronic probability distribution along the x, y,
and z orientations; repulsion between the oxygen anions and the electrons in the orbitals raises
their energy. In a pyroxene M2 site, distortion causes further splitting of the two eg orbitals: 3dx2y2 has most of its electron probability in the plane of the x and y axes, while the 3dz2 orbital has
most of its distribution along the z axes. Each M2 site is surrounded by four non-bridging
11
oxygens (O1 and O2) that coordinate to the Fe atom only, and two-four bridging oxygens (O3)
that bond to Si4+ tetrahedra (Fig. 14). It is known from single-crystal refinements that the Fe-O1
and Fe-O2 distances decrease as the smaller Fe2+ substitutes for the larger Ca2+ cation. At the
same time, the Fe-O3 bond length increases (but only slightly) with Fe2+/Ca2+. It seems possible
that the 1 m band might be more affected by the Fe-O1 and Fe-O2 bond length changes, while
the 2 m absorption, resulting from a transition to an orbital with a fundamentally different
electronic distribution, responds more to changes in the Fe-O3 bond distance. In fact, as shown
in Fig. 14, the z direction is closest to the Fe-O2 bond, so it might be more likely to have its 3dz2
orbital energy increased by such proximity. The x direction is not very close to any of the bonds,
and the y orientation splits the angle between the O1 oxygens (and the O3 oxygens on the other
side of the polyhedron), so these would be expected to have less proximity to oxygen anions, and
thus have lower energies, consistent with the energy of the 2 m band (ca. 5000 cm-1).
6. Implications for Remote Sensing
One of the primary objectives in studying pyroxene spectra in-depth is to be able to
determine what can and cannot be concluded when a pyroxene-rich spectrum is measured on a
planet or asteroid. Currently, most global mapping of planetary surfaces only differentiates
between the broad groupings of high and low Ca2+ pyroxenes (Bibring et al., 2005; Mustard and
Sunshine, 1995; Mustard et al., 2005; Pieters, 1993). Some asteroid studies, however, use a
series of empirically-derived equations to estimate specific pyroxene mineralogies for asteroids
(Gaffey et al., 2002; Hardersen et al., 2004; Hardersen et al., 2006).
Though previous studies of natural pyroxenes has led to the suggestion that the entire
pyroxene mineral series exhibits a smooth trend of 1 and 2 m band positions (Adams and
McCord, 1972; Adams, 1974; Cloutis and Gaffey, 1991) our investigation of pure, single-phase
pyroxenes indicates that this is not the case. Mineral structure is one of the most important
factors affecting the band parameters of pyroxenes. Structures in orthopyroxenes and pigeonites
can be easily distinguished from augites on the basis of their 1 and 2 m band positions.
However, to evaluate structure from near-infrared spectra, it is critical to ensure that the 1 and 2
m bands belong to a single pyroxene, and do not represent an average of a mixture. This
typically requires careful deconvolution of pyroxenes using a method such as the MGM.
Gaffey et al. (2002) recognized that the entire quadrilateral cannot be treated as a single
system for determining the composition, and thus proposed seven equations for determining
composition. Each of the equations has a stated Wo or Fs content for which it is valid. If we
apply the correct equations from Gaffey et al. (2002) to our synthetic pyroxene data, we find that
orthopyroxenes and pigeonites that are within the compositional range that is valid are predicted
within the allowable error range with the exception of one which is correct to within Fs10.
Unfortunately, twelve of the synthetic pyroxenes are outside of the chemical range for which
equations are available (the Fe is too high). The results when the formulas are applied to the
synthetic augites are significantly less accurate. Five of the pyroxenes are outside of the range
for which there are equations available, 2 of 27 are within the expected error range for the
equations, and 5 of 27 are within Fs10 and Wo10 of the correct composition. This result is not
unexpected, due to the stationary nature of the 2.3 m band in the endmember pyroxene data.
Based on our pyroxene analyses, compositional remote analysis cannot ignore structural
constraints. We propose that when a pyroxene spectrum is observed, it should be analyzed using
a decision-tree that reflects the constraints from this larger suite of well-characterized synthetic
12
pyroxenes. If the 2 m band occurs between 2.25-2.3 m, the pyroxene is dominated by augite,
and the 2 m band is no longer useful for assessing the specific composition, but the 1 m band
can be used to determine the Ca content. If the 2 m band occurs between 2.20 and 2.25 m, it
is likely that the target represents a mixture of pyroxene compositions, either intimately or
mechanically mixed. If the 2 m band occurs shortwards of 2.2 m, the pyroxene is dominated
by pigeonite and orthopyroxene and both the positions of the 1 and 2 m bands can be used to
estimate the composition. Additional constraints can be added based on the width of the bands,
which help to determine whether the target is dominated by one or multiple pyroxenes, and the
relative strengths of the M1 and M2 bands.
7. Conclusions
For clinopyroxenes, it had been previously recognized that the addition of the large Ca2+
cation to the structure causes the 1 and 2 m bands to shift to longer wavelengths. However, all
other things being equal, Fe2+ can have a similar effect. We examined pyroxenes of similar Mg
number but varying Ca2+ content and clinopyroxenes of roughly constant Ca2+ content but
varying Fe2+-Mg2+ ratio to isolate the effect of Ca2+ on the NIR spectrum and to determine
whether the effects of Ca2+ and Fe2+ can be distinguished from one another. We find that for all
compositions of clinopyroxene in this study, the 1 m band shifts regularly to longer
wavelengths as a function of Ca2+. The 2 m band also shifts to longer wavelengths with the
addition of Ca2+ at low Ca2+ contents, but after ~Wo30 the Ca2+ dependency of the band position
drops significantly because of the structural transition from pigeonite to augite. This suggests
that that for augites, the addition of Ca2+ to a pyroxene has little to no effect on the bond lengths
in the direction responsible for the lower energy splitting of the M2 site.
Among Ca2+-saturated pyroxenes, where almost all of the Fe2+ resides in the M1 site, it is
still the trace amounts of iron in the M2 site that have the most dramatic effect on the appearance
of the spectrum. Distinctive M2 absorption bands are observed, even when 2% or less of the M2
sites contain Fe2+. This is because the M2 site is significantly more distorted than the M1 site,
and crystal field band intensity is directly a result of the symmetry of the cation site.
The specific structure of a pyroxene is one of the most important factors controlling the
properties of visible and near-infrared spectra. Contrary to a continuous trend of band positions
suggested from previous studies of spectra of natural pyroxenes, we find that pyroxenes of
different structural groups are separated from one another on a plot of 1 and 2 µm band
positions. It is likely that previous diagrams convolved the spectral changes caused by the
structure and composition of the pyroxene with a mixing trend between the low and high Ca endmember pyroxenes. Because structural transitions are controlled essentially by Ca content,
changes in Fe content are obscured when large amounts of Ca present are in the structure. In
low-Ca clinopyroxenes, the 1 and 2 m bands can be used to assess the Fe content of the
pyroxene, but in but in augites the most information can be extracted from the position of the 1
m band, which reflects the Ca content of the pyroxene.
13
References
Adams, J. B., and McCord, T. B., 1972. Electronic spectra of pyroxenes and interpretation of
telescopic spectral reflectivity curves of the moon, Proceedings, 3rd Lunar Science
Conference, 3021-3034.
Adams, J. B., 1974. Visible and near-infrared diffuse reflectance spectra of pyroxenes as applied
to remote sensing of solid objects in the solar system, Journal of Geophysical Research,
79:4829-4836.
Adams, J. B., 1975, Interpretation of visible and near-infrared diffuse reflectance spectra of
pyroxenes and other rock-forming minerals, in Infrared and raman spectroscopy of lunar
and terrestrial minerals, edited by C. J. Karr, C. J. New York: Academic Press Inc., pp.
91-116.
Bell, P. M., Mao, H. K. and Weeks, R. A., 1976. Optical spectra and electron paramagnetic
resonance of lunar and synthetic glasses: A study of the effects of controlled atmosphere,
composition, and temperature. Proceedings, 7th Lunar and Planetary Science Conference,
pp. 2543-2559.
Bibring, J. P., Langevin, Y., Gendrin, A., Gondet, B., Poulet, F., Berthé, M., Soufflot, A.,
Arvidson, R., Mangold, N., Mustard, J., Drossart, P. and the OMEGA team, 2005. Mars
surface diversity as revealed by the OMEGA/Mars Express observations, Science, 307:15761581.
Burns, R. G. (1993), Mineralogical applications of crystal field theory, 2nd ed., Cambridge:
Cambridge University Press. 551 p.
Cameron, M., and Papike, J. J., 1980. Crystal chemistry of silicate pyroxenes, In Pyroxenes,
edited by Prewitt, C. T. Washington, D.C.: Mineralogical Society of America, pp. 5-92.
Cloutis, E. A., and M. J. Gaffey, 1991. Pyroxene spectroscopy revisited - Spectral-compositional
correlations and relationship to geothermometry, Journal of Geophysical ResearchPlanets, 96:22809-22826.
Denevi, B. W., Lucey, P. G., Hochberg, E. J., and Steutel, D., 2007. Near-infrared optical
constants of pyroxene as a function of iron and calcium content, Journal of Geophysical
Research, 112:doi10.1029/2006JE002802.
Dowty, E., and Lindsley, D. H., 1973. Mössbauer spectra of synthetic hedenbergite-ferrosilite
pyroxenes, American Mineralogist, 58:850-868.
Dyar, M. D., 1985. A review of Mössbauer data on inorganic glasses: the effects of composition
on iron valency and coordination, American Mineralogist, 70:304-316.
Gaffey, M., Cloutis, E., Kelley, M. and Reed, K., 2002. Mineralogy of asteroids, in Asteroids III,
edited by Bottke, W. F. et al. Tucson: University of Arizona Press. pp. 183-204.
Hardersen, P. S., Gaffey, M. J. and Abell, P. A., 2004. Mineralogy of asteroid 1459 Magnya and
implication for its origin, Icarus, 167:170-177.
Hardersen, P. S., Gaffey, M. J., Cloutis, E. A., Abell, P. A., Reddy, V., 2006. Near-infrared
spectral observations and interpretations for S-asteroids 138 Tolosa, 306 Unitas, 346
Hermentaria and 480 Hansa, Icarus, 181:94-106.
Hazen, R. M., Bell, P. M. and Mao, H. K., 1978. Effects of compositional variation on
absorption spectra of lunar pyroxenes, Proceedings, 9th Lunar and Planetary Science
Conference, pp. 2919-2934.
Klima, R. L., 2008. Integrated spectroscopy of Mg-Fe-Ca pyroxenes: A foundation for modern
compositional analysis of planetary surfaces. Ph.D. thesis, Brown University,
Providence, RI, U.S.A.
14
Klima, R. L., Pieters, C. M. and Dyar, M. D., 2007. Spectroscopy of synthetic Mg-Fe pyroxenes
I: Spin-allowed and spin-forbidden crystal field bands in the visible and near-infrared,
Meteoritics and Planetary Science, 42:235-253.
Klima, R. L., Pieters, C. M. and Dyar, M. D., 2008. Characterization of the 1.2 um M1 pyroxene
band: Extracting cooling history from near-IR spectra of pyroxenes and pyroxenedominated rocks, Meteoritics and Planetary Science, 43:1591-1604.
Lindsley, D. H., 1983. Pyroxene thermometry, American Mineralogist, 68:477-493.
Mustard, J. F. and Sunshine, J. M., 1995. Seeing through the dust: Martian crustal heterogeneity
and links to the SNC meteorites, Science, 267:1623-1626.
Mustard, J. F., Poulet, F., Gendrin, A., Bibring, J. P., Langevin, Y., Gondet, B., Mangold, N.,
Bellucci, G and Altieri, F., 2005. Olivine and pyroxene diversity in the crust of Mars,
Science, 307:1594-1597.
Ohashi, Y., Burnham, C. W., and Finger, L. W., 1975. The effect of Ca-Fe substitution on the
clinopyroxene crystal structure. American Mineralogist, 60:423-434.
Pieters, C. M., 1993. Compositional diversity and stratigraphy of the lunar crust derived from
reflectance spectroscopy, in Remote geochemical analysis: Elemental and mineralogical
composition, edited by Pieters, C. M. and Englert, P. A. J. Cambridge: Cambridge
University Press. pp. 309-339.
Rossman, G. R., 1980. Pyroxene Spectroscopy. In Pyroxenes, edited by Prewitt, C. T.
Washington, D.C.: Mineralogical Society of America, pp. 93-116.
Rutstein, M.S., and Yund, R.A., 1969. Unit-cell parameters of synthetic diopsde-hedenbergite
solid solution series. American Mineralogist, 54:238-245.
Schade, U., Wäsch, R. and Moroz, L., 2004. Near-infrared reflectance spectroscopy of Ca-rich
clinopyroxenes and prospects for remote spectral characterization of planetary surfaces,
Icarus, 168:80-92.
Sueno, S., Cameron, M., and Prewitt, C. T., 1976. Orthoferrosilite: High-temperature crystal
chemistry. American Mineralogist, 61:38-53.
Sunshine, J. M., and Pieters, C. M., 1993. Estimating modal abundances from the spectra of
natural and laboratory Pyroxene mixtures using the modified Gaussian model, Journal of
Geophysical Research-Planets, 98:9075-9087.
Sunshine, J. M., Pieters, C. M. and Pratt, S. F., 1990. Deconvolution of Mineral AbsorptionBands - an Improved Approach, Journal of Geophysical Research-Solid Earth and
Planets, 95:6955-6966.
Sunshine, J. M., Pieters, C. M., Pratt, S. F. and McNaron-Brown, K. S., 1999. Absorption band
modeling in reflectance spectra: Availability of the modified Gaussian model (abstract
#1306). 30th Lunar and Planetary Science Conference. CD-ROM.
Turnock, A. C., Lindsley, D. H. and Grover, J. E., 1973. Synthesis and unit cell parameters of
Ca-Mg-Fe pyroxenes, American Mineralogist, 58:50-59.
Wang, L., Moon, N., Zhang, Y., Dunham, W. R., Essene, E. J., 2005. Fe-Mg disorder in
orthopyroxenes, Geochimica et Cosmochimica Acta, 69:5777-5788.
15
8. Figure Captions
16
Fig. 1. Configurations of the M1 and M2 polyhedra in different space groups within the
pyroxene quadrilateral, viewed down the a axis, with c horizontal and b vertical. Diopside,
hedenbergite, and augite are all in the C2/C space group and have somewhat similar geometries
in their M1 and M2 polyhedra. Enstatite and ferrosilite are very similar both at the end-members
and across the orthopyroxene composition range. Pigeonite is in the P21/c space group but has
coordination polyhedral that are more similar to the Pbca orthopyroxenes than the C2/C augites.
17
Fig. 2. Graphical illustration of the difference in kinking between SiO tetrahedra in the P21/c
(e.g., pigeonite) and C2/C (e.g., augite) space groups for pyroxenes; the linkages are outlined in
thick black line for clarity. The chains are parallel to the z orientation in the pyroxenes.
Variations in the angle of kinking, which is more extreme for the pigeonite, are related to
distortions in the adjacent M1 and M2 polyhedra that may contain Fe2+, Mg, or Ca. The c axis
lies along the length of the chains in all the structures.
18
Fig. 3. Idealized 6-coordinated polyhedron around a hypothetical Fe atom, along with the crystal
field splitting diagram. When the Fe atom is in octahedral coordination (as in pyroxene), the
energies of the Fe 3d orbitals change as a function of proximity to electronic distributions of the
surrounding oxygen anions. When the octahedron is not perfectly symmetrical, as shown in the
pyroxene examples in Fig. 1, further splitting occurs among the eg and t2g orbitals. Such splitting
of the energy levels makes possible multiple electronic transitions that can give rise to absorption
features such as the 1 and 2 m bands seen in pyroxenes.
19
Fig. 4. Pyroxene quadrilateral showing the compositions of pyroxenes used in this study. Note
that the pyroxene 088 could not be microprobed and the plotted position is the synthesis
composition.
20
Fig. 5. Pyroxene spectra of transects cutting vertically through the pyroxene quadrilateral.
Pyroxenes in each panel are of approximately constant Mg number. (A) Mg0 (B) Mg25 (C) Mg50
(D) Mg80.
21
Fig. 6. To-scale renderings of M1 (left) and M2 (right) polyhedra from single crystal
refinements of synthetic pyroxenes ranging down the Hd-Fs join. The Fs100 data are from Sueno
et al. (1976) and the other data are from Ohashi et al. (1975). These compare with the data
shown in Fig. 5A over the same edge of the quadrilateral.
22
Fig.7. Spectra of pyroxenes along horizontal transects through the pyroxene quadrilateral. Each
panel contains pyroxenes of approximately constant Ca content. (A) Ca-saturate pyroxenes
(Wo50) (B) augites with around Wo40 (C) augites with around Wo40 (D) augites with around
Wo30 (E) pigeonites with near Wo10.
23
Fig. 8. Example MGM fits for the four basic types of pyroxenes in this study. The data are
shown as a thick grey line, and the modeled fits is a thin black line superimposed. (A) Fit to a
typical pigeonite sample (B) Fit to a subcalcic augite (C) Fit to a type B hedenbergite and (D) a
type A hedenbergite.
24
Fig. 9. Variation of 1 and 2 m band positions as a function of Ca content (Mg number held
constant).
25
Fig. 10. Variation of 1 and 2 m band positions as a function of iron content (Ca held
approximately constant)
26
Fig. 11. Varations in the positions of the 1 and 2 m bands of all clinopyroxenes as a function of
Ca and Fe content.
27
Fig. 12. Band 1 and Band 2 diagram of synthetic pyroxenes compared to natural pyroxenes
measured by Adams and Cloutis and Gaffey.
28
Fig. 13. Pyroxene quadrilaterals showing the variation trends for the 1 and 2 m bands. Note
that the slopes within the pigeonite field (medium grey) are approximate and based on samples
with a restricted range of Ca content (~Wo10). The augite field for the 2 m band could not be
explicitly contoured as all samples exhibit 2 m bands clustering around 2.3 m.
29
Fig. 14. Two different views of the M2 coordination polyhedron from Fs80Wo20 from Ohashi et
al. (1975); the top view mimics that of Fig. 3.
30