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Raman study of EETA79001 Martian meteorite

Compositional features of olivine, pyroxene, and opaque minerals

A. Studied rock chips -- Raman point-counting measurements were made on unprepared surfaces of the rock chips with surface relief on the order of several mm.

eeta_10.jpg (270036 bytes) eeta_rock13.jpg (42951 bytes) eeta_rock22.jpg (60352 bytes)
eeta_rock20.jpg (48862 bytes) eeta_rock12.jpg (46795 bytes)
EETA79001_476 lithology A EETA79001_482 Lithology B

B. Major minerals (pyroxene and olivine) -- The continuous range of Mg' in all rock chips indicates that the pyroxenes are mainly rapidly cooled, unequilibrated pyroxenes typical of basalt; the histogram for 79001,482 pyroxenes spans a broader range, however, and extends to more Fe-rich composi-tions, indicating more rapid crystallization. Using Raman spectral data published by Guyot et al. [7] and Chopelas [8], we developed a correlation to calculate Fo values (Mg/(Mg+Fe) mole fraction) of oli-vine based on the frequencies of their strongest doublet at ~850 cm-1 and ~820 cm-1 (Fig. 1b).

eeta_px1.gif (12271 bytes) eeta_px2.gif (21499 bytes) eeta_px3.gif (19566 bytes)
From the major Raman peak positions of the py-roxenes (Fig.1a), we calculated Mg' (Mg/(Mg+Fe)) using the correlation developed in [3]. High-Ca and low-Ca pyroxene can be distinguished, but here we fo-cus on Mg'. The histograms in Figure 2a and 2c show the abundance of pyroxene of different Mg' in rock chips 79001,476 and 79001,482. The distributions of Mg' for pyroxenes of both samples are consistent with those from EMPA data published in previous petrologic studies [5,6]. On the basis of Figure 3, we surmise that there are three types of olivine that represent three stages of formation of EETA79001.

C. Minor and rare mineral phases identified in point-counting procedure
eeta_calcite.jpg (104907 bytes)A calcite grain was identified during a linear traverse. Calcite was identified by other techniques in lithology C of this meteorite. Three types of phosphates were identified. eeta_phosphates.jpg (281463 bytes)

D.  Raman features of opaque minerals --how the structural and compositional information of these minerals can be extracted from their Raman spec-tral patterns and peak positions.

 

eeta_chromite.jpg (168464 bytes) eeta_chromite2.jpg (142726 bytes) eeta_spinel.jpg (364491 bytes) eeta_hematite.jpg (257592 bytes)

The degree of covalency of a chemical bond, A-O, or the compressibility of a polyhedron (indicated by the bulk modulus) in a crystal structure is commonly used to predict the major ionic group responsible for Raman spectral features.In Fe-Ti oxides, however, differences in bond strength between different types of polyhedra are less pronounced. Thus, only approximate "major contribu-tors" for the Raman spectral features can be assigned, and these roles can be changed by cation substitutions. In following Table, the suggested major contributors of Raman features for each oxide are marked in bold typeface.

 

  Corundum structure Spinel structure
Minerals Ilmenite Hematite Ulvospinel Magnetite Chromite
formula (Mg<0.5, Fe>0.5)TiO3 alpha-Fe2O3 Fe2+ (Ti, Fe2+)2O4 Fe3+(Fe2+, Fe3+)2O4 (Mg,Fe)(Cr,Al,Fe)2O4
Structure pseudo-corundum corundum normal spinel inverse spinel ormal spinel
C3i2 -- R3- D6(3d) -- R3-C Oh 7 -- Fd3m Oh 7 -- Fd3m Oh 7 -- Fd3m
polyhedra Ti4+O6 Fe3+O6 Ti4+O6, Fe2+O6 Fe3+O6, Fe2+O6 Cr3+O6, Al3+O6, Fe3+O6
Fe2+O6, Mg2+O6   Fe2+O4 Fe3+O4 Mg2+O4, Fe2+O4
TiO2 (wt%) 46-57 7-11 29-30 19 ~0
Raman peak 676-692 274-293, 214-226 660-665 662-667 680-770

Reference: Wang A., Kuebler, K.E , Jolliff B. L., Haskin L. A., Mineral features of EETA79001 martian meteorite revealed by point-counting Raman measurements as anticipated for in-situ exploration on planetary surfaces, 31th LPSC, 2000.
                        Wang A., Kuebler, K.E., Jolliff B. L., Raman spectroscopy of opaque minerals and applications to EETA79001 martian meteorite, 32th LPSC, 2001.

 

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