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Structure & Chemistry of pyroxene

The pyroxene mineral group is of particular interest in planetary sciences. Determining the compositional and structural characteristics of pyroxenes in rocks can be crucial to understanding their petrogenesis. In this study, we evaluate the feasibility to determine both structure and composition of quadrilateral pyroxene [(Mg,Fe,Ca)Si2O6] using only Raman spectra as would be obtained from on-surface planetary measurements. Such characterization would also be useful in terrestrial field and laboratory investigations. The studied samples include 5 lunar rocks, 2 Martian meteorites, and 7 sets of terrestrial rocks and minerals grains.

A. CharacteristicRaman spectra of pyroxene

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The detail features in Raman spectral patterns can be used to distinguish among different structures  of quadrilateral pyroxene. Raman spectral peak positions are the key to determining pyroxene cation mole fractions. Note the deconvolved peaks in spectrum a. Correlations are shown among the positions of three Raman peaks and the molar ratios of the three principal cations in pyroxene octahedral sites. Histograms indicate the number of cases as a function of parameter value.

B. Determination of pyroxene cation mole fraction using Raman data

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Predicted values of cation molar ratios Mg/(Mg+Fe+Ca) and Ca/(Mg+Fe+Ca) obtained by Raman measurements are compared with EMP-determined values for basaltic pyroxenes in a Martian meterite and in the lunar rock used to calibrate the Raman peak positions. The lower graphs show the absolute values of discrepancies between Raman and EMP values. Note the large percent errors for Ca2+ mole fraction at values <0.08. These erroneous values are all overestimates and can be detected and corrected because the structural pattern of the spectra allows their identification as low-Ca orthopyroxene. Predicted values obtained from Raman measurements are compared with EMP-determined values of cation mole fraction for equilibrated pyroxenes in two lunar rocks and five terrestrial samples. Errors for Mg2+ cation fraction are of the same magnitude as for basaltic pyroxenes (~0.1). The larger errors for both low-Ca orthopyroxene and hedenbergitic high-Ca pyroxenes can be detected from the structural patterns and corrections made.

C. Distinguish basaltic pyroxene, equilibrated pyroxene, and non-quadrilateral pyroxene

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The monotonic change in position of Raman peak 3 relative to peak 2 is shown. The central dashed line in a and b is the combined result from the regressions of the positions of peaks 2 and 3 against cMg2+  [Mg/(Mg+Fe+Ca)] for the Raman and EMPA data of pyroxenes from lunar sample 15273,7039. Peak positions of orthorhombic pyroxenes plot at the highest values of Mg2+ cation fraction and peak positions of triclinic pyroxenoids plot at the lowest values. The dashed lines perpendicular to the central line separate regions of different cMg2+. The gap between points for the equilibrated pyroxenes in b indicates the presence of a host pyroxene and exsolution lamellae and identifies the pyroxenes as equilibrated. Part c contains the same correlation curve, but on a smaller scale, and shows that non-quadrilateral pyroxenes can deviate strongly from the calibration for quadrilateral pyroxenes.

 

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Raman spectra of several non-quadrilateral pyroxenes Frequency distributions of peaks in the 300-400 cm-1 region (peak 3) obtained by point-counting Raman experiments are shown for pyroxenes from different rock types.

References: Wang A., Jolliff B. L., Viskupic K. M., Haskin L. A., “Raman spectroscopic characterization of different types of pyroxene”, Abstracts of Papers Submitted to the Twenty-eighth Lunar and Planetary Science Conference, Part 3, P1491-1492, 1997.

                    Wang A., Jolliff B. L., Haskin A. L., Kuebler E. K., "Raman spectral features of pyroxene - Application to Martian meteorites Zagami & EETA79001", Thirty Lunar and Planetary Science Conference, (1999) Huston, Texas, USA.

                   Wang A., Haskin L. A., Jolliff B. L., Kuebler, K.E., Characterization of structure and compositions of quadrilateral pyroxenes by Raman spectroscopy -- implication for future planetary exploration, 31th LPSc, 2000.

                    Wang A., Jolliff B. L., Haskin L. A., Kuebler K. E., Viskupic K. M., (2001), Characterization and comparison of structural and compositional features of planetary quadrilateral pyroxenes by Raman spectroscopy, American Mineralogists. V86, p790-806.

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