Cation distribution in low-calcium pyroxenes: Dependence on temperature and calcium content and the thermal history of lunar and terrestrial pigeonites |
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| Authors: | Surendra K Saxena Subrata Ghose AC Turnock |
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| Institution: | Department of Geological Sciences, Virginia Polytechnic Institute, Blacksburg, Va. U.S.A.;Department of Geological Sciences, University of Washington, Seattle, Wash. U.S.A.;Department of Earth Sciences, University of Manitoba, Winnipeg, Manitoba ,Canada |
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| Abstract: | Four pyroxenes with compositions En48Fs48Wo4, En47·5Fs47·5Wo5, En45Fs45Wo10 and En40Fs40Wo20, synthesized at 1200°C at atmospheric pressure, were heat-treated at 500, 600, 700, and 800°C for various lengths of time. These pyroxenes are variously ordered with respect to Fe2+ and Mg2+ without unmixing. The Fe2+-Mg2+ distribution over the two nonequivalent sites M1 and M2, determined through Mössbauer spectroscopy, is found to be a function of both temperature and concentration of Ca2+ at the M2 site. The preference of Fe2+ for the M2 site increases with decreasing temperature and increasing Ca2+. These data can be used to determine cation equilibration temperatures of lunar and terrestrial pigeonites. The lunar pigeonites usually indicate equilibration temperatures of 700–860°C, except the pigeonite from rock 14053, which may have been subjected to shock heating due to meteoritic impact. |
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