Structural systematics of Ge substitution in primitive pyroxenes
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- 作者：M. D. Welch ; A. R. Pawley
- 关键词：Pyroxene ; Si–Ge ordering ; Transformation
- 刊名：Physics and Chemistry of Minerals
- 出版年：2016
- 出版时间：March 2016
- 年：2016
- 卷：43
- 期：3
- 页码：161-169
- 全文大小：973 KB
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A. R. Pawley (2)
1. Department of Earth Sciences, Natural History Museum, London, SW7 5BD, UK
2. School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Oxford Road, Manchester, M13 9PL, UK - 刊物类别：Earth and Environmental Science
- 刊物主题：Earth sciences
Mineralogy
Crystallography
Geochemistry
Mineral Resources - 出版者：Springer Berlin / Heidelberg
- ISSN：1432-2021
文摘
Pyroxenes of general stoichiometry Mg(Ge x Si1−x )O3 were encountered in attempts to synthesise Ge-substituted talcs at 0.2 GPa, 650–700 °C. Orthopyroxenes (Pbca) of compositions x = 0.21, 0.30, and 0.34 were identified, and also a P21/c clinopyroxene of composition x = 0.63, and C2/c clinopyroxenes of compositions x = 0.91 and 1. End-member clinoenstatite MgSiO3-P21/c synthesised at 16 GPa, 1300 °C and transformed from C2/c was also included in the study. Crystal structure refinements using single-crystal XRD data showed that unit-cell parameters vary linearly with Si–Ge for the Pbca and P21/c pyroxenes, both of which have two symmetrically non-equivalent tetrahedral chains. Refinement of Si–Ge occupancies at tetrahedral sites showed that the two chains of all primitive pyroxenes have very different compositions, with XGe(TB) ≫ XGe(TA). This difference arises from the greater flexibility of the B-chain to rotate in response to tetrahedral expansion due to increasing Ge content. The TA-M2 shared polyhedral edge imposes significant constraints on the flexibility of the A-chain, which can accommodate much less Ge than the B-chain. Linear trends of cell parameters, site occupancies, and structural parameters for the primitive pyroxenes, when extrapolated to published data for MgGeO3–Pbca, extend across the entire Si–Ge join.