Structural systematics of Ge substitution in primitive pyroxenes

M. D. Welch, A. R. Pawley

    Research output: Contribution to journalArticlepeer-review

    Abstract

    Pyroxenes of general stoichiometry Mg(GexSi1−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.

    Original languageEnglish
    Pages (from-to)161-169
    Number of pages9
    JournalPhysics and Chemistry of Minerals
    Volume43
    Issue number3
    Early online date2 Jan 2016
    DOIs
    Publication statusPublished - 1 Mar 2016

    Keywords

    • Pyroxene
    • Si–Ge ordering
    • Transformation

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