Crystal structure of a new high-pressure polymorph of topaz-OH

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    Abstract

    A new high-pressure form of topaz-OH (denoted here topaz-OH II) was obtained at 14 GPa and 1400 °C. The X-ray diffraction patternof this phase can be indexed by an orthorhombic cell with a = 4.72318(5), b = 8.91480(9), and c = 2.77276(3) Å. The lengths of a andb are similar, but c is approximately a third of that for a previously reported topaz-OH (denoted topaz-OH I). The structural formula of topaz-OH II can be written as (Al 0.68Si0.32(O0.66(OH)0.34) 2, suggesting significant cation disorder in the structure. The crystal structure of topaz-OH II is solved using powder synchrotron X-ray diffraction data, and refined with constraints provided by a separate multi-nuclear NMR study. The structure has similarities with topaz-OH I and diaspore (α-AlOOH) structures, having partially occupied double edge-shared octahedral chains, and 2 × 1 tunnels with partially occupied tetrahedral and octahedral sites.

    Original languageEnglish
    Pages (from-to)1349-1352
    Number of pages4
    JournalAmerican Mineralogist
    Volume95
    Issue number8-9
    DOIs
    Publication statusPublished - 2010

    Keywords

    • Crystal structure
    • High-pressure studies
    • Order-disorder
    • Phase transition

    ASJC Scopus subject areas

    • Geophysics
    • Geochemistry and Petrology

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