Unique crystal chemistry of two polymorphs of topaz-OH: A multi-nuclear NMR and Raman study

Xianyu Xue, Masami Kanzaki, Hiroshi Fukui

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    12 Citations (Scopus)

    Abstract

    A new polymorph of topaz-OH (denoted as topaz-OH II) was recently discovered at higher P-T conditions than has been known thus far (denoted as topaz-OH I). High-resolution 1H, 29Si, and 27Al nuclear magnetic resonance (NMR) and micro-Raman spectroscopy are applied to shed light on the crystal chemistry of both polymorphs. Topaz-OH I, synthesized at 7 GPa and 640 °C, is stoichiometric (Si/Al = 0.5) with a largely ordered local structure. Higher P-T topaz-OH I synthesized at conditions close to the polymorphic phase transition boundary, on the other hand, shows lower Si/Al ratios (0.44-0.45) and greater local structural disorder [including a small fraction (∼3%) of octahedral Si with a unique 29Si chemical shift near -133 ppm]. The latter may be accounted for by the development of defects (Si/Al in normally vacant octahedral sitesand vacancies in the tetrahedral sites) at higher P-T conditions. Topaz-OH II synthesized at 13.5∼14 GPa and 1300∼1400 °C similarly exhibits low Si/Al ratios (0.41-0.46). The NMR and Raman spectra for these topaz-OH II are, in general, broader and revealed a substantial fraction (33-37%) of octahedral Si with a range of 29Si chemical shifts (-130 to -190 ppm), a small fraction (2-3%) of tetrahedral Al, and a range of (and overall shorter) hydrogen-bonding distances than topaz-OH I. Therefore, the phase transition from topaz-OH I to II is characterized by both a significant increase in the occupied octahedral/tetrahedral site ratio as well as disordering of cation distribution, which is unique from the viewpoint of crystal chemistry.

    Original languageEnglish
    Pages (from-to)1276-1293
    Number of pages18
    JournalAmerican Mineralogist
    Volume95
    Issue number8-9
    DOIs
    Publication statusPublished - 2010

    Keywords

    • Al-Si disorder
    • Crystal structure
    • High pressure
    • Hydrogen bonding
    • NMR spectroscopy
    • Phase transition
    • Raman spectroscopy
    • Vacancy
    • Vacant site

    ASJC Scopus subject areas

    • Geophysics
    • Geochemistry and Petrology

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