Si-Al distribution in high-pressure CaAl4Si2O 11 phase: A 29Si and27Al NMR study

Xianyu Xue, Shuangmeng Zhai, Masami Kanzaki

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    10 Citations (Scopus)

    Abstract

    High-resolution 29Si and 27Al NMR techniques have been, applied to resolve the Si-Al distribution, and coordination in the high-pressure CaAl4Si2O11 (CAS) phase, a potentially important mineral in subducted crustal materials in the deep mantle that has a unique hexagonal ferrite structure containing two octahedral (M1; M2) and one trigonal bipyramidal sites. The 29Si MAS NMR spectra of the CAS phase synthesized at 20 GPa and 1400∼1600 °C show two broad, asymmetric peaks near -92.7 and -182.7 ppm with an intensity ratio of 1:3, suggesting that 1/4 of the Si are in tetrahedral coordination and 3/4 in octahedral coordination. Therefore, the trigonal bipyramidal and M1 octahedral sites are each occupied by equal proportions of Si and Al, and the former are effectively half-occupied face-sharing tetrahedra (at least for Si). The 27Al MAS and 3Q MAS NMR, spectra contain only one unresolved peak typical, of octahedral Al with a range of quadrupolar coupling constants.

    Original languageEnglish
    Pages (from-to)1739-1742
    Number of pages4
    JournalAmerican Mineralogist
    Volume94
    Issue number11-12
    DOIs
    Publication statusPublished - 2009

    Keywords

    • Al and si coordination
    • CAS phase
    • High pressure
    • NMR spectroscopy
    • Si-Al disorder

    ASJC Scopus subject areas

    • Geophysics
    • Geochemistry and Petrology

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