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Si vacancies in the 10-Å phase

¹ Department of Mineralogy, The Natural History Museum, London SW7 5BD, U.K.
² Department of Earth Sciences, Cambridge University, Cambridge CB2 3EQ, U.K.
³ School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Manchester M13 9PL, U.K.
⁴ Department of Geosciences, SUNY Stony Brook, Stony Brook, New York 11794-2100, U.S.A.

Correspondence: ^* E-mail: mdw{at}nhm.ac.uk

²⁹Si MAS NMR spectroscopy on samples of 10-Å phase synthesized from oxides (6.0 GPa/600 °C/400 h) and from partial transformation of talc (6.5 GPa/650 °C/12.5 h) reveals that this phase contains Q²-type Si sites in a ratio Q³:Q² of 5.33:1. It is proposed that the Q² arise from adjacent vacancies in the tetrahedral sheets for which charge balance is most likely achieved by hydroxylation via a hydrogarnet-like substitution involving the formation of Q² silanol groups. Variable-contact-time ²⁹Si {¹H} CP/MAS NMR spectra of the talc/10-Å phase product support the assignment of Q² Si to the proposed SiO₃(OH) groups. Electron microprobe analysis, including oxygen, gives the following empirical formula normalized to three Mg apfu and inferring a hydrogarnet component Si 4H associated with Si vacancies: Mg₃Si_3.83(8)O_9.32(OH)_2.68.1.1(4)H₂O. The observed Mg:Si indicates a significant Si deficiency relative to talc. Comparison of the ²⁹Si MAS NMR and microprobe data indicates that Si vacancies likely occur as single isolated entities, rather than as pairs or clusters, and that between 1 in 18 and 1 in 23 Si sites is vacant. The results suggest new and intriguing possibilities for the incorporation of excess H into the 10-Å phase and, potentially, other phyllosilicates under upper-mantle conditions.

Key Words: 10-Å phase • Si vacancies • ²⁹Si NMR spectroscopy • silanol

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