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: Mg3Si3.83(8)O9.32(OH) 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.

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