Si vacancies in the 10-A phase

29 Si MAS NMR spectroscopy on samples of 10-A 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 Q 2 -type Si sites in a ratio Q 3 :Q 2 of 5.33:1. It is proposed that the Q 2 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 Q 2 silanol groups. Variable-contact-time 29 Si { 1 H} CP/MAS NMR spectra of the talc/10-A phase product support the assignment of Q 2 Si to the proposed SiO 3 (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: Mg 3 Si 3.83(8) O 9.32 (OH) 2.68. 1.1(4)H 2 O. The observed Mg:Si indicates a significant Si deficiency relative to talc. Comparison of the 29 Si 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-A phase and, potentially, other phyllosilicates under upper-mantle conditions.