Unravelling the incorporation mechanisms of water in aluminous orthoenstatite: II. comprehensive 1H, 29Si and 27Al NMR measurements

Aluminum has been shown to significantly enhance the water incorporation capacity of orthoenstatite (OEn), but the incorporation mechanisms remained to be clarified. We performed a comprehensive one- and two-dimensional ¹H, ²⁹Si and ²⁷Al NMR study on four hydrous aluminous OEn samples containing 1–8 wt% Al₂O₃ synthesized at 1.5 GPa and 900 °C to clarify the issue. The combined ¹H MAS and static (non-spinning) NMR, ¹H double-quantum and triple-quantum MAS NMR, and ²⁷Al→¹H CP MAS NMR and HETCOR results, in particular, unambiguously revealed that a large part of the incorporated water are present as proton pairs in Mg vacancies adjacent to Al, with one proton of each pair for the dominant proton pairs exhibiting significantly weaker hydrogen bonding than those in Al-free OEn. Proton pairs in Mg vacancies remote from Al are minor or absent for samples with 4–8 wt% Al₂O₃, and more abundant for a sample with 1 wt% Al₂O₃. Isolated protons due to coupled substitutions of Al + H for 1Si and 2Mg (both with weak hydrogen bonding) were also detected, but are less abundant than hitherto considered. The observed NMR peaks match well with those predicted for the corresponding OH defect models from our first-principles calculations. Thus, the enhancement of water solubility by Al for OEn are due to not only coupled substitutions of Al + H for 1Si and 2Mg, but also interactions of Al with proton pairs in Mg vacancies. These mechanisms may also be important in other nominally anhydrous aluminous silicate minerals.