Aluminum substitution in stishovite and MgSiO3 perovskite: High-resolution 27Al NMR

This is the authors' abstract. We don't add key points for this paper.

Aluminum is an important minor constituent of a number of high-pressure mantle silicates in which it substitutes for octahedrally coordinated silicon. In several cases, its solid solution may be linked to the presence of oxygen vacancies; in others, to charge balance with H + . Here we present new data from high-resolution, high-field (18.8 Tesla) 27 Al NMR of aluminous stishovite and of a non-stoichiometric perovskite with nominal composition MgSi 0.95 Al 0.05 O 2.975 . For the stishovite, we characterize the local structure of the symmetrical, octahedral site for Al. These results, combined with 27 Al{ 1 H} REDOR NMR, are consistent with hypothesized H + charge balance, although the presence of a significant fraction of randomly distributed oxygen vacancies could remain undetected. As in a recent previous study of a related perovskite composition, the observed ratio of Al at symmetrical octahedral B sites to that of Al at large, central A sites is about 2:1, indicating the presence of oxygen vacancies to account for charge neutrality in this phase. Such vacancies are not preferentially associated with the Al octahedra, however, suggesting a random distribution in the structure.