• journal_title：American Mineralogist
• Contributor：Jonathan F. Stebbins ; Lin-Shu Du ; Kimberly Kelsey ; Hiroshi Kojitani ; Masaki Akaogi ; Shigeaki Ono
• Publisher：Mineralogical Society of America
• Date：2006-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2006.1988
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：91
• issue：2-3
• firstpage：337
• section：Articles

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) ²⁷Al NMR of aluminous stishovite and of a non-stoichiometric perovskite with nominal composition MgSi_0.95Al_0.05O_2.975. For the stishovite, we characterize the local structure of the symmetrical, octahedral site for Al. These results, combined with ²⁷Al{¹H} 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.