H IN RUTILE-TYPE COMPOUNDS .2. CRYSTAL-CHEMISTRY OF AL SUBSTITUTION IN H-BEARING STISHOVITE

The crystal structures of a synthetic H-bearing aluminous stishovite, containing approximately 1.6 wt% Al2O3, and a pure-silica stishovite h ave been refined from three-dimensional, single-crystal X-ray diffract ion data. The space group of both is P4(2)/mnm, and the unit cell para meters of the aluminous crystal are a = 4.1839(8) and c = 2.6684(6) An gstrom, which are slightly larger than those of pure silica [a = 4.177 3(4) and c = 2.6652(2) Angstrom]. The Al-bearing octahedron is slightl y larger and more regular than that of pure-silica stishovite. Unit ce ll and atom position refinement are consistent with 1.6% substitution of Al for Si. The observation of 1.9% Al substitution for Si-[6], even without clear charge balance, is evidence that this substitution may be significant in many lower mantle silicate phases.