H in rutile-type compounds: II. 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 have been refined from three-dimensional, single-crystal X-ray diffraction data. The space group of both is P4₂/mnm, and the unit cell parameters of the aluminous crystal are a = 4.1839(8) and c = 2.6684(6) Å, which are slightly larger than those of pure silica [a = 4.1773(4) and c = 2.6652(2) Å]. The Al-bearing octahedron is slightly larger and more regular than that of pure-silica stishovite. Unit cell and atom position refinement are consistent with 1.6% substitution of Al for Si. The observation of 1.9% Al substitution for ^[6]Si, even without clear charge balance, is evidence that this substitution may be significant in many lower mantle silicate phases. -Authors