The crystal-chemistry of high-aluminium titanites

Abstract

A crystal-chemical study of high-aluminium titanites , where X Al =Al/(Al+Ti+Fe 3+ )≥0.25, has been made using single-crystal X-ray diffractometry on four samples from three different rock-types and localities. The first refinement of a natural pure CaTiOSiO 4 titanite in the P2 1 /a space group is also presented. All of the high-aluminium titanites studied have A2/a space group symmetry with linear trends between geometrical and chemical parameters. Increasing Al content provokes a strong contraction of the octahedral site geometry : as X Al approaches 0.50, the mean distance in titanites reaches that of the chemical end-member vuagnatite, in which the octahedral chains are not corner-sharing but edge-sharing. The distance decreases linearly with , but the SiO 4 tetrahedra remain constant in size. The contraction of the octahedron does not explain by itself the apparent structural limit to Al substitution at around X Al =0.50 previously suggested from experimental studies and natural observations; this limit, if it is real, is probably related to problems with local charge balancing and/or with structural matching