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A crystal-chemical study of "high-aluminium titanites", where XA1 = AI/(AI + Ti + Fe3+) �0.25, has been made using single-crystal X-ray diffractometry on four sampies from three different rock-types and localities. The first refinement of a natural pure CaTiOSi04 titanite in the Y2j/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 AI content provokes a strong contraction of the octahedral site geometry : as XA1 approaches 0.50, the mean < oct-O > distance in titanites reaches that of the chemical end-member vuagnatite, in which the octahedral chains are not corner-sharing but edge-sharing. The < Ca-O > distance decreases linearly with < oct-O >, but the Si04 tetrahedra remain constant in size. The contraction of the octahedron does not explain by itself the apparent structural limit to AI substitution at around XA1 = 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.

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