We investigated the crystal structural changes in titanite solid-solution Ca(Ti,Al)(O,F)SiO 4 along the binary join TiO-AlF, on the basis of X-ray powder data and Rietveld refinement of seven synthetic titanites of intermediate compositions. Investigations with the transmission electron microscope allow us to narrow down the space group transition from P2 1 /a to A2/a to compositions between X Al = 0.09 and X Al = 0.18 [X Al = Al/(Al+Ti)]. The changes in most of the unit-cell dimensions along the binary join are non-linear, resulting in a small excess volume of mixing with a maximum at X Al = 0.54. The commonly observed trend of positive deviation of the excess volume of mixing near the large end-member, and negative deviation towards the small end-member seems to be reversed in this case. At AlF-contents larger than X Al = 0.6 the Ca-site and the O1-site in the titanite structure become increasingly over-bonded with Al-F substitution. At about X Al = 0.4 the octahedral cation-oxygen distances change significantly, indicating that the titanite structure undergoes a major atomic rearrangement at high AlF-contents in order to accommodate the increasingly different ionic size and charge. Generally, with increasing AlF content the polyhedra are being deformed rather than rotated. The changes in unit-cell dimensions, bond lengths and bond valence sums along the binary join suggest the presence of structural strain in AlF-rich titanite, especially at Al-F contents exceeding X Al = 0.4. The structural problems are obviously not significant enough to prevent the formation of Al-rich titanite in simple chemical systems as in our experiments. However, the structural strain may be significant enough to decrease the thermodynamic stability of Al-rich titanite in natural rocks compared to other Al- and F-bearing phases. This could partly explain the rare natural occurrence of titanite with X Al >0.54.

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