Crystal structure changes in titanite along the join TiO-AlF

We investigated the crystal structural changes in titanite solid-solution Ca(Ti,Al)(O,F)SiO (sub 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 (sub 1) /a to A2/a to compositions between X (sub Al) = 0.09 and X (sub Al) = 0.18 [X (sub 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 (sub 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 (sub 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 (sub 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 (sub 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 (sub Al) >0.54.