Crystal structural changes in titanite along the join TiO-AlF

We investigated the crystal structural changes in titanite solid-solution C a(Ti,Al)(O,F)SiO4 along the binary join TiO-AlF, on the basis of X-ray powd er data and Rietveld refinement of seven synthetic titanites of intermediat e compositions. Investigations with the transmission electron microscope al low us to narrow down the space group transition from P2(1)/a to A2/a to co mpositions between X-Al = 0.09 and X-Al = 0.18 [X-Al = Al/(Al + Ti)]. The c hanges in most of the unit-cell dimensions along the binary join are non-li near, 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 volu me 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 becom e increasingly overbonded with Al-F substitution. At about X-Al = 0.4 the o ctahedral cation-oxygen distances change significantly, indicating that the titanite structure undergoes a major atomic rearrangement at high AlF-cont ents in order to accommodate the increasingly different ionic size and char ge. 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 XAl = 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 expe riments. However, the structural strain may be significant enough to decrea se the thermodynamic stability of Al-rich titanite in natural rocks compare d to other Al- and F-bearing phases. This could partly explain the rare nat ural occurrence of titanite with X-Al > 0.54.