The synthesis and crystal structure of CaAlFSiO₄, the Al-F analog of titanite

Aluminum-rich titanites [Ca(Ti,Al)(O,F)SiO₄] with X(Al) > 0.53 [X(Al)=Al/(Al+Ti)], including the pure end-member CaAlFSiO₄, were synthesized for the first time in a high-pressure experimental study. The crystal structure of CaAlFSiO₄ was determined by Rietveld analysis of an X-ray powder diffraction pattern. CaAlFSiO₄ is monoclinic, belongs to the space group A2/a, and has the unit-cell dimensions a = 6.9149(2) Å, b = 8.5064(1) Å, c = 6.4384(2) Å, and β = 114.684(2)°. The unit-cell volume is less than 93% of CaTiOSiO₄, which is consistent with the natural occurrence of Al-rich titanite in high-P rocks. Although previous studies suggested that titanite with X(Al) > 0.5 is possibly not stable, this study demonstrates that complete solid solution occurs between CaTiOSiO₄ and CaAlFSiO₄. The similarity of the crystal structures of titanite and CaAlFSiO₄ explains why in natural Al-rich titanite the end-member CaAlFSiO₄ generally dominates over the hypothetical end-member CaAlOHSiO₄, which under geological conditions is stable in a different crystal structure.