Aluminum-rich titanites [Ca(Ti,Al)(O,F)SiO 4 ] with XAl>0.53 [XAl = Al/(Al+Ti)], including the pure end-member CaAlFSiO 4 , were synthesized for the first time in a high-pressure experimental study. The crystal structure of CaAlFSiO 4 was determined by Rietveld analysis of an X-ray powder diffraction pattern. CaAlFSiO 4 is monoclinic, belongs to the space group A2/a, and has the unit-cell dimensions a = 6.9149(2) Aa, b = 8.5064(1) Aa, c = 6.4384(2) Aa, and beta = 114.684(2) degrees . The unit-cell volume is less than 93% of CaTiOSiO 4 , 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 CaTiOSiO 4 and CaAlFSiO 4 . The similarity of the crystal structures of titanite and CaAlFSiO 4 explains why in natural Al-rich titanite the end-member CaAlFSiO 4 generally dominates over the hypothetical end-member CaAlOHSiO 4 , which under geological conditions is stable in a different crystal structure.

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