Abstract

A synthetic analogue, Ca(Ti0.6Al0.2Ta0.16Nb0.04)OSiO4, of an aluminoan tantalian titanite previously described from Craveggia pegmatite (Piemonte, Italy) has been prepared by a ceramic synthesis technique and its crystal structure determined by Rietveld analysis of the powder X-ray diffraction pattern. The synthetic Al-Ta-Nb-rich titanite adopts space group A2/a implying that substitutions at the single octahedral site destroy the coherence of the off-centering of octahedral chains in the titanite structure resulting in a P21/aA2/a phase transition. Unit-cell dimensions obtained for the Al-Ta-Nb- rich titanite are: a = 7.0649(1) Å; b = 8.7187(1) Å ; c = 6.5701(1) Å ; ß = 113.755(1)°, V = 370.41(1) Å3. The extensive replacement of Ti by Al, Ta and Nb results in a considerable decrease in the distortion of all coordination polyhedra in the structure of this titanite. These structural data suggest that a solid solution CaTi1–x(Alx/2[Ta,Nb]x/2)OSiO4 (0 ⩽ x ⩽ 0.4) adopting the titanite structure might exist.

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