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Crystal structure of a synthetic aluminoan tantalian titanite: a reconnaissance study

R. P. Liferovich and R. H. Mitchell^*

Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, Canada P7B 5E1

^* E-mail: rmitchell{at}lakeheadu.ca

A synthetic analogue, Ca(Ti_0.6Al_0.2Ta_0.16Nb_0.04)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/a A2/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) ų. 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 CaTi_1–x(Al_x/2[Ta,Nb]_x/2)OSiO₄ (0 x 0.4) adopting the titanite structure might exist.

KEYWORDS: titanite, Rietveld analysis, crystal structure, distortion, tantalum, niobium, aluminium