TANTALUM-BEARING TITANITE: SYNTHESIS AND CRYSTAL STRUCTURE DATA

Abstract

Synthetic titanite, CaTiOSiO4, and the series of (Ca1-xNax)(Ti1-xTax) OSiO4 and Ca(Ti1-2xTaxAlx) OSiO4 solid solutions have been prepared by ceramic methods, and their crystal structure determined by the Rietveld analysis. At ambient conditions, titanite can contain up to 20 mol% NaTaOSiO4 or 60 mol% Ca(Al0.5Ta0.5) OSiO4. These limits might differ in natural samples due to combination with substitutions involving fluorine and/or hydroxyl replacing oxygen together with vacancies at cationic sites. All cations located at the viiX- and viY-sites in the structures of tantalian titanite are disordered. Expansion of the <si-o> bond from 1.618 to 1.621 Å in CaTi0.8 Ta0.1Al0.1OSiO4 and CaTi0.6 Ta0.2Al0.2OSiO4 to 1.644 Å in the CaTi0.4Ta0.3Al0.3OSiO4 titanite suggests the possible presence of some Al3+ in the tetrahedral site replacing Si4+ in the latter. All tantalian titanites crystallize in the space group A2/a. This implies that both single-site and complex double-site substitutional schemes induce P21a → A2/a phase transition(s). The (Ca1-x Nax)(Ti1-xTax)OSiO4 substitution scheme incorporates larger cations at both the viiX and viY sites, whereas the Ca(Ti1-2xTaxAlx)OSiO4 scheme involves only viY-site (Al3+,Ta5+) cations with a slightly smaller "average" radius. Unit cell dimensions change insignificantly or increase incrementally with increase of average cationic radii in the (Ca1-x Nax)(Ti1-xTax)OSiO4 series, and with an insignificant decrease in the viRY average cationic radii in the Ca(Ti1-2xTax Alx)OSiO4 series. Both Ta-doped titanite and CaTiOSiO4 consist of distorted polyhedra with the XO7, YO6 coordination polyhedra and the SiO4 tetrahedron in tantalian titanite being less distorted compared to those of the pure CaTiOSiO4. © Springer-Verlag 2006.