ORIGINAL PAPER Ruslan P. Liferovich Æ Roger H. Mitchell Tantalum-bearing titanite: synthesis and crystal structure data Received: 12 July 2005 / Accepted: 18 September 2005 / Published online: 8 March 2006 Ó Springer-Verlag 2006 Abstract Synthetic titanite, CaTiOSiO 4 , and the series of (Ca 1x Na x )(Ti 1x Ta x )OSiO 4 and Ca(Ti 12x Ta x Al x ) OSiO 4 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% NaTaOSiO 4 or 60 mol% Ca(Al 0.5 Ta 0.5 )OSiO 4 . 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 vii X- and vi Y-sites in the structures of tantalian titanite are disordered. Expansion of the <Si–O> bond from 1.618 to 1.621 A ˚ in CaTi 0.8 Ta 0.1 Al 0.1 OSiO 4 and CaTi 0.6 Ta 0.2 Al 0.2 OSiO 4 to 1.644 A ˚ in the CaTi 0.4 Ta 0.3 Al 0.3 OSiO 4 titanite suggests the possible presence of some Al 3+ in the tetrahedral site replacing Si 4+ 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 P2 1 /a fi A2/a phase transition(s). The (Ca 1x Na x )(Ti 1x Ta x )OSiO 4 substi- tution scheme incorporates larger cations at both the vii X and vi Y sites, whereas the Ca(Ti 12x Ta x Al x )OSiO 4 scheme involves only vi Y-site (Al 3+ ,Ta 5+ ) cations with a slightly smaller ‘‘average’’ radius. Unit cell dimensions change insignificantly or increase incrementally with increase of average cationic radii in the (Ca 1x Na x ) (Ti 1x Ta x )OSiO 4 series, and with an insignificant decrease in the vi R Y average cationic radii in the Ca(Ti 12x Ta x Al x )OSiO 4 series. Both Ta-doped titanite and CaTiOSiO 4 consist of distorted polyhedra with the XO 7 , YO 6 coordination polyhedra and the SiO 4 tetra- hedron in tantalian titanite being less distorted com- pared to those of the pure CaTiOSiO 4 . Keywords Titanite Æ Tantalum Æ Aluminium Æ Natrium Æ Crystal structure Introduction Titanite, CaTiOSiO 4 , is an important accessory phase in igneous and metamorphic rocks and a major rock- forming mineral in highly differentiated agpaitic nephe- line syenite. Typically, the composition of igneous tita- nite approaches the theoretical stoichiometry, whereas in contrast, titanite formed during diverse postmagmatic processes is known to incorporate significant amounts of Zr, Nb and Ta (Sahama 1946;C ˇ erny´ and Riva di Sanseverino 1972; Clark 1974; Paul et al. 1981; Woolley et al. 1992; Russel et al. 1994;C ˇ erny´ et al. 1995; Chakhmouradian et al. 2003, 2004; Brigatti et al. 2004; Liferovich and Mitchell 2005a). Niobian titanite occurs primarily in postmagmatic parageneses in nepheline syenite and carbonatite (Chakhmouradian et al. 2003; Chakhmouradian 2004; Brigatti et al. 2004; Liferovich and Mitchell 2005a). Tantalian titanite, commonly also enriched in Nb, is known from siliceous pegmatites, where it typically forms as a product of subsolidus alteration of primary tantalo-niobates (columbite-tat- natlite, fersmite, ixiolite, microlite, stru¨verite, etc.) and (Nb,Ta)-rich rutile (C ˇ erny´ and Riva di Sanseverino 1972; Clark 1974; Paul et al. 1981;C ˇ erny´ et al. 1995). Both niobium and tantalum enter the structure of tita- nite according to two major substitutional schemes (Table 1). In natural titanite these schemes can occur in conjunction with the presence of minor vacancies and/or fluorine or hydroxyl replacing up to one atom of oxygen per a formula unit (apfu). The greatest concentration of tantalum in a natural titanite described to date is 21.5 wt% Ta 2 O 5 (0.22 apfu Ta) in combination with up to 9.49 wt% Nb 2 O 5 (0.16 apfu Nb), with a maximum of 0.36 apfu (Ta,Nb) 2 O 5 (C ˇ erny´ et al. 1995). Natural samples of tantalian and niobian titanite are commonly inhomogeneous (Clark 1974;C ˇ erny´ et al. 1995; Chakhmouradian 2004; Liferovich and Mitchell R. P. Liferovich Æ R. H. Mitchell (&) Department of Geology, Lakehead University, 955 Oliver Road, P7B 5E1, Thunder Bay, Ontario, Canada E-mail: rmitchel@lakeheadu.ca Tel.: +1-807-3438287 Fax: +1-807-3467853 Phys Chem Minerals (2006) 33: 73–83 DOI 10.1007/s00269-006-0069-y