Solid solution of rare earth elements in synthetic titanite: a reconnaissance study

Summary Titanite varieties doped with rare earth elements (REE) have been prepared by ceramic synthesis and quenching in air. Their crystal structure was determined by Rietveld analysis of the powder X-ray diffraction patterns. Two different substitution schemes, Ca_1–xNa_x/2Sm_x/2TiSiO₅ and Ca_1–xDy_xTi_1–x SiO₅, are studied at x=0.2. Both synthetic varieties of titanite adopt space group A2/a. This implies that both single-site and complex multivalent substitutional schemes 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 REE-bearing titanite varieties are as follows: a=7.0541(1)Å; b=8.7247(1)Å; c=6.5664(1)Å; =113.732(1)° for Ca_0.8Na_0.1Sm_0.1TiSiO₅; and a=7.0021(1)Å; b=8.7256(1)Å; c=6.5427(1)Å; =113.294(1)° for . Both REE-doped titanite samples and a control sample of the pure titanite end member have similar unit cell parameters and consist of polyhedra distorted to a similar extent with the exception of more-distorted SiO₄ tetrahedron in CaSiTiO₅. The structural data suggest that the Ca_1–xNa_x/2Sm_x/2TiSiO₅ and Ca_1–xDy_xTi_1–xFe_xSiO₅ solid solutions adopting the titanite structure might extend to x sufficiently greater than 0.2 and involve both heavier and lighter trivalent rare earth elements.Permanent address: Geological Institute KSC RAS, 14 Fersmana St., Apatity, 184200 Russia