SYNTHESIS METHODS AND UNIT-CELL VOLUME OF END-MEMBER TITANITE (CATIOSIO4)

Unit-cell parameters of synthetic, end-member titanite (CaTiOSiO4) cri tically depend on the synthesis conditions, as is shown for studies re ported in the literature and for new samples reported here. Our study suggests that phase-pure samples are likely to be obtained only if the y are synthesized entirely below the solidus. In contrast, samples syn thesized either directly from melt or by annealing of glass tend to ha ve higher unit-cell volumes, contain Si-rich and Ca-Si-rich phase impu rities, and may be nonstoichiometric. The observed variations in cell parameters among the samples strongly correlate with synthesis methods and can be explained by vacancies in the Ca or Si site or both. This result is particularly important because the thermodynamic properties currently in use for titanite are based on samples synthesized from me lts of stoichiometric composition and thus are suspect even though the y have been determined carefully. To establish a reference point for f uture studies concerned with the chemical and physical properties of t his material we report our findings along with a redetermination of th e unit-fell parameters [alpha = 7.062(1), b = 8.716(2), and c = 6.559( 1) Angstrom; beta = 113.802(9)degrees, V = 369.4(3) Angstrom(3)] from powder X-ray data of synthetic, stoichiometric titanite.