Structural phase transition near 825 K in titanite: Evidence from infrared spectroscopic observations

We report the direct experimental observation of a structural anomaly near 825 K in synthetic and natural titanite samples by high-temperature, hard-mode infrared spectroscopy. The anomaly in titanite, CaTiSiO₅, is characterized by a break of the temperature dependence of the 562 cm^–1 Si-O bending mode, the 675 cnr^–1 Ti-O band, and the 900 cm^–1 Si-O stretching modes, and the rapid decrease of the IR signal at 873 cm^–1. The order parameter, as determined from the temperature evolution of the frequencies of the absorption bands in the middle infrared (MIR) region follows a second-order Landau behavior with an order-parameter exponent β = ½. At T > 825 K, the Ti-O band shows further softening, whereas the Si-O bands at 562 and 900 cm^–1 show hardening with increasing temperature. In natural samples, the effects of impurities on the high-temperature transition are weak. For pure titanite, the transition temperature, T_c, is about 825 K and increases with increasing impurity concentration.