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, Tc, is about 825 K and increases with increasing impurity concentration.

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