The beta-gamma phase transition in titanite and the isosymmetric analogue in malayaite

X-ray diffraction technique and hard mode infrared spectroscopy were used t o study-the thermal behaviour of synthetic titanite, CaTiSiO5, two natural titanite samples and isostructural malayaite, CaSnSiO5. Both pure minerals show a thermal anomaly near 500 K. Specific heat measurements show an inter mediate phase in synthetic titanite. Impurities in titanite tend to suppres s the discontinuity near 500 K. The beta-gamma transition near 820 K is not affected by Fe and Al defects in the natural material. Infrared studies in malayaite show a change of the temperature dependence of the phonon freque ncies. X-ray experiments show weak structural rearrangements with heating b ut no evidence of a symmetry breaking phase transition near 500 K The chara cter of the 500 K anomaly in malayaite is more similar to the beta-gamma tr ansition (825 K) than the alpha-beta transition (496 K) in synthetic titani te.