Natural titanite and malayaite: Structural investigations and the 500 K anomaly

Titanite, CaTiSiOS, and malayaite, CaSnSiOs, are isostructural. Both minerals exhibit a thermal anomaly near 500 K but diffuse reflections only occur in titanite. Titanite crystals from Rauris, Austria (Fe 1.8%, A1 3.8%) were studied using synchrotron X-ray diffraction, IR spectroscopy and high-resolution transmission electron microscopy (HRTEM). The phase transformation near 500 K is smeared out by impurities. The diffuse scattering disappears above 830 K. No antiphase boundaries were found in natural titanite. High-temperature X-ray diffraction and IR spectroscopy were used to study the thermal anomaly in malayaite. Structure determinations of malayaite at different temperatures reveal a discontinuity in the evolution of the anisotropic Debye-Waller factors of the Ca atoms near 500K. At this point IR spectra show a break in the temperature dependence of the intensity of the 533cm−1mode and the peak positions of the 499cm−1mode and the Si-0 stretching mode near 908cm−1