Phase transformation of natural titanite: An infrared, Raman spectroscopic, optical birefringence and X-ray diffraction study

Abstract The thermal behaviour of natural titanite has been studied using infrared and Raman spectroscopy, linear birefringence and synchrotron X-ray diffraction. Natural single crystals from Rauris, Austria (Fe 1.8%. Al 3.8%) were investigated and two additional samples from the Smithonian Institute Washington (Fe 2.8%, Al 0.5% and Fe 8.8%, Al 13.9%) served for IR and birefringence measurements. The coupled substitution of Fe and Al for Ti and OH and F for O leads to the formation of long ranging defect fields which are assumed to account for an increased density of antiphase boundaries. Infrared absorption spectra show increased bandwidths with increasing defect concentration. The phase transition near 500 K is clearly seen in the IR spectra of samples with low defect concentrations but not for those with high defect densities. X-ray diffraction shows strong anisotropic diffuse scattering in the Rauris sample leading to the average symmetry A2/a at room-temperature. A second anomaly at ca. 850 K was fou...