PHASE-TRANSFORMATION OF NATURAL TITANITE - AN INFRARED, RAMAN-SPECTROSCOPIC, OPTICAL BIREFRINGENCE AND X-RAY-DIFFRACTION STUDY

The thermal behaviour of natural titanite has been studied using infra red and Raman spectroscopy, linear birefringence and synchrotron X-ray diffraction. Natural single crystals from Rauris, Austria (Fe 1.8%, A l 3.8%) were investigated and two additional samples from the Smithoni an Institute Washington (Fe 2.8%, Al 0.5% and Fe 8.8%, Al 13.9%) serve d for LR and birefringence measurements. The coupled substitution of F e and Al for Ti and OH and F for O leads to the formation of long rang ing defect fields which are assumed to account for an increased densit y of antiphase boundaries. Infrared absorption spectra show increased bandwidths with increasing defect concentration. The phase transition near 500 K is clearly seen in the TR spectra of samples with low defec t concentrations but not for those with high defect densities. X-ray d iffraction shows strong anisotropic diffuse scattering in the Rauris s ample leading to the average symmetry A2/a at room-temperature. A seco nd anomaly at ca. 850 K was found by X-ray diffraction techniques usin g synchrotron radiation and by a break in the temperature evolution of the IR spectra.