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

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1998

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Titanite, CaTiSiO5, and malayaite, CaSnSiO5, 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%, Al 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 500 K. At this point IR spectra show a break in the temperature dependence of the intensity of the 533 cm-1 mode and the peak positions of the 499 cm-1 mode and the Si-O stretching mode near 908 cm-1. © 1998 OPA (Overseas Publishers Association) N.V. Published by license under the Gordon and Breach Science Publishers imprint.

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Forschungsinformationssystem der UHH

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