Local Phenomena in metamict Titanite
|U. Bismayera, C. Paulmanna, L. Groatb and M. Zhangc
a Mineralogisch-Petrographisches Institut, Universität Hamburg, Grindellee 48, 20146 Hamburg, Germany
b Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada
c Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, U.K.
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|Pure titanite is a mineral with chemical composition CaTiSiO5. Its structure consists of corner linked TiO6-octahedra, SiO4-tetrahedra and sevenfold coordinated Ca positions. In nature various impurities like U and Th can be incorporated in the structure showing alpha and beta radiation. On metamictization, a process which occurs in nature when a mineral is exposed to radioactive irradiation, strong modifications of the structure are observed. Recoil processes due to alpha radiation change over geological time scales the originally periodically structured material into a quasi-amorphous state with persisting short-range order but destroyed long-range order. We report IR and Raman spectra as well as diffraction data of metamict and heat treated titanite from the Cardiff mine, Canada. IR as well as the Raman modes are strongly broadened in the metamict material and sharpen on annealing. The OH-stretching mode at 3486 cm-1 indicates strong changes in the local environment of OH in metamict titanite. The appearance of the Raman excitations between 620 and 750 cm-1 in metamict titanite, which in IR spectra are due to Ti-O stretching from TiO6 octahedra, indicates the breakdown of the Raman selection rules. This points to the breaking of the octahedral symmetry of TiO6 polyhedra.|
PACS numbers: 23.60.+e, 61.43.-j, 62.23.-c, 78.30.-j, 81.40.-z