A high-temperature and high-pressure Raman spectroscopic study of CaGeO3 garnet |
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| Authors: | T D Chaplin N L Ross B Reynard |
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| Institution: | (1) Research School of Geological and Geophysical Sciences, University College London, Gower Street, London WC1E 6BT, UK e-mail: t.chaplin@ucl.ac.uk Fax: +44 171 387 1612 Tel.: +44 171 504 2397, GB;(2) Laboratoire des Sciences de la Terre CNRS UMR 5570, école Normale Supérieure de Lyon, 46 Allée d'Italie, 69364 Lyon Cedex 07, France, FR |
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| Abstract: | High-pressure and high-temperature Raman spectra of CaGeO3 tetragonal garnet have been collected to 11.5 GPa and 1225 K, respectively, in order to investigate possible intrinsic anharmonic
behaviour in this phase. The Raman peak positions were observed to vary linearly with pressure and temperature within the
ranges studied, with the higher-energy peaks showing larger P- and T-induced shifts than the low energy modes. The observed T-induced shifts are similar to those reported for grossular and andradite, while the observed P-induced shifts are generally larger than those of aluminosilicate and MgSiO3 majorite garnets (Gillet et al. 1992; Rauch et al. 1996) due to the larger bulk modulus of CaGeO3 garnet. The observed mode shifts of CaGeO3 garnet were used to determine the isothermal and isobaric mode Grüneisen parameters for this phase. These parameters are
similar in value to those reported previously for grossular and andradite (Gillet et al. 1992). The calculated intrinsic anharmonic
parameters, a
i
, for CaGeO3 garnet were determined to be nonzero, indicating significant anharmonic behaviour for this phase. These values, which range
from −3.8 × 10−5 K−1 to −1.3 × 10−5 K−1, are also similar to those reported for andradite and grossular, but smaller than those determined for pyrope (Gillet et al.
1992). Hence, we expect MgSiO3 majorite to show greater anharmonicity than the germanate analogue studied by us. The anharmonic parameters determined for
CaGeO3 tetragonal garnet may now be introduced into quasiharmonic vibrational heat capacity models to account for the observed anharmonic
behaviour.
Received: 21 April 1999 / Revised, accepted: 11 September 1999 |
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| Keywords: | Raman spectroscopy CaGeO3 garnet Intrinsic anharmonicity |
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