High-temperature properties of geikielite (MgTiO3-ilmenite) from high-temperature high-pressure Raman spectroscopy — Some implications for MgSiO3-ilmenite |
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| Authors: | B Reynard F Guyot |
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| Institution: | 1. Laboratoire de Minéralogie Physique, Géosciences Rennes, UPR 4661, CNRS, Campus de Beaulieu, 35042, Rennes Cédex, France
2. Laboratoire des Géomatériaux, Institut de Physique du Globe, 5 Place Jussieu, 75262, Paris cédex, France
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| Abstract: | The Raman spectra of geikielite (MgTiO3-ilmenite) have been recorded at high pressure (up to 27 GPa) and at high temperature (up to 1820 K). No phase transitions could be evidenced in both cases. In particular, no cation disordering can be evidenced from the high temperature spectra. The observed Raman wavenumber shifts with pressure and with temperature are used to calculate the intrinsic mode anharmonic parameters. The low absolute values of these parameters indicate that geikielite has a nearly quasi-harmonic behaviour, at least to moderate temperatures. However, systematics and the temperature evolution of Raman linewidths suggest that the absolute values of the anharmonic parameters increase at high temperatures. Anharmonic corrections are applied to Kieffer modelling of the constant volume heat capacity of geikielite. They amount to +4 J · mol-1 · K-1 at 1800 K, i.e. are much lower than those inferred for, for instance, olivine and garnet structures. These results are used to discuss some implications on the phase relations of the high-pressure MgSiO3-ilmenite, and the factors controlling the occurence of order-disorder transitions in ilmenite structures. |
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