Anharmonic properties of Mg2SiO4-forsterite measured from the volume dependence of the Raman spectrum
Gillet, Philippe; Daniel, Isabelle; Guyot, Francois
European Journal of Mineralogy Volume 9 Number 2 (1997), p. 255 - 262
published: Jun 26, 1997
manuscript accepted: Oct 30, 1996
manuscript received: Jun 12, 1996
ArtNo. ESP147050902003, Price: 29.00 €
Abstract The effect of temperature on the Raman spectrum of Mg2SiO4-forsterite is investigated between 100 and 2000 K. The data, combined with those obtained at high pressure, are used to discuss the intrinsic anharmonicity of the Raman-active modes. Anharmonicity of a mode of frequency Vj is measured through a parameter (a1) = (əln Vj/əT)v calculated from the measured temperature and pressure shifts of the vibrational frequency. In forsterite, these parameters are negative, of the order of -10-5 K-1. Those related to the internal modes of the SiO4 tetrahedra are significantly different (-0.5 10-5 K-1) from those of the lattice modes (-3 10-5 K-1). Because of the large temperature range investigated in this study, the temperature and volume dependencies of the ai parameters could be estimated for the first time in a mineral, and are shown to be negligible, at least up to 2000 K. The significant non-zero values of the ai parameters combined with their small volume dependencies imply that intrinsic anharmonicity in forsterite contributes significantly to entropies and specific heats, even at high pressures, but has negligible effects on thermal pressure and high-temperature equations of state.