Anharmonic properties of Mg2SiO4-forsterite measured from the volume dependence of the Raman spectrum
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 v(i) is measured through a parameter (a(i)) = (partial derivative ln v(i)/partial derivative T)(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 a(i) 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 al 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.