Anharmonic properties of Mg (sub 2) SiO (sub 4) -forsterite measured from the volume dependence of the Raman spectrum

The effect of T on the Raman spectrum of synthetic forsterite was studied between 100 and 2000 K, and the data, together with those obtained at high P, are used to discuss the intrinsic arharmonicity of the Raman-active modes. For forsterite, the anharmonicity parameters related to the internal modes of the SiO (sub 4) tetrahedra are significantly different (-0.5/K) from those of the lattice modes (-3 X 10 (super -5) /K). Because of the large T range investigated, the T and V dependencies of the a (sub 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 a (sub i) parameters combined with their small volume dependencies imply that intrinsic anharmonicity in forsterite contributes significantly to entropies and specific heats, even at high P, but has negligible effects on thermal P and high-T equations of state.