Raman spectra of forsterite and fayalite at high pressures and room temperature

Raman spectra of the two pure end-members of olivine (forsterite and fayalite) were studied at high pressures and room temperature in a diamond-anvil cell using both single-crystals and polycrystalline samples in pressure mediums of either an ethanol-methanol mixture or H2O. Variations with pressure were studied up to 170kbar for fayalite and to 300kbar for forsterite. Two intensive Raman bands of fayalite were definitely observable at high pressures, but only one of them can be reliably determined. Both have a linear variation within experimental uncertainty. Because of interference from the high spectral background, we found that nearly all the weak bands of forsterite could not be reliably determined at high pressures. However, the pressure variations of all bands of forsterite which can be reliably determined are non-linear. The rates of frequency change for the intense bands of forsterite determined in the present experiment are consistent with those of natural forsterites determined by Besson et al. 1 and Gillet et al. 2, but are in a slight discordance with those reported by Chopelas3. Furthermore, there is no evidence for the olivine ↔ spinel phase transition occurring at room temperature.