Raman spectra of hydrous γ-Mg2SiO4 at various pressures and temperatures

One well-defined OH Raman band at 3651 +/- 1 cm(-1) and one weak feature near 3700 +/- 5 cm(-1) are recognized for the hydrous gamma-phase of Mg(2)SiO(4). Like the hydrous beta-phase, the H(2)O content in the gamma-phase shifts most of the corresponding silicate modes towards lower frequencies. Variations in Raman spectra of the hydrous beta-phase were investigated up to about 200 kbar at room temperature and in the range 81-873 K at atmospheric pressure. Unlike the anhydrous gamma-phase, which remains intact up to at least 873 K, the hydrous gamma-phase sometimes converts to a defective forsterite structure above 800 K. Although the hydrous gamma-phase remains intact up to at least 800 K. Raman signals of the OH bands disappear completely above 423 K. The Raman frequency of the well-defined OH band decreases linearly with increasing temperature between 81 and 423 K. In the region of the silicate vibrations, the Raman frequencies of the two most intense bands increase nonlinearly with increasing pressure, and decrease with increasing temperature. The frequencies for all other weak bands, however, decreased linearly with increasing temperature. The latter most likely reflects the larger scatter of the data for the weak bands.