Micro-Raman and infrared spectral study of forsterite under high pressure

Raman and mid-IR spectra of synthetic forsterite (Mg₂SiO₄) have been recorded under pressure to about 200 kbar. Multichannel micro-Raman spectroscopy has made it possible to measure the forsterite spectra with high signal to noise ratio. Twenty-one bands are identified in the Raman spectrum of forsterite. Pressure dependencies of the frequencies of two of the Raman bands have been determined for the first time. All of the Raman bands move progressively to higher frequencies with increasing pressure. The frequency shift displays a linear relation with respect to pressure up to 70 kbar. The Raman peak at 585 cm⁻¹, usually attributed to a single vibrational mode, is in fact a doublet. The pressure derivatives of the bands in 585-cm⁻¹ doublet are slightly different but do not change abruptly in the pressure range studied in our work. We also found that two Raman modes (424 and 441 cm⁻¹) are still present in our spectra in the pressure range of 100–200 kbar, in contrast to previously reported results. There is a slight break in the slopes of the five Raman bands in the pressure range 70–90 kbar. These changes in the slope are attributed to a change in the compression mechanism of forsterite at high pressures. Frequencies of six infrared-active internal modes of forsterite change linearly with pressure to 184 kbar, in agreement with the results of previous workers. The pressure dependencies of the infrared bands reported by various workers, however, show much larger variations than those of corresponding Raman bands. Observed changes in the relative intensities of the infrared bands at high pressures are attributed to the deformation of the SiO₄ tetrahedra above 81 kbar.