Raman and infrared spectra of phase E, a plausible hydrous phase in the mantle

Abstract

Micro-Raman and micro-infrared spectra of the silicate and principal OH-stretching regions have been obtained from a well-characterized crystal of phase E (Mg2.17Si1.01H3.62O6) containing approximately 18 wt.% H2O. The number of observed infrared and Raman bands exceeds the number predicted by factor-group analysis for a crystal of phase E with space group R3̄m. This finding suggests that some long-range order may exist in phase E, or that it possesses a superstructure that lowers the overall symmetry of the crystal. Although phase E contains layers of brucite-type units linked by SiO4 tetrahedra and MgO6 octahedra, the lower frequencies and the broadness of the OH-stretching bands of phase E (in comparison to brucite) indicate that the hydrogen bonding is stronger than that in brucite. There is a close similarity between the Raman spectra of phase E and forsterite below 1000 cm-1. This may be explained by assuming that the vibrations of the isolated SiO4 tetrahedra dominate the Raman spectrum of phase E below 1000 cm-1. Tentative assignments of the observed bands are made by comparison with computed spectral modes and the corresponding spectra of brucite and forsterite. Show more