Raman spectra of phase E at various pressures and temperatures with geophysical implications

Variations in Raman spectra of phase E were investigated up to 340 kbar at room temperature and in the range 83–870 K at atmospheric pressure. Phase E appears to remain intact up to at least 870 K. However, the OH Raman band of phase E was not observable at temperatures above 473 K during heating, and it was not detected in the recovered sample. The temperature variation in Raman frequencies of phase E also showed abnormal behavior above 473 K. Thus, it is suggested that phase E dehydrated and converted to an imperfect forsterite above 473 K. Only the two most intense Raman bands of phase E were reliably measured in the high- pressure experiments. With the exception of the OH Raman band, the Raman frequencies of phase E increase with increasing pressure but decreased with increasing temperature. The frequency of the OH Raman band increases with increasing temperature, which parallels that observed in both phase A and the hydrous β-phase. The temperature dependency was observed to be linear below 500 K and the pressure dependency to be non-linear within the experimental uncertainties and the range investigated. It appears that phase E may be regarded as the hydrous form of forsterite and it is further suggested that phase E and the hydrous β-phase may be competitive hydrous phases in the upper mantle and transition zone.