Raman Spectroscopic Studies of Hydrous and Nominally Anhydrous Deep Mantle Phases

This chapter deals with Raman spectroscopic measurements on both hydrous and nominally anhydrous silicates that are relevant to water cycling and storage in the Earth's mantle. Raman spectroscopy is a sensitive tool for probing structure and chemistry of silicates. It is particularly useful in the identification of OH and H₂O in minerals and in the discrimination between structurally similar phases. Here we present a collection of room-pressure and -temperature Raman spectra and structure models followed by low-temperature Raman spectra of lawsonite, the only known phase containing molecular water groups that is stable below ~150 km depth. We compare the behavior of the Raman active OH stretching frequencies in chlorite across the pressure-induced transition at 9-10 GPa with the results of first-principles calculations, and we also present high-pressure Raman spectroscopic studies of transition zone silicate spinelloids and spinel. The latter studies show that the incorporation of iron (Fe²⁺) into hydrous wadsleyite and ringwoodite may allow resonance electronic Raman scattering processes.