On protons, iron and the high-pressure behavior of ringwoodite

Raman spectra of a single crystal of hydrous Fo(89)-ringwoodite have been measured in a diamond-anvil cell at room temperature to 60 GPa. Strong peaks corresponding to asymmetric and symmetric stretching vibrations of the SiO4 tetrahedra are observed at 796 and 841 cm(-1) respectively. Additional bands occur in the range 709-939 and 100-250 cm(-1). The SiO4 stretching modes of the spinel structure shift continuously up to 60 GPa. Near 40 GPa new Raman bands emerge in the region 550-680 cm(-1) gaining intensity to the maximum pressure, a change that is reversible on pressure release. This pressure-induced modification in the structure appears to be related to structural complexity generated by both Fe and proton substitution.