High-pressure Raman spectroscopic studies of hydrous wadsleyite II

Raman spectra in the range 80 to 4000 cm^-1 of wadsleyite II (Fo₉₀ with 2.0 wt% H₂O and Fo₈₈ with 2.7 wt% H₂O) have been measured in a diamond-anvil cell with solid rare-gas pressure-transmitting media to 51.4 GPa at room temperature. The ambient Raman spectrum of wadsleyite II is closely similar to wadsleyite modified with bands in frequency regions where the SiO₄ tetrahedral and OH stretching vibrations of hydrous ringwoodite occur. The most intense, characteristic wadsleyite II modes at 709 and 911 cm^-1 (Si₂O₇ and SiO₃ symmetric stretching vibrations, respectively) shift continuously to 51.4 GPa showing no evidence for a change in the crystal structure. A striking feature in the high-pressure Raman spectra of wadsleyite II is a significant growth in intensity in the mid-frequency range (300-650 cm^-1 at 10^-4 GPa and 400-750 cm^-1 at 51.4 GPa) under compression accompanied by the appearance of new Raman modes near 40 GPa, perhaps a result of resonance electronic Raman scattering. In the OH stretching frequency range, the Raman spectrum of wadsleyite II exhibits at least six modes and their high-pressure behavior agrees with that of Fo₉₀ hydrous wadsleyite: OH stretching modes above 3530 cm^-1 remain approximately constant up to at least 21.8 GPa whereas OH modes at frequencies <3530 cm^-1 decrease with increasing pressure. The OH stretching modes are consistent with protonation of the non-silicate oxygen O2 and the O atoms surrounding the partially vacant tetrahedral site Si2, as suggested from X-ray diffraction data.