High-pressure Raman spectroscopic studies of hydrous wadsleyite II

Raman spectra in the range 80 to 4000 cm(-1) of wadsleyite II (Fo(90) with 2.0 wt% H2O and Fo(88) 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 Fo(90) 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.