Raman spectroscopic study of hydrous wadsleyite (beta-Mg2SiO4) to 50 GPa

Raman spectra of hydrous beta -Mg2SiO4 (1.65 wt% H2O) have been measured in a diamond-anvil cell with helium as a pressure-transmitting medium at room temperature to 50 GPa. We observe three OH-stretching modes, a doublet wit h components at 3329 and 3373 cm(-1), which decrease linearly with pressure , and a single mode at 3586 cm(-1), which remains nearly constant up to 24 GPa before decreasing at higher pressures. Assessment of the mode frequenci es and their pressure dependence, together with previous results from X-ray and IR data, are consistent with protonation of the O1 site in agreement w ith previous studies. Strict assignment of Raman activity awaits detailed s tructural models. The nature of the protonation in wadsleyite may require m ore specific experimental probes for full solution of the hydrogen-site pro blem.