High-pressure and high-temperature Raman spectroscopic study of hydrous wadsleyite (β-Mg2SiO4)

In situ Raman spectra of hydrous wadsleyite (beta-Mg2SiO4) with similar to 1.5 wt% H2O, synthesized at 18 GPa and 1,400 degrees C, have been measured in an externally heated diamond anvil cell up to 15.5 GPa and 673 K. With increasing pressure (at room temperature), the three most intense bands at similar to 549, 720 and 917 cm(-1) shift continuously to higher frequencies, while with increasing temperature at 14.5 GPa, these bands generally shift to lower frequencies. The temperature-induced frequency shifts at 14.5 GPa are significantly different from those at ambient pressure. Moreover, two new bands at similar to 714 and similar to 550 cm(-1) become progressively significant above 333 and 553 K, respectively, and disappear upon cooling to room temperature. No corresponding Raman modes of these two new bands were reported for wadsleyite at ambient conditions, and they are thus probably related to thermally activated processes (vibration modes) at highpressure and temperature conditions.