HIGH-TEMPERATURE RAMAN-SPECTROSCOPY OF SILICATE AND ALUMINOSILICATE HYDROUS GLASSES - IMPLICATIONS FOR WATER SPECIATION

Unpolarized Raman spectra of hydrous silica and quartzofeldspathic gla sses containing 2.8 and 2.0 wt% H2O, respectively, have been recorded at 1 arm between room temperature and 700 degrees C in order to invest igate the effect of temperature (T) on the speciation of water. High-T spectra and room-ir spectra were collected alternatively and compared together, allowing the determination of the temperature at which wate r started to diffuse out of the glasses. For both compositions this te mperature was found to be > 480 degrees C, In the temperature range 25 -480 degrees C (no loss of water), the low-frequency range spectra (50 0-1400 cm(-1)) do not show important variations with T. In the silica glass, the integrated intensity of the 970-cm(-1) band is not affected by T. In contrast, important variations are observed in the high-freq uency range (2800-3800 cm(-1)) at temperatures as low as 160 degrees a nd 120 degrees C in silica and quartzofeldspathic glasses, respectivel y. In both samples the intensities of the bands in the range 3200-3500 cm(-1), attributed to molecular water, are decreasing with T. Concomi tantly, an intensity increase is observed for the narrow band at 3597 cm(-1) in the silica glass and for a band centered at similar to 3580 cm(-1) in the quartzofeldspathic glass. These variations are fully rev ersible (time scale of < 1 min) when T decreases. The observed changes can be interpreted to be due to a collapse of hydrogen bonding involv ing molecular water. In the silica glass, the 3597-cm(-1) band, showin g increasing intensities and frequencies with T, may be assigned to a symmetric mode involving at least two stretching vibrations from both hydroxyl group (SiO-H) and molecular water. This vibrational mode is e nhanced by the reaction leading to the breaking of hydrogen bonds.