High-temperature Raman spectroscopy of silicate and aluminosilicate hydrous glasses: Implications for water speciation

Abstract

Unpolarized Raman spectra of hydrous silica and quartzofeldspathic glasses containing 2.8 and 2.0 wt% H 2O, respectively, have been recorded at 1 atm between room temperature and 700°C in order to investigate the effect of temperature ( T) on the speciation of water. High- T spectra and room- T spectra were collected alternatively and compared together, allowing the determination of the temperature at which water started to diffuse out of the glasses. For both compositions this temperature was found to be > 480°C. In the temperature range 25–480°C (no loss of water), the low-frequency range spectra (500–1400 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-frequency range (2800–3800 cm −1) at temperatures as low as 160° and 120°C in silica and quartzofeldspathic glasses, respectively. In both samples the intensities of the bands in the range 3200–3500 cm −1, attributed to molecular water, are decreasing with T. Concomitantly, an intensity increase is observed for the narrow band at 3597 cm −1 in the silica glass and for a band centered at ∼ 3580 cm −1 in the quartzofeldspathic glass. These variations are fully reversible (time scale of < 1 min) when T decreases. The observed changes can be interpreted to be due to a collapse of hydrogen bonding involving molecular water. In the silica glass, the 3597-cm −1 band, showing 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 enhanced by the reaction leading to the breaking of hydrogen bonds.