Water in Silicate Glasses: An Infrared Spectroscopic Study

Abstract

Infrared and near-infrared transmission spectra have been taken on 19 volcanic and synthetic silicate glasses with known H_2O contents (0.06–6.9 wt. %). Absorption peaks were observed at wavelengths of 1.41 µm, 1.91 µm, 2.22 µm, 2.53 µm, and 2.8 µm. These peaks have been attributed to the first overtone of the OH stretching vibration, the combination stretching + bending mode of H_2O molecules, the combination stretching + bending mode of X-OH groups, a combination mode of the fundamental OH stretch + a low energy lattice vibration, and the fundamental OH stretching mode, respectively. Molar absorptivities of the peaks have been determined to be 0.2, 1.8, 1.0, 0.9, and 67 l/mol-cm. These values apply over the full range of glass compositions studied (albite, rhyolite, basalt). Quantitative determinations of total H_2O contents and of the concentrations of molecular water and hydroxyl groups in silicate glasses are possible using these molar absorptivities, although they are limited in their accuracy by the accuracy of the reported water contents of the glasses used to calibrate these molar absorptivities. The most important uses of this technique may stem from its applicability to microsamples (≥ 100 µm) and to the determination of the concentrations of hydroxyl groups and molecular water in quenched silicate melts. Hydroxyl groups are the dominant hydrogen-bearing species in water-bearing glasses at low total water contents, but molecular H_2O was detected in all samples with ≥ 0.5 weight percent total water. The concentration of hydroxyl groups increases rapidly with total water content at low total water contents, but more slowly at higher (>3 wt. %) total water contents; it may level off or even decrease at high total water contents. The concentration of molecular water increases slowly at low total water contents and more rapidly at high total water contents. More water is dissolved as molecular water than as hydroxyl groups at total water contents greater than ~4 wt. %. Molecular water in these glasses is probably structurally bound rather than present as fluid inclusions as a separate phase, since ice bands were not observed in spectra taken at 78K and since samples were free of visible bubbles. It is proposed that the speciation of water in silicate glass formed by rapid quenching from melt equilibrated at high temperatures reflects that of the melt. According to this hypothesis, neither high water contents nor high pressures are needed to stabilize substantial quantities of molecular water in melts. This hypothesis, that water dissolves in silicate melts as both molecular water and hydroxyl groups in proportions similar to those measured in waterbearing glasses, can explain the variations in viscosity, electrical conductivity, diffusivity of "water", diffusivity of cesium, and phase relationships that are observed in melts as functions of total water content. It also explains the observation that at vapor-saturation at high pressures, where most of the dissolved water is expected to be present as molecular water, water solubilities are similar for all melts but that at low pressures and water contents, where most dissolved water is present in dissociated form as hydroxyl groups, vapor-saturated water solubilities differ for different melt compositions. The linear relationship between water fugacity and the square of the mole fraction of total dissolved water observed for silicate melts at low water contents and the observed deviations from this linear relationship at high total water contents can be accounted for by this hypothesis.

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