Water speciation in oxide glasses and melts

Silicic melts in magma chambers below active volcanoes typically contain several wt% H2O. The dissolved water has drastic effects on the chemical and physical properties of the melts and, hence on processes like magma differentiation, ascent and degassing. But interaction of water with glasses and melts is also of great importance for the production and application of glasses. The effect of water is closely related to its speciation. OH groups bond to tetrahedral cations and H2O molecules are predominant species in polymerized melts. In strongly de polymerized melts free OH groups may also be present. In this paper we review the data on water speciation in oxide melts, considering new data for hydrous silica and basaltic melts. SiO2 glasses containing up to 4 wt% H2O were produced at temperatures of 1573-1673 K and pressures of 100-200 MPa in an internally heated gas pressure vessel. Water speciation in the quenched glasses was determined by near-infrared spectroscopy. The set of samples used for the calibration of the absorption coefficients for the combination bands of OH and H2O also included a natural hyalite and water-poor commercial silica glasses. Fictive temperatures of the glasses were estimated based on viscosity and cooling rate. The derived K values (=[OH](2)/([O]center dot[H2O]),) indicate, at given melt temperature, higher abundance of molecular H2O for hydrous silica melts than for other oxide melts. For tholeiitic basalt melts, water speciation was inferred using speciation data for glasses from Shishkina et al. (2010), and T-f was estimated using the viscosity model of Giordano et al. (2008). Some general trends can be derived by considering literature data for water speciation in oxide melts. At constant p,T, little variation of K values is observed for melts of the pseudo join from rhyolite to basalt, implying that composition has minor effect for most natural aluminosilicate melts. However, this applies only to melts with Al/Si << 1. The network can be hydrolyzed much more easily if alternating Al-O-Si bonds are present, i.e. when Al/Si approaches 1. K values are higher for silicate melts than for aluminosilicate melts, implying that non-bridging oxygen support the dissociation of H2O. The type of network former also has a strong influence on the dissociation of H2O in the melt as well. Replacement of Al by B favors the formation of OH groups, and OH contents are particularly high in borate and phosphate melts. This can be explained by easy hydrolysis of P-O and B-O bonds.