EFFECT OF BORON ON THE WATER SPECIATION IN (ALUMINO)SILICATE MELTS AND GLASSES

Abstract

The investigation of hydrous boro(alumino)silicate melts and glasses with near infrared (NIR) spectroscopy revealed an important effect of boron on the water speciation. In the NIR spectra of B-bearing glasses new hydroxyl-related bands develop at the high frequency side of the 4500 cm-1 peak. In NaAlSi3O8 + B2O3 glasses this new peak is present as a shoulder at 4650 cm-1, and in NaAlSi3O8-NaBSi3O8 (Ab-Rd) glasses it appears as a resolved peak at 4710 cm-1. These bands increase with increasing boron concentration, suggesting that they are due to B-OH complexes. Furthermore, the variations in the NIR spectra indicate that with increasing B-content, but constant total water concentration, the amount of structurally bonded hydroxyl groups increases at the expense of molecular H2O. For example, at a total water concentration of 4 wt.%, pure Rd-glass contains ˜50% more water as hydroxyl groups than pure Ab-glass. In-situ NIR spectroscopy at high P and T using a hydrothermal diamond-anvil cell was used to gain information about the temperature dependence of the water speciation in NaBSi3O8 melts. The data demonstrate the conversion of molecular H2O to hydroxyl groups with increasing temperature. However, a fully quantitative evaluation of the high T spectra was hampered by problems with defining the correct baseline in the spectra. As an alternative approach annealing experiments on a Rd-glass containing 2.8 wt.% water were performed. The results confirm the conversion of H2O to OH groups with increasing T, but also suggest that the OH groups represented by the 4710 cm-1 peak (B-OH) participate much less in the conversion reaction compared to X-OH, represented by the 4500 cm-1 peak.