Solution mechanisms of H2O in depolymerized peralkaline melts

Solubility mechanisms of water in depolymerized silicate melts quenched from high temperature (1000 degrees-1300 degrees C) at high pressure (0.8-2.0 GPa) have been examined in peralkaline melts in the system Na2O-SiO2-H2O with Raman and NMR spectroscopy. The Na/Si ratio of the melts ranged from 0.25 to 1. Water contents were varied from similar to 3 mol% and similar to 40 mol% (based on O = 1). Solution of water results in melt depolymerization where the rate of depolymerization with water content, partial derivative(NBO/Si)/partial derivative X-H2O, decreases with increasing total water content. At low water contents, the influence of H2O on the melt structure resembles that of adding alkali oxide. In water-rich melts, alkali oxides are more efficient melt depolymerizers than water. In highly polymerized melts, Si-OH bonds are formed by water reacting with bridging oxygen in Q(4)-species to form Q(3) and Q(2) species. In less polymerized melts, Si-OH bonds are formed when bridging oxygen in Q(3)-species react with water to form Q(2)-species. In addition, the presence of Na-OH complexes is inferred. Their importance appears to increase with Na/Si. This apparent increase in importance of Na-OH complexes with increasing Na/Si (which causes increasing degree of depolymerization of the anhydrous silicate melt) suggests that water is a less efficient depolymerizer of silicate melts, the more depolymerized the melt. This conclusion is consistent with recently published H-1 and Si-29 MAS NMR and H-1-Si-29 cross polarization NMR data. Copyright (c) 2005 Elsevier Ltd.