Ionization and speciation of water in silicate melts

Water in silicate melts is commonly assumed to take the form of molecular water (H2Om) and hydroxyl groups (OH), the latter bounded to network formers (T) as T-OH groups. Although free hydroxyls (OH-) were only recently ascertained in depolymerized melts (Xue and Kanzaki, 2004), the amphoteric behavior of water, was already suggested long ago (Fraser, 1977) and preliminarily modelled considering redox data (Moretti, 2005). However, autoprotolysis of water has not been assessed in silicate melts hitherto, because of the rather complex nature of such a solvent. Here, a simple theoretical frame is developed based on the extended two-sublattice hypothesis of Temkin. The theory accounts for acid-base properties of the melt phase, and reconciles differing notations. An independent proof of our appraisal is given by an algorithmic approach to chemical equilibria between O and H in polymeric silicate melts. The assessment of the ionization constant of water in melts can explain the conditions at which precipitation of hydrous minerals can occur. Because of the amphoteric behavior, melt polymerization can be affected less than that normally perceived and assumed, depending on bulk composition. This has implications for the redox behavior, which is strictly connected to melt acid-base properties, hence polymerization. These connections must be accounted for when modelling magmatic equilibria. REFERENCES: FRASER DG (1977) In: Thermodynamics in Geology (D.G. Fraser, ed.), D. Reidel Pub. Co. MORETTI R. (2005) Ann. Geophysics, 48, 503-608. XUE, X. AND KANZAKI, M. (2004) Geochim. Cosmochim. Acta, 68, 5027-5057.