MELT SPECIATION IN THE SYSTEM NA2O-SIO2

Abstract

The applicability of a speciation model to quantify the thermodynamic properties of silicate liquids was evaluated in the Na2O–SiO2 system. Based on spectroscopic data, four sodium-silicate species with various numbers of bridging oxygen atoms were considered. We used a thermodynamic model that assumed ideal mixing between the species and temperature-independent, additive heat capacities of the species. Enthalpies and entropies of the melt species were determined by fitting experimental data.Enthalpies and entropies for each species were determined by fitting either speciation data from spectroscopic investigations or experimental data on mineral–melt equilibrium assemblages. Results were evaluated by comparing the calculated species abundances and mineral–melt equilibrium temperature with the experimental data. No temperature or compositional dependence of the quality of the fit was observed. An independent check of the applicability of the model is provided by the fact that enthalpies and entropies of the species could be determined by fitting phase equilibrium data and used to reproduce the speciation data. The excellent overlap of the experiments and the calculated values shows that in this system, the speciation approach accurately reproduces melt properties. This evaluation of a speciation model suggests that it has a significant potential to describe thermodynamic and physicochemical properties of silicate liquids. Moreover, for melt compositions in which the speciation cannot be determined, information from phase diagrams can be used to resolve the speciation of the melt and, potentially, speciation-dependent properties (e.g., the viscosity).