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In situ, high-temperature Raman spectroscopy was used to study the [Q.sup.n] speciation in binary Na-silicate glasses and melts. Over 300 Raman spectra in the compositional range from 25 to 40 mol% [Na.sub.2]O were collected at room and high temperatures between 800 and 1200 K. Quantitative information on the relative abundances of species in melts was obtained from the Raman spectra through a quantification procedure that does not require any a priori assumptions about the line shapes or external calibration of the Raman scattering efficiencies for the various [Q.sup.n] species. The [DELTA]H[degrees] associated with the speciation reaction 2[Q.sup.3] = [Q.sup.4]+ [Q.sup.2] was found to be 20.3 [+ or -] 7.9 kJ/mol. For a given temperature, the speciation is more disordered in sodium than in potassium silicate melts. Because of the smaller temperature dependence of the speciation in the sodium silicate system, the difference in the speciation for the sodium and potassium silicate system decreases with increasing temperature. In addition to the speciation data, the partial Raman spectra for the different species were obtained. The experimentally observed variation of the partial Raman spectra with temperature, and, to a minor extent, with composition, should stimulate future theoretical studies on the vibrational properties of silicate glasses and melts. Keywords: Silicate melts, Na-silicate glasses, speciation, Raman spectroscopy, partial Raman spectra

Abstract: A 29Si NMR spectrum with high signal to noise ratio was collected from a well-characterized, isotopically-enriched SiO2 glass with a single scan after full relaxation of the magnetization. From the spectrum, the average Si–O–Si bond angle was estimated at 150° with a minimum fwhm of the bond angle distribution of 16°. The 29Si spin-lattice relaxation times were measured by saturation recovery and differential T1 relaxation was observed for the various Si environments. [Copyright &y& Elsevier]

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). [Copyright &y& Elsevier]