Effect of water and network connectivity on glass elasticity and melt fragility

The sound velocities and elastic properties of haplogranitic glasses with excess Na2O were determined by Brillouin scattering spectroscopy at ambient conditions. Qualitatively, excess Na2O has a similar effect on the elastic properties to that previously reported for water: both lead to an increase of the Poissonʹs ratio, indicative of a decrease in the degree of polymerization. Thus, as for Na2O, water can be considered a network modifier in haplogranitic glasses. A fit to the current and literature data allows the parameterization of the Poissonʹs ratio as a function of water content and degree of polymerization (NBO/T) for a wide range of geologically relevant composition. This model for the Poissonʹs ratio, combined with the observed correlation between the melt fragility and Poissonʹs ratio for our peralkaline granitic glasses, enables the quantitative prediction of the fragility for a broad spectrum of anhydrous melt compositions.