Water speciation in hydrous sodium tetrasilicate and hexasilicate melts: Constraint from high temperature NIR spectroscopy

Changes of near-infrared (NIR) absorption peaks at ~ 4500 cm− 1 due to SiOH groups and at ~ 5200 cm− 1 due H2O molecules upon heating were investigated in sodium tetrasilicate glasses (Na2O · 4SiO2, 1.0–5.2 wt.% H2O) and sodium hexasilicate glasses (Na2O · 6SiO2, 2.3–7.0 wt.% H2O) at ambient pressure. Temperature was varied between 25 °C and 475 °C using a heating stage fitted to a Fourier-transform micro-infrared spectrometer. Dissolved water is stable in the glasses during such a heat treatment as evidenced by NIR spectra recorded after heating. Changes in the spectra below 200 °C are assigned to decreasing hydrogen bonding of water species and to the T-dependence of the molar absorption coefficients for the NIR combination bands. At higher temperature a rapid increase in intensity of the 4500 cm− 1 band on expense of the 5200 cm− 1 band was observed indicating the onset of species interconversion Si–O–Si + H2O = 2Si–OH. The onset temperature for these changes increase with decreasing total water content of the glasses. Water species concentrations could not be determined directly from the intensities of the NIR combination bands for the melt because the molar absorption coefficients cannot be calibrated for the sodium silicate melts at high temperatures. Therefore, the heating experiments were used only to determine the fictive temperatures (also denoted as apparent equilibrium temperatures) for the water speciation measured on the glasses at room temperature.Assuming ideal mixing of quasi-oxygen species (OH group, H2O molecule, non-bridging oxygen NBO, bridging oxygen BO) the equilibrium constant K for interconversion reaction of water species was calculated. The temperature dependence of the speciation equilibrium in the melt state can be expressed by ln K = 5.39–3427 / T for sodium tetrasilicate and ln K = 5.80–3731 / T for sodium hexasilicate. At magmatic temperatures the equilibrium constant for water speciation in sodium silicate melts is larger than in polymerized aluminosilicate melts by a factor of 5 or more, depending on melt composition. This large difference of ln K is most likely owing to the high degree of melt depolymerization and the high alkali concentration in sodium silicate melts.