Carbon dioxide speciation in silicate melts: a restart

Using infrared spectroscopy, we investigated the speciation of carbon dioxide in albitic and synthetic iron-free dacitic glasses after annealing the glasses below the glass transition temperature in the temperature range 673–973 K at 0.5 GPa. Samples were rapidly quenched under constant pressure. The experiments show that the ratio of the integrated intensities of the 2350 cm−1 band (due to molecular CO2) to that of the band doublet at 1700–1375 cm−1 (due to CO32−) increases with higher annealing temperatures for both glass compositions. Our study demonstrates that in contrast to recent suggestions, the equilibrium of the CO2 species reaction CO2+O2−=CO32− in silicate glasses/melts shifts towards molecular CO2 with increasing temperature. The CO2 species concentrations and an ideal solution model were used to determine equilibrium constants for the homogeneous species reaction. We derived preliminary values for the enthalpy ΔH0 and entropy ΔS0 of this reaction, assuming that the species concentrations reflect those at experimental temperatures (ΔH0=−12±2 kJ mol−1 and ΔS0=−23±3 J mol−1 K−1 for albitic composition; ΔH0=−29±2 kJ mol−1 and ΔS0=−32±3 J mol−1 K−1 for dacitic composition).