Carbon dioxide speciation in silicate melts: a restart

Abstract

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 CO 2) to that of the band doublet at 1700–1375 cm −1 (due to CO 3 2−) increases with higher annealing temperatures for both glass compositions. Our study demonstrates that in contrast to recent suggestions, the equilibrium of the CO 2 species reaction CO 2+O 2−=CO 3 2− in silicate glasses/melts shifts towards molecular CO 2 with increasing temperature. The CO 2 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 Δ H 0 and entropy Δ S 0 of this reaction, assuming that the species concentrations reflect those at experimental temperatures (Δ H 0=−12±2 kJ mol −1 and Δ S 0=−23±3 J mol −1 K −1 for albitic composition; Δ H 0=−29±2 kJ mol −1 and Δ S 0=−32±3 J mol −1 K −1 for dacitic composition).