, Volume 91, Issue 2, pp 105-121

The speciation of carbon dioxide in sodium aluminosilicate glasses

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Abstract

Infrared spectroscopy has been used to study the speciation of CO2 in glasses near the NaAlO2-SiO2 join quenched from melts held at high temperatures and pressures. Absorption bands resulting from the antisymmetric stretches of both molecular CO2 (2,352 cm−1) and CO 3 2− (1,610 cm−1 and 1,375 cm−1) are observed in these glasses. The latter are attributed to distorted Na-carbonate ionic-complexes. Molar absorptivities of 945 liters/mole-cm for the molecular CO2 band, 200 liters/mole-cm for the 1,610 cm−1 band, and 235 liters/mole-cm for the 1,375 cm−1 band have been determined. These molar absorptivities allow the quantitative determination of species concentrations in the glasses with a precision on the order of several percent of the amount present. The accuracy of the method is estimated to be ±15–20% at present.

The ratio of molecular CO2 to CO 3 2− in sodium aluminosilicate glasses varies little for each silicate composition over the range of total dissolved CO2 content (0–2%), pressure (15–33 kbar) and temperature (1,400–1,560° C) that we have studied. This ratio is, however, a strong function of silicate composition, increasing both with decreasing Na2O content along the NaAlO2-SiO2 join and with decreasing Na2O content in peraluminous compositions off the join.

Infrared spectroscopic measurements of species concentrations in glasses provide insights into the molecular level processes accompanying CO2 solution in melts and can be used to test and constrain thermodynamic models of CO2-bearing melts. CO2 speciation in silicate melts can be modelled by equilibria between molecular CO2, CO 3 2− , and oxygen species in the melts. Consideration of the thermodynamics of such equilibria can account for the observed linear relationship between molecular CO2 and carbonate concentrations in glasses, the proposed linear relationship between total dissolved CO2 content and the activity of CO2 in melts, and observed variations in CO2 solubility in melts.