The speciation of carbon dioxide in sodium aluminosilicate glasses

Abstract

Infrared spectroscopy has been used to study the speciation of CO_2 in glasses near the NaAlO_2-SiO_2 join quenched from melts held at high temperatures and pressures. Absorption bands resulting from the antisymmetric stretches of both molecular CO_2 (2,352 cm^(-1)) and CO^(2-)_3 (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 CO_2 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 CO_2 to CO^(2-)_3 in sodium aluminosilicate glasses varies little for each silicate composition over the range of total dissolved CO_2 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 Na_2O content along the NaAlO_2-SiO_2 join and with decreasing Na_2O content in peraluminous compositions off the join. Infrared spectroscopic measurements of species concentrations in glasses provide insights into the molecular level processes accompanying CO_2 solution in melts and can be used to test and constrain thermodynamic models of CO_(2-) bearing melts. CO_2 speciation in silicate melts can be modelled by equilibria between molecular CO_2 , CO^(2-)_3, and oxygen species in the melts. Consideration of the thermodynamics of such equilibria can account for the observed linear relationship between molecular CO_2 and carbonate concentrations in glasses, the proposed linear relationship between total dissolved CO_2 content and the activity of CO_2 in melts, and observed variations in CO_2 solubility in melts.

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