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Published October 1985 | public
Journal Article

The speciation of carbon dioxide in sodium aluminosilicate glasses


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.

Additional Information

© 1985 Springer-Verlag. Received October 22, 1984; Accepted May 21, 1985; Published Oct 1, 1985. The spectroscopic measurements used in this study were performed in the laboratory of Professor George Rossman at Caltech. The guidance of Professor Rossman and the help of Roger Aines, Stephanie Mattson, and Martin Ruzek is gratefully acknowledged. Professor A. Boettcher, R. Luth and Dr. S. Sharma provided samples. S. Epstein performed C analyses. Professors D. Burnett and T. Tombrello, and Drs. M. Mendenhall and R. Livi, all of Caltech, made analysis of C using nuclear reaction techniques possible. P. Ihinger provided invaluable laboratory assistance. Reviews by Art Boettcher, John Holloway, Björn Mysen and one anonymous reviewer are appreciated. Supported by NSF grant EAR-8212765. Caltech Division of Geological and Planetary Sciences Contribution Number 4147.

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