The solubility behavior of CO2 in melts on the join CaAl2Si2O8−NaAlSi3O8−CO2 has been determined at high pressures and temperatures by Raman spectroscopy and beta-track autoradiography. The carbon dioxide dissolves predominantly as a metal carbonate complex in such melts and is 25 percent more soluble in CaAl2Si208 than in NaAlSi3Oe melt at the same pressure and temperature. The Raman spectra of quenched NaAlSi3Oe + CO2 melt indicate that some carbon dioxide is also dissolved as molecular CO2.

The CO2-free melts consist of two 3-dimensional aluminosilicate network units. In both CaAl2Si2O8 and NaAlSi3Og melt these two units become enriched in Si relative to Al as carbon dioxide is dissolved. In addition, there is a third structural unit with both Si and Al in tetrahedral coordination and with nonbridging oxygens. A consequence of the formation of a metal carbonate complex is that some of the aluminum in the melt is not in tetrahedral coordination in melts of CaAl2Si2O8 + CO2 and NaAlSi3O8 + CO2 composition.

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