Structural controls on the solubility of CO2 in silicate melts: Part II: IR characteristics of carbonate groups in silicate glasses

The characteristics of carbonate groups dissolved in silicate glasses have been investigated using FTIR spectroscopy. Glasses of natural melt compositions are compared with simple analogues. This approach allows systematic investigation of the role of each major oxide component found in the more complex compositions. Only Ca- and Fe2+-bearing systems display the characteristic spectral feature which is dominant in all carbonate-bearing natural compositions, although other cations are important in producing more subtle effects. Spectra of Mg-related carbonate groups suggest the associated Mg is in a network-forming role (T site) and this may explain the lower solubility for MgO-rich compositions noted in Part I. Several spectral features also favour the assignment of Fe2+ as well as Fe3+ to a network-forming role, consistent with the solubility data reported in Part I. As the solubility of CO2 is controlled largely by the availability of non-bridging oxygens (NBO) in depolymerised compositions, this implies that carbonate is more likely to be associated with Ca than with Fe2+ in natural melts.