Structural controls on the solubility of CO2 in silicate melts Part II: IRcharacteristics 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 composit ions are compared with simple analogues. This approach allows systematic in vestigation of the role of each major oxide component found in the more com plex compositions. Only Ca- and Fe2+-bearing systems display the characteri stic spectral feature which is dominant in all carbonate-bearing natural co mpositions, although other cations are important in producing more subtle e ffects. Spectra of Mg-related carbonate groups suggest the associated Mg is in a network-forming role (T site) and this may explain the lower solubili ty for MgO-rich compositions noted in Part I. Several spectral features als o 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 o f CO, is controlled largely by the availability of non-bridging oxygens (NB O) in depolymerised compositions, this implies that carbonate is more likel y to be associated with Ca than with Fe2+ in natural melts. (C) 2001 Elsevi er Science B.V. All rights reserved.