Abstract

The bulk diffusivity of dissolved carbon dioxide (CO2 and CO32−) in NaAlSi3O8 + nNa2O (n = 0 – 6.87 wt%) and in NaAlSi3O8 + nH2O (n = 0–2 wt%) melts was investigated at 1523 K and 0.5 GPa using the diffusion couple technique. CO2 contents of the starting glass pairs varied between 0 and 0.2 wt%. Symmetrical concentration-distance profiles of bulk CO2 were determined by infrared spectroscopy. An error function was fitted to the profiles to obtain apparent chemical diffusion coefficients of bulk CO2. In the investigated compositional range, the diffusivity of bulk CO2 increases exponentially with Na2O and H2O content and thus exponentially with the ratio of non-bridging oxygen atoms per tetrahedral cations (NBO/ T). The bulk CO2 diffusivity increases from logDCO2 = −11.38 (DCO2 in m2/s) in NaAlSi3O8 melt to logDCO2 = −10.92 in NaAlSi3O8 melts containing 6.87 wt% Na2O excess, and to logDCO2 = −10.91 in NaAlSi3O8 melts containing 2 wt% H2O. These data imply that either: (1) the diffusivities of the CO2 species (molecular CO2 and CO32−) are very similar, or (2) the speciation of CO2 in the quenched glasses is very different from the speciation in the melt.

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