The influence of bulk composition on the diffusivity of carbon dioxide in Na aluminosilicate melts

摘要

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 logD_CO2 = –11.38 (D_CO2 in m2/s) in NaAlSi₃O₈ melt to logD_CO2 = –10.92 in NaAlSi₃O₈ melts containing 6.87 wt% Na₂O excess, and to logD_CO2 = –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.