Carbon dioxide in silica-undersaturated melt Part II: Effect of CO2 on quenched glass structure

We observe that in such depolymerized nephelinite compositions the exchange between Na and K in volatile-free glasses induces a change in the glass structure attributed to the Mixed Alkali Effect (MAE) invoked in previous works. The observed changes might be related to geometric changes in the glass structure (change in network species bond lengths or angles). The addition of H2O only produces a negligible change in the degree of polymerization being probed by the constant NBO/T (Non-Bridging Oxygen per Tetrahedron) as H2O is added to volatile-free glasses. In contrast, we observed that the addition of CO2 induces a strong change in the glass structure which might be interpreted as an increase in polymerization. Alternatively, considering the identified CO2 dissolution mechanism (mostly as non-bridging carbonates units), the glass structure changes might reflect an apparent glass polymerization through the creation of Si-O-C bonds rather a true polymerization through the creation of Si-O-Si bonds.