Carbon dioxide in silica-undersaturated melt. Part I: The effect of mixed alkalis (K and Na) on CO2 solubility and speciation

The 13C NMR spectra show that carbonate (CO32−) environments can be attributed to carbonate species associated to non-bridging oxygen in agreement with the depolymerized nature of the investigated compositions. Two singular additional carbonated species were also identified with 13C NMR signatures at 161 and 165 ppm. Those species are assigned to isolated K+…CO32−…H+ and Na+…CO32−…H+ carbonate species. The presence of such isolated carbonate species is interpreted as possible precursors to carbonatitic melt genesis.