Effect of pressure on the short-range structure and speciation of carbon in alkali silicate and aluminosilicate glasses and melts at high pressure up to 8 GPa: C-13, Al-27, O-17 and Si-29 solid-state NMR study

Despite the pioneering efforts to explore the nature of carbon in carbon-bearing silicate melts under compression, experimental data for the speciation and the solubility of carbon in silicate melts above 4 GPa have not been reported. Here, we explore the speciation of carbon and pressure-induced changes in network structures of carbon-bearing silicate (Na2O-3SiO(2), NS3) and sodium aluminosilicate (NaAlSi3O8, albite) glasses quenched from melts at high pressure up to 8 GPa using multinuclear solid-state NMR. The Al-27 triple quantum (3Q) MAS NMR spectra for carbon-bearing albite melts revealed the pressure-induced increase in the topological disorder around 4 coordinated Al (Al-[4]) without forming Al-[5,Al-6]. These structural changes are similar to those in volatile-free albite melts at high pressure, indicating that the addition of CO2 in silicate melts may not induce any additional increase in the topological disorder around Al at high pressure. C-13 MAS NMR spectra for carbon-bearing albite melts show multiple carbonate species, including Si-[4](CO3)Si-[4], Si-[4](CO3)Al-[4], Al-[4](CO3)Al-[4], and free CO32-. The fraction of Si-[4](CO3)Al-[4] increases with increasing pressure, while those of other bridging carbonate species decrease, indicating that the addition of CO2 may enhance mixing of Si and Al at high pressure. A noticeable change is not observed for Si-29 NMR spectra for the carbon-bearing albite glasses with varying pressure at 1.5-6 GPa. These NMR results confirm that the densification mechanisms established for fluid-free, polymerized aluminosilicate melts can be applied to the carbon-bearing albite melts at high pressure.