Carbonate speciation in depolymerized and polymerized (alumino)silicate glasses: Constraints from ¹³C MAS and static NMR measurements and ab initio calculations

Knowledge of the dissolution mechanisms of carbon dioxide in silicate melts/glasses is indispensable for understanding its effects on physical and thermodynamic properties. Carbon dioxide is generally known to dissolve as molecular CO₂ and CO₃ ^2– species, with the latter dominant for depolymerized compositions. However, less is agreed upon about how the CO₃ ^2– groups are incorporated, especially for depolymerized silicate melt compositions relevant to natural mafic and ultramafic magmas. Here we report ¹³C MAS and static NMR results on a series of ¹³CO₂-bearing glasses (quenched from melts) of diverse silicate compositions, including nominally fully polymerized sodium aluminosilicate and calcium aluminosilicate, depolymerized sodium silicate and sodium aluminosilicate, and depolymerized calcium‑magnesium silicate and calcium aluminosilicate compositions (with varying degrees of polymerization), as well as ab initio calculations, to provide new constraints on the speciation of carbonates in silicate melts/glasses as a function of composition. The ab initio calculation revealed that both vibrational frequencies and ¹³C chemical shift tensor are sensitive to the local environments of carbonates. The splittings of the asymmetric stretching doublets (Δν₃) for CO₃ ^2– groups bonded to one or two tetrahedral Si/Al via two oxygens (network carbonates) are all relatively large (around 180–480 cm⁻¹), contrary to previous speculations. In comparison, experimental data for CO₃ ^2– groups bonded only to metal cations (free carbonates) in minerals show zero to moderate Δν₃ (up to ~100 cm⁻¹). Our calculations also showed that network carbonates bonded to one or two tetrahedral Si/Al both show ¹³C chemical shift tensor parameters (especially skew and isotropic chemical shift) that are distinctly different from those of free carbonates. Our ¹³C MAS and static NMR data, as well as infrared spectroscopic data (moderate Δν₃ of 60–100 cm⁻¹) from the literature, for depolymerized silicate and aluminosilicate glasses are all indicative of free carbonates as the dominant species. Data for nominally fully polymerized aluminosilicate compositions, on the other hand, are consistent with carbonate groups bonded to two Si/Al via two oxygens (network carbonate) as the dominant species. The quantitative ¹³C MAS NMR data also revealed the coexistence of a small amount of the other type of carbonate species, especially for Ca aluminosilicate glasses. These new structural insights should be valuable in helping better understand physical properties (e.g. viscosity) of CO₂-bearing silicate melts of diverse compositions.