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

Abstract

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 CO32– species, with the latter dominant for depolymerized compositions. However, less is agreed upon about how the CO32– 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 CO32– 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 CO32– 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.