Carbonate speciation in silicate melts (glasses): New insights from ab initio calculations and NMR measurements

Abstract

Knowledge of the dissolution mechanisms of CO₂ in silicate melts/glasses is indispensible for understanding how it affects physical and thermodynamic properties. CO₂ is generally known to dissolve in silicate melts/glasses as molecular CO₂ and CO₃^2- species, but how the latter groups are incorporated in the melt and its effect on the silicate structure have been less certain. Here we report ab initio calculation (vibrational frequencies, ¹³C chemical shift tensors) and multinuclear NMR study on ¹³CO₂-bearing glasses of diverse silicate compositions. Our calculation suggests that both vibrational frequencies and ¹³C chemical shift tensor are sensitive to the local environments of carbonates. The experimental data indicate that carbonates are present dominantly as free carbonates (not bonded to tetrahedral Si/Al) in depolymerized melts, and as network carbonates (bonded to two tetrahedral Si/Al via two oxygens) in polymerized glasses. The formation of free carbonates would lead to polymerization of the silicate structure.