First-principles modeling of sulfate incorporation and 34S/32S isotopic fractionation in different calcium carbonates

Incorporation mechanism of sulfate groups in major calcium carbonates (calcite, aragonite and vaterite) is investigated using first-principles quantum-mechanical calculations. For each mineral, the stable structure of the substituted site is determined. In calcite and aragonite, a tilting of the sulfate-group with respect to the orientation of planar carbonate groups is observed. The theoretical vibrational properties of the sulfate-bearing carbonates are determined together with those of anhydrite (CaSO4) and isolated SO2 molecule. Comparison with experimental data supports the substitution of sulfate for carbonate groups in carbonate minerals. The energetic of sulfate incorporation is increasingly unfavorable in the order vaterite, calcite, and aragonite. Regarding isotopic fractionation properties, our calculations suggest that the equilibrium 34S/32S isotopic fractionation factor between vaterite and calcite is small (~ 0.5‰ at 0 °C). A slightly larger fractionation factor (~ 1‰ at 0 °C) is expected between aragonite and calcite; whereas a negative fractionation (~ − 1.5‰ at 0 °C) is expected between anhydrite and calcite. Finally, it is suggested that the isotopic fractionation between calcite and aqueous sulfate is smaller than 4‰.