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Project Progress

Massive layers of sedimentary dolomite formed at Earth’s surface or near-surface temperatures (< ~ 50 °C) are found in strata of all ages from the Precambrian to the Cenozoic. Modern dolomite formation is observed only in specific environments, such as highly saline lakes, deep-sea sediments and supersaturated saline lagoons, and only in minor quantities, but laboratory synthesis at low temperatures have been difficult. More generally, the synthesis of mixed cation carbonates ((Fe, Ca, Mg)CO₃)) at Earth’s near-surface temperatures is difficult inorganically, although such phases are known geologically as fine-grained early diagenetic phases in sedimentary rocks, as cements in oil reservoir rocks, and in the Martain meteorite ALH84001. Microbial involvement in the formation of mixed-cation carbonates has been proposed in the literature, with a role for sulfate-reducing bacteria, which remove SO₄^2- an inhibitor of dolomite formation. Thus, the occurrence of dolomite or other mixed-cation carbonates may serve as a biosignature on Earth and on other solid worlds such as Mars. Based on the proposed promoting role of HS- in Mg²⁺ desolvation, Zhang et al. synthesized high-magnesium calcite and disordered dolomite successfully at room temperature by mixing NaHCO₃ + Na₂S, and the effects of increasing pH were properly accounted for. The authors suggested that the low dielectric constant of aqueous sulfide somehow promoted Mg²⁺ dehydration, but no detailed mechanism was provided. Zhang et al.'s experimental work serves as a direct motivation for our present computational study.

Chemical intuition, based on considerations of charge density, chemical hardness and softness, or molecular electronic structures, does not suggest any particular characteristics of HS- that would render it an effective promoter. In the present work, therefore, we examined the purported role of sulfide in facilitating water dissociation from the Mg²⁺ center. We used a combination of computational methods ranging from classical molecular dynamics (MD) simulations to ab initio quantum mechanical calculations with model clusters (Evans et al., 2008). In the “real world”, such a promotion effect may occur in the aqueous bulk solution or at the solution-solid interface. In the present work, as a first step, we study the mechanism in bulk solution; future work will address the mechanism at the solution-solid interface.

We conducted ab initio reaction path ensemble (RPE) study for hydrated Mg²⁺ to determine the energy penalty for removal of a single water molecule from the first solvation shell compared to that for hydrated Mg²⁺ in the presence of HS- in the second coordination shell of Mg²⁺. We used large cluster models, where up to the second layer of Mg²⁺ hydration and the first solvation-shell of HS- are included, in order to address the solvent effect and specific hydrogen-bond interactions from water beyond the first-solvation shell. We found that HS- has little, if any, effect on lowering the Mg²⁺ dehydration barrier in aqueous solution. We speculated, then, that empirical observations of HS- promoted dolomite formation may indicate a potential role for HS- in promoting Mg²⁺ desolvation at a solid precursor phase that ultimately leads to a proto-dolomite phase.