Crystal chemical controls on the dissolution kinetics of the isostructural sulfates: Celestite, anglesite, and barite

Abstract

The isostructural sulfates, barite (BaSO 4), celestite (SrSO 4), and anglesite (PbSO 4), share the crystallographic space group Pnma. With their crystal structure and bulk stoichiometry invariant, we investigate controls on the kinetics of sulfate mineral dissolution. Dissolution rates were measured in hydrothermal mixed flow reactors with solutions of pH 2 to 10 and at 25–140°C. Data were analyzed using a first order rate law. In general, dissolution rates follow the order celestite > anglesite > barite. Although the chemistry of lead differs from strontium and barium, anglesite has intermediate reactivity. All minerals exhibit a similar pH dependence of dissolution where rates decline with increasing pH for the range 2 to 5 and are approximately independent of pH over the range of 5 to 9. Above pH 9, anglesite dissolution rate increases sharply with increasing pH. An analysis of the temperature dependence of dissolution at near-neutral pH shows that the reactivity trend is controlled by differences in the preexponential component of the rate constant. Thus, reaction frequency, not energetics, primarily determines relative reactivity and suggests steric and/or solvation controls on reaction rate. Dissolution rates at near-neutral pH correlate inversely with both ionic radius and average bond length of the structural divalent atom. This leads us to consider the nature of the aqueous metal complexes that are released to solution upon hydrolysis. We find a positive correlation between solvation number and dissolution rate. This correlation extends to describe reported rates of anhydrite (CaSO 4) dissolution. Combining evidence, we suggest that rates of isostructural sulfate mineral dissolution are limited by the relative solvation affinity of the divalent metal atoms for near-surface water. Hydrolysis of the sulfate group is probably not rate-limiting since anionic hydration is extremely fast and approximates rates of aqueous diffusion. Dissolution rates are limited by the mineral component having the lowest solvation affinity. This model predicts dissolution rates of sulfate minerals within a solid-solution compositional series and may describe other minerals with strong anisodesmic character. Our study reiterates the role of near-surface solvent properties in controlling mineral reactivity in aqueous solutions.