General model for calcite growth kinetics in the presence of impurity ions

Abstract

The concentrations of Sr, Mg and other elements in calcite are widely used to infer the conditions of mineral growth. However, such inferences are dependent on the mechanisms that govern the incorporation of minor constituents into the calcite lattice during growth. A particularly confusing observation is that both Sr and Mg are readily incorporated into growing calcite crystals at low concentrations but inhibit calcite growth at higher concentrations. Here we show that the growth rate dependence of Sr and Mg incorporation into calcite, as well the inhibitory effects on calcite growth of both incorporating and non-incorporating ions, can be predicted with an ion-by-ion crystal growth model where ion attachment is confined to kink sites on the crystal surface.The exchange of ions between active growth (kink) sites on the mineral surface and aqueous solution governs both the efficiency of incorporation of minor constituents and the kinetics of mineral precipitation. Ions such as Sr and Mg in calcite, that are not stoichiometric constituents, may attach to kink sites and impede crystal growth by either blocking propagation of the kink (kink blocking), or if incorporated into the growing mineral, straining the local crystal lattice, and hence increasing the mineral solubility (incorporation inhibition). Here we investigate the effects of including these growth inhibition mechanisms into a microscopic model for crystal growth based on kink creation, propagation and collision (CPC) theory. This model predicts that kink blocking by either incorporated or non-incorporated ions causes an exponential decrease in mineral growth rate with increasing impurity concentration, while incorporation inhibition results in more complicated functional forms of the growth rate effect depending on the thermodynamics of the solid solution. Applying this model to existing data on the partitioning of strontium and magnesium into calcite and the simultaneous effects on growth kinetics and mineral composition, we find that strontium uptake inhibits growth by enhancing mineral solubility while magnesium inhibits growth primarily by kink blocking. Our model should be widely applicable to understanding the impurity content of a large range of sparingly soluble minerals that form by precipitation from aqueous solutions.