2018 MSA AWARD LECTURE - HOW ION EXCHANGE DRIVES THERMODYNAMIC AND KINETIC FEEDBACKS IN GEOCHEMICALLY IMPORTANT SOLID SOLUTIONS

Natural aqueous solutions consist of complex mixtures of charged and neutral species, such that coexisting solid minerals can never be pure phases. The substitution and exchange of ions at the mineral-aqueous interface controls the composition of precipitating or exchanging solids as well as the rate of exchange of species at that interface. Importantly, solid composition is often tightly coupled to energetics, so small amounts of substitution or exchange can lead to extreme changes in both solid solution stability and subsequent exchange rates. These feedbacks present a major hurdle in the development of geochemical models that can accurately predict both mass transport and state properties of natural systems.

In this lecture, I discuss how couplings between composition and structure generate feedbacks that control bulk composition and mass transport kinetics in carbonate and 2:1 clay minerals, as well as approaches to modeling these coupled systems. Solid solution formation during calcite growth in the presence of impurities (e.g., Sr, Mg, and Mn) alters cation-carbonate bond strengths, leading to growth rate inhibition. Results of chemostat calcite growth experiments are presented alongside a process based ion by ion model for crystal growth to demonstrate how ion substitution at the surface and in the bulk crystal can dramatically impact the growth rate. In a separate study, I discuss the role of ion exchange in driving 2:1 clay mineral swelling and collapse. We find that the tight coupling between layer spacing and counterion composition controls both exchange kinetics and selectivity in the bulk clay. A new thermodynamic model for Na-K exchange driven swelling in smectite is introduced that explicitly links local basal spacing to ion exchange selectivity within a given layer. This model reproduces the coexistence of multiple hydration states of smectite, which can contain 0, 1, 2 or 3 discrete water layers. Exchange of K for Na in the bulk clay shifts the distribution of coexisting layer states towards smaller layer numbers, leading to water expulsion. Together, these findings illustrate the importance of feedbacks between composition, stability and kinetics in natural minerals, and explicitly accounting for this complexity can improve the predictive capacity of geochemical models.