Carbonate minerals are ubiquitous on Earth, and their diverse production through biomineralization has had profound geologic consequences. At high supersaturation, carbonate minerals form through hydrated amorphous intermediates that dehydrate upon crystallization. This process controls the nature of crystalline carbonate end-products and can be profoundly affected by factors such as the presence of impurities or water activity. A key example is nanoconfined environments, which often lead to water-limited conditions and thus affect the water content and crystallization pathways of amorphous carbonate intermediates. However, the molecular-scale mechanisms underlying dehydration and crystallization pathways remain poorly understood, in large part due to the difficulties in probing the short-range order of transient amorphous structures. In this work, we use ab initio molecular dynamics (AIMD) simulation to shed light on the structure and crystallization pathways of amorphous carbonate intermediates. Physically realistic ensembles of amorphous carbonate structures are generated via a melt-quench procedure. In this presentation, we will show AIMD results obtained with amorphous calcium carbonate (ACC), amorphous magnesium carbonate (AMC), and amorphous strontium carbonate (ASC) at four water contents (MCO3 nH2O where M = Ca, Mg, or Sr and n = 0, 0.5, 1, or 2). In addition to being common amorphous carbonate intermediates, the divalent cations of these carbonates have widely ranging hydration enthalpies and water exchange rates allowing an exploration of the effects of cation-water interactions upon dehydration. Simulated and measured signals from neutron and X-ray pair distribution function experiments, X-ray absorption spectroscopy, and infra-red spectroscopy are compared directly to validate the simulations and interpret experimental measurements. Moving beyond single-cation carbonates, we also explore amorphous calcium-strontium carbonate solid solutions as a function of water content (CaxSr1-xCO3 nH2O where x = 0, 0.25, 0.5, 0.75, or 1 and n = 0.5 or 1) to determine whether cations compete for water molecules as amorphous carbonates dehydrate.

Ab initio molecular dynamics simulation of the short-range structure of amorphous carbonates

ACS Spring 2022

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