Structure and Transformation of Amorphous Calcium Carbonate: A Solid-State ⁴³Ca NMR and Computational Molecular Dynamics Investigation

Amorphous calcium carbonate (ACC) is a metastable precursor to crystalline CaCO 3 phases that precipitates by aggregation of ion pairs and prenucleation clusters. (1, 2) We use 43 Ca solid-state NMR spectroscopy to probe the local structure and transformation of ACC synthesized from seawater-like solutions with and without Mg 2+ and computational molecular dynamics (MD) simulations to provide more detailed molecular-scale understanding of the ACC structure. The 43 Ca NMR spectra of ACC collected immediately after synthesis consist of broad, featureless resonances with Gaussian line shapes (FWHH = 27.6 ± 1 ppm) that do not depend on Mg 2+ or H 2 O content. A correlation between 43 Ca isotropic chemical shifts and mean Ca–O bond distances for crystalline hydrous and anhydrous calcium carbonate phases indicates indistinguishable maximum mean Ca–O bond lengths of ∼2.45 Å for all our samples. This value is near the upper end of the published Ca–O bond distance range for biogenic and synthetic ACCs obtained by Ca-X-ray absorption spectroscopy. (3-5) It is slightly smaller than the values from the structural model of Mg-free ACC by Goodwin et al. (6) obtained from reverse Monte Carlo (RMC) modeling of X-ray scattering data and our own computational molecular dynamics (MD) simulation based on this model. An MD simulation starting with the atomic positions of the Goodwin et al. (6) RMC model using the force field of Raiteri and Gale (2) shows significant structural reorganization during the simulation and that the interconnected carbonate/water-rich channels in the Goodwin et al. model shrink in size over the 2 ns simulation time. The distribution of polyhedrally averaged Ca–O bond distances from the MD simulation is in good agreement with the 43 Ca NMR peak shape, suggesting that local structural disorder dominates the experimental line width of ACC.