Affinity for OH^- Produces Four-Coordinated Zn²⁺ Impurities in Hydrated Amorphous Calcium Carbonate

Using ab initio based molecular dynamics and electronic structure calculations, we show that Zn impurities in hydrated amorphous calcium carbonate (ACC) have a much lower coordination number than other divalent impurities due to covalent interactions between the 3d Zn shell and the oxygen atoms of the carbonate and water groups. The local structure around Zn in ACC, including the predicted low coordination number, is confirmed by X-ray absorption spectroscopy of synthetic Zn-bearing ACC. The strong Zn-O chemical interaction leads to substantial water dissociation and slightly disrupts the hydrogen bonding network. Implications of Zn²⁺ incorporation for ACC stability are discussed.