Nanodomain Structure of Magnesium‐Stabilized Amorphous Calcium Carbonate Prepared in Aqueous Solution

ABSTRACT Magnesium‐stabilized amorphous calcium carbonate (Mg‐ACC) is a key precursor for biogenic Mg‐calcite. Biogenic Mg‐calcite is not a single phase but comprises Mg‐rich calcitic nanodomains embedded within an Mg‐depleted calcitic matrix, a structural heterogeneity that enhances fracture toughness compared with geological calcite. In a recent study of Mg‐ACC monoliths prepared in ethanol, Mg‐rich amorphous nanodomains (MgR) within an Mg‐depleted amorphous matrix (MgD) were identified; however, it remains unclear whether the same structural heterogeneity exists in aqueous systems. Here, advanced solid‐state NMR techniques were applied to demonstrate that MgD and MgR nanodomains coexist in Mg‐ACC powders prepared in aqueous solution. Notably, MgR and MgD domains are well integrated only when the total Mg content in Mg‐ACC is below ~16 mol%. In addition, a hydroxyl structural motif is identified in Mg‐ACC, with its abundance increasing at higher Mg content.