Structural Characterization of Mg-Stabilized Amorphous Calcium Carbonate by Mg-25 Solid-State NMR Spectroscopy

Biogenic amorphous calcium carbonates (ACCs) play a crucial role in the mineralization process of calcareous tissue. Most biogenic ACCs contain Mg ions, but the coordination environment of Mg, which may influence the kinetics of the phase transformation of an ACC, remains poorly understood. We demonstrate that Mg-25 solid-state NMR can be used to probe the coordination shells of Mg in synthetic ACCs. The variation in Mg-25 chemical shifts suggests that Mg-O bond lengths increase as Mg content increases. On the basis of the Van Vleck second moments obtained from the double-resonance NMR experiments, we infer that the average number of carbonates surrounding the central Mg ion is in the range of 4-4.5 and that there is at least one water molecule coordinated to each Mg ion for the synthetic Mg-ACC samples. We suggest that the stability of Mg-ACC is owing to the structural water bound to Mg ions, which increases considerably the activation energy associated with the dehydration of Mg-ACC.