Structural order of Mg-stabilized amorphous calcium carbonate and its associated phase transformation

Biominerals formed by marine organisms exhibit intricate structures and support a remarkable range of functionalities. Recent advances in our understanding of biomineralization highlight the pivotal role of magnesium-stabilized amorphous calcium carbonate (Mg-ACC) as a transient precursor in the formation of calcareous biominerals. This feature article reviews recent in vitro studies of Mg-ACC, illustrating the concepts of particle attachment, secondary nucleation, domain segregation, and mesocrystal formation. Some conceptual issues associated with the ongoing debate between classical nucleation theory and non-classical nucleation theory are discussed. We suggest that solid-state NMR measurements of the van Vleck second moment provide a stringent test for any proposed atomic model of Mg-ACC. The coordination environment of Mg²⁺ ions and the significance of bicarbonate ions in Mg-ACC are discussed. The diffusion of Mg²⁺ ions within the calcitic lattice of high-Mg calcite offers a mechanistic insight into the 'dolomite problem'. The fusion of neighboring high-Mg calcite spherulites highlights their potential role as versatile building blocks for calcareous biomineral skeletal frameworks. Altogether, this article aims to provide a comprehensive understanding of the structural complexity and transformation pathways of Mg-ACC.