Unveiling the Formation Pathway of Vaterite from Amorphous Calcium Carbonate Using Metadynamics Simulations

Calcium carbonate is a compound that is widely distributed throughout the Earth as a natural mineral and a material produced by biological activities. The crystal structure of calcium carbonate has three polymorphs: the most thermodynamically stable calcite, followed by aragonite and vaterite. Of the three crystalline phases, the formation process and structure of the most unstable, vaterite, remain mysterious. In this study, the pathway for forming the vaterite crystal structure from amorphous CaCO₃ (ACC) is reproduced using well-tempered metadynamics molecular dynamics simulations. The structures sampled at multiple minima on the energy landscape are refined through first-principles calculations based on density functional theory. The sampled structures are assigned space groups and classified as calcite- and vaterite-like structures according to the arrangement of CO₃^2– and Ca²⁺ sheets. The initial crystal structure produced from ACC is a monoclinic crystal with Ca²⁺ sheets and CO₃^2– lying in the interlayer; however, it does not exhibit the 3-fold symmetry of calcite. Calcite structures with 3-fold symmetry, or orthorhombic crystals with rotated CO₃^2– units, as found in vaterite, can be derived from this structure. The orthorhombic structure then transitions to the more stable monoclinic form, which is likely vaterite. The understanding of phase transitions based on the diverse crystal structures of calcium carbonate revealed in this study provides a predictive view of biomineralization and carbonation reactions of cementitious materials.