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

Area of Science:

• Crystallography and Materials Science
• Computational Chemistry
• Geochemistry

Background:

• Calcium carbonate (CaCO3) exists in multiple crystal polymorphs, including calcite, aragonite, and vaterite.
• The formation pathway and structural details of vaterite, the least stable polymorph, are not well understood.

Purpose of the Study:

• To elucidate the formation pathway of vaterite from amorphous calcium carbonate (ACC).
• To investigate the structural evolution and phase transitions involved in vaterite formation.
• To provide a predictive understanding of calcium carbonate biomineralization and cement carbonation.

Main Methods:

• Well-tempered metadynamics molecular dynamics simulations were employed to reproduce the formation pathway of vaterite from ACC.
• First-principles calculations based on density functional theory were used to refine sampled structures.
• Space groups were assigned, and structures were classified based on the arrangement of calcium (Ca2+) and carbonate (CO32-) sheets.

Main Results:

• The initial crystal structure formed from ACC was a monoclinic crystal, lacking the 3-fold symmetry of calcite.
• Calcite-like structures with 3-fold symmetry and vaterite-like orthorhombic structures with rotated CO32- units were derived.
• The orthorhombic structure transitioned to a more stable monoclinic form, identified as likely vaterite.

Conclusions:

• This study reveals the formation pathway and phase transitions of vaterite from amorphous calcium carbonate.
• Understanding these diverse crystal structures and transitions offers predictive insights into biomineralization processes.
• The findings are applicable to understanding carbonation reactions in cementitious materials.