Solid‐State Transformation of Amorphous Calcium Carbonate to Aragonite Captured by CryoTEM

Early‐stage reaction mechanisms for aragonite‐promoting systems are relatively unknown compared to the more thermodynamically stable calcium carbonate polymorph, calcite. Using cryoTEM and SEM, the early reaction stages taking place during aragonite formation were identified in a highly supersaturated solution using an alcohol–water solvent, and an overall particle attachment growth mechanism was described for the system. In vitro evidence is provided for the solid‐state transformation of amorphous calcium carbonate to aragonite, demonstrating the co‐existence of both amorphous and crystalline material within the same aragonite needle. This supports non‐classical formation of aragonite within both a synthetic and biological context.