Water is the key to nonclassical nucleation of amorphous calcium carbonate.

Calcium carbonate is a ubiquitous mineral that represents one of the most significant biominerals, a major contributor to carbon sequestration through geological deposits, and a technological hindrance as a result of scale formation. Amorphous calcium carbonate is intimately involved in the nucleation and growth of this material, yet much remains undiscovered regarding the atomic detail. Through dynamical simulation we demonstrate that nucleation of amorphous calcium carbonate follows a nonclassical pathway. This arises from the addition of ion pairs to clusters exhibiting a consistently exothermic free energy that persists with increasing particle size. Furthermore, the disruption of the surrounding water of solvation by the atomically rough surface reduces the barrier to growth to the order of ambient thermal energy, thereby allowing the amorphous phase to grow faster than crystalline polymorphs. Amorphous calcium carbonate nanoparticles are also found to exploit size-dependent water content to render itself more stable than the favored bulk phase, calcite, below a critical diameter of close to 4 nm.