A classical view on nonclassical nucleation

Understanding and controlling nucleation is important for many crystallization applications. Calcium carbonate (CaCO3) is often employed as a model system to investigate nucleation mechanisms. Despite its great importance in geology, biology, and many industrial applications, CaCO3 nucleation is still a topic of intense discussion, with new pathways for its growth from ions in solution proposed in recent years. These new pathways include the so-called “non-classical” nucleation mechanism via the assembly of thermodynamically stable prenucleation clusters, as well as the formation of a dense liquid precursor phase through a liquid-liquid phase separation. Here, we present results from a combined experimental and computational investigation on the precipitation of CaCO3 in dilute aqueous solutions. We propose that a dense liquid phase (containing 4–7 H2O per CaCO3 unit) forms in supersaturated solutions through the association of ions and ion-pairs without significant participation of larger ion clusters. This liquid acts as the precursor for the formation of solid CaCO3 in the form of vaterite, which grows via a net transfer of ions from solution according to z Ca2+ + z CO32-  z CaCO3. The results show that all steps in this process can be explained according to classical concepts of crystal nucleation and growth, and that long-standing physical concepts of nucleation can describe multi-step, multi-phase growth mechanisms.