Prenucleation Cluster Pathway is Inconsistent with CaCO 3 Kinetics.

It has been debated whether CaCO 3 nucleates classically with the attainment of a critical cluster size or nonclassically with the restructuring of a prenucleation cluster (PNC). Here, we determine from the nucleation kinetics of CaCO 3 that the transition state is composed of about 10 formula units, irrespective of the supersaturation. Crucially, the size of the transition state is considerably smaller than the average PNC size estimated from experimental characterization. This size discrepancy suggests the PNCs are uninvolved in nucleation, and the kinetics indicate that if CaCO 3 nucleates classically, the transition state must be an abnormally unstable (antimagic) cluster.