A classical view on nonclassical nucleation

Significance Nucleation is the process by which constituent building blocks first assemble to form a new substance. In the case of mineral formation from initially free ions in solution, the emergence of intermediary phases often determines the thermodynamics and kinetics of formation for the most stable phase. Our work on CaCO 3 mineralization reevaluates a topic of intense discussion: Can nucleation be explained by theories established over a century ago, or should new physical concepts, as recently proposed, be adopted? Our data show that classical theories can indeed be used to describe complex mechanisms of crystallization. In addition, we provide information about the properties of intermediate phases, which will aid in the design of additives to control mineralization.