Calcium Carbonate Prenucleation Cluster Pathway Observed via In Situ Small-Angle X-ray Scattering

For more than 150 years, our understanding of solid-phase mineral formation from dissolved constituent ions in aqueous environments has been dominated by classical nucleation theory (CNT). However, an alternative paradigm known as non-classical nucleation theory (NCNT), characterized by the existence of thermodynamically stable and highly hydrated ionic "prenucleation clusters" (PNCs), is increasingly invoked to explain mineral nucleation, including the formation of calcium carbonate (CaCO₃) minerals in aqueous conditions, which is important in a wide range of geological and biological systems. While the existence and role of PNCs in aqueous nucleation processes remain hotly debated, we show, using in situ small-angle X-ray scattering (SAXS), that nanometer-sized clusters are present in aqueous CaCO₃ solutions ranging from thermodynamically under- to supersaturated conditions regarding all known mineral phases, thus demonstrating that CaCO₃ mineral formation cannot be explained solely by CNT under the conditions examined.