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 CaCO3solutions ranging from thermodynamically under- to supersaturated
conditions regarding all known mineral phases, thus demonstrating
that CaCO3mineral formation cannot be explained solely
by CNT under the conditions examined.