A nanostructured amorphous precursor confines the nucleation of iron oxide nanocrystals Crystallization from solution is commonly FIGURE 1 described by the classical nucleation theory, ignoring that crystals often form “non- classically” via disordered nanostructures. Alternatives to the classical nucleation theory exist, but still lack experimental validation at proper time and spatial scales (ms, nm). Here, we used in situ X-ray scattering to follow the formation of magnetite nanocrystals, and found that the self-confinement by an amorphous precursor slows down crystal growth by two orders of magnitude once the crystal size reaches the amorphous particle size (ca. 3 nm). NON-CLASSICAL NUCLEATION Crystallization of solids from dilute solutions governs the formation of many synthetic, biogenic and geologic minerals. This ubiquitous process has long been described by “classical” nucleation theory (CNT): crystal nuclei capture or release solute monomers through thermal fluctuations; nuclei that reach a critical size, where lattice energy overcomes interfacial energy, grow faster than they dissolve and finally reach macroscopic sizes. [1] However, the CNT is questioned since recent experimental evidence has pointed towards crystals forming through amorphous intermediate states: [2] droplets, polymers, clusters and nanoparticles that only crystallize later, leading to a so-called “non-classical” nucleation scenario. The hypotheses of the CNT are incompatible with the observed amorphous intermediate states, which would explain why the predicted nucleation rates are inconsistent by several tens of orders of magnitude [3]. A common correction is to describe non-classical crystallization as a multi-step process where CNT still applies to each phase transition, i.e. from the metastable solution, to the amorphous phase, to the crystals. But this alternative theory has been tested only incompletely; in particular, it requires well-defined 40