From Non-Classical to Classical: Crystallization Seeds Reshape Nucleation Mechanisms

Crystalline seeds are widely employed in crystallization to accelerate nucleation and control product polymorphs, yet their impact on nucleation mechanisms remains poorly understood. While homogeneous nucleation of crystals from solution often proceeds through non-classical pathways involving amorphous intermediates, it is unclear how seeds that promote heterogeneous nucleation reshape these mechanisms and govern polymorph selection. Here, we provide the first direct evidence that crystalline seeds can bypass amorphous intermediates, converting non-classical nucleation mechanisms into classical, monomer-by-monomer crystallization pathways. Using molecular dynamics simulations of zeolite synthesis, we uncover a complex reaction network of competing nucleation processes mediated by intermediate interfacial polymorphs. The interplay between thermodynamic stability and kinetic favorability of these interfacial polymorphs dictates nucleation outcomes, creating a dynamic tension between interfacial polymorph stability and crystallization rates. Fur-thermore, we show that the synthesis environment—whether monomers or aggregates serve as reactants—profoundly impacts these pathways. At moderate supersaturation, seeds eliminate amorphous intermediates and promote classical nucleation, whereas high super-saturation or aggregate-based reactants favor non-classical pathways, even in the presence of a seed. These findings establish a general framework for understanding how seeds govern crystallization mechanisms, with broad implications for controlling nucleation kinetics, polymorph selection, and material properties. While focused on zeolites, this work reveals insights applicable to biominerals, pharmaceu-ticals, functional materials, and catalysts, providing a basis for engineering crystallization pathways in diverse applications.