Framboid Self-Assembly and Self-Organization

The formation of framboids involves two distinct processes. First, pyrite microcrystals aggregate into spherical groups through surface free energy minimization. The self-assembly of framboid microcrystals to form framboids is consistent with estimations based on the classical Derjaguin-Landau-Verwey-Overbeek (DVLO) theory, which balances the attraction between particles due to the van der Waals forces against the interparticle electrostatic repulsive force. Second, the microcrystals rearrange themselves into ordered domains through entropy maximization. Icosahedral symmetry tends to minimize short-range attractive interactions and maximize entropy. The physical processes which facilitate this rearrangement are Brownian motion and surface interactions. Curved framboid interface enforce deviation from the cubic close packed structure. In the absence of a curved surface, weakly interacting colloidal particles preferentially self-assemble into a cubic close packed structure, and this is observed in irregular, non-framboidal aggregates of pyrite micro- and nanocrystals.