Nucleation of Framboids

Framboid microcrystals, which are intrinsically similar in size and habit within any individual framboid, must have all nucleated and grown at the same time. The formation of many thousands of equidimensional and equimorphic microcrystals in framboids is the fundamental evidence for burst nucleation. This is conventionally described by the LaMer model, which is characterized by (1) a lag phase before nucleation becomes significant; (2) burst nucleation where the rate of nucleation increases exponentially and may be completed in seconds; and (3) a short growth phase where nucleation becomes again insignificant. The growth phase is limited by the diffusion of Fe and S in stagnant, diffusion limited environments. By contrast, individual pyrite crystals evidence isolated nucleation and unlimited growth in advecting systems. The reaction with surface =FeS provided by sulfidized iron oxyhydroxides may a major route for producing individual pyrite crystals, rather than framboids, especially in sediments. Framboid formation by the nucleation of pyrite in solution can be described by classical nucleation theory (CNT), which leads to results consistent with observed critical supersaturation ranges, critical nucleus radius, and surface energies.