Aragonite-Calcite Nucleation Kinetics: An Application and Extension of Avrami Transformation Theory

Information on nucleation rates and processes in the solid-state transformation of aragonite to calcite is derived by fitting Avrami models of nucleation and growth to existing data on the overall rate of reaction, using recent measurements of calcite growth rates. Aragonite samples of diverse origins and morphologies exhibit extremely rapid nucleation at the onset of the transformation, with negligible subsequent nucleation. Preferential nucleation on surfaces of transforming domains produces systematic discrepancies between observed reaction rates and the kinetics calculated from standard Avrami theory; these discrepancies may be resolved in large part by reformulating the kinetic equations to encompass both surficial and internal nucleation in domains of idealized shape and finite size. When fitted to a model of this sort, the data require that surficial nucleation greatly dominates internal nucleation, in agreement with petrographic observations and theoretical expectations. These models demonstrate that transformation functions may display a first-or second-order time dependence, even though all grains are growing isotropically in three dimensions. Consequently, growth mechanisms or product morphologies may not be inferred solely from the order of time dependence of transformation.