Chapter 11. Nucleation in geological materials
Nucleation is the initial process of most phase transformations and is of fundamental importance for the kinetics of mineral reactions. A departure from equilibrium is required to overcome the energy barrier associated with nucleation, which is a function of the structural and compositional differences between the nucleus and the metastable reactant, and the level of elastic deformation experienced by the nucleus as it forms in the host lattice. Nucleation in geological materials almost always takes place at grain boundaries, crystal defects or impurities, which catalyse the nucleation process and influence the chemical composition, size, shape, lattice orientation and spatial distribution of nuclei with important implications for the texture and microstructure evolution of rocks. Nuclei are microscopic in most systems and thus are too small to be observed in experiment. This is why nucleation has been intensively studied theoretically and through numerical simulations. In those treatments, nucleation integrates more elementary processes such as chemical diffusion and interface motion.
This chapter provides the essential physics of the thermodynamics and kinetics of nucleation. It reviews the fundamentals of the classical nucleation theory including the chemical driving force for nucleation in partitioning systems and the interfacial area of clusters, discusses possible nucleus/substrate interactions and their influence on the free energy of the nucleus and the energy barrier to nucleation, and presents expressions for the classical nucleation rate. In a second part, extensions to CNT are outlined that couple long-range diffusion with the kinetics of interface processes in order to address the formation of nucleation exclusion or depleted zones around supercritical clusters and the enrichment of the precipitated components in the vicinity of subcritical clusters. Finally, the reader is introduced to non-classical gradient-energy continuum approaches to nucleation in inhomogeneous systems, and to the phase field method for the simulation of microstructure evolution.
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