PNNL

Abstract

Although calcite is an important mineral for many processes, there ae been relatively few simulations of it's growth and dissolution behavior. Such simulations are complicated by the multitude of defect types and by the asymmetry of the crystal. The present work combined a kinetic Monte Carlo (KMC) technique with the Kossel crystal (100) simple cubic concept and the Blasius boundary layer model to simulate the simultaneous growth and dissoution of the (1014)calcite cleavage surface in flowing water. The objective was to determine the activation energies of the back reaction (growth) from those of the forward reaction (dissolution) by obtaining agreement with cleavage-step morphologies and step dissolution velocities previously measured using an atomic force microscope (AFM). Blasius boundary layer conditions for the flowing fluid defined a model that treated the solid, the dissolution/growth interface, and the fluid kinetics. Microscopic reversibility and the laws of large numbers gave an expression for the back reaction activation energies in terms of the forward reaction energies and the entropy of mixing, a quantity estimated from the concentration of desorbates in a very small fluid layer adjacent to the interface. The KMC simulations produced cleavage-step morphologies that were in qualitive agreement with observations from AFM. The kinetics were dominated by diffusion events on the solid/fluid interface and in the fluid, as expected. The relative magniyudes of the desorption and adsorption activation energies were consistent with experimental data, entropic arguments, and crystal roughening theories. Quantitive agrrement with measured step velocities was best when the boundary layer parameters were given physically reasonable values, indicating that the model is self consistent.