Modeling the growth of an altered layer in mineral weathering

A stochastic reaction-diffusion model on a lattice is introduced to describe the growth kinetics of an altered layer in the weathering of a mineral. Particles R represent H2O that permanently fills the outer surface and diffuse on M (mineral) and A (altered) sites with coefficients D-M and D-A, respectively. The transformation M vertical bar R -> A occurs with rate r, representing the irreversible formation of the altered material in a region of molecular size, viz. the lattice site of size a. These assumptions agree with predictions of the interfacial dissolution-reprecipitation mechanism, although the model does not describe the chemistry of dissolution reactions or precipitation processes. Scaling concepts are used to distinguish kinetic regimes and their crossovers, and are supported by simulation results. In the short time reactive regime, the thickness of the altered layer increases linearly in time and filling of that layer by particles R is high. In the long time diffusive regime, the altered layer thickness grows as (Dt(A))(1/2). Modeling of single crystals require very small values of D-M, which produces atomically narrow interfaces between the altered material and the mineral and absence of R in the latter, in agreement with recent experimental results. If r < D-M/a(2), an interface of thickness l(AM) similar to (D(M/)r)(1/2) is found, with subsurface fluid penetration and increased growth velocity of the altered layer by a factor l(AM)/a, but no fluid in the bulk mineral. Estimates of the order of magnitude of transformation rates and of diffusion coefficients are obtained by application of the model to some recently studied systems: calcite dissolution, labradorite weathering, and silicate glass weathering. Effects of dissolution of the altered layer are analyzed. Significant differences between the model and leached layer theories are discussed. (C) 2015 Elsevier Ltd. All rights reserved.