Despite the fact that reactive transport modeling has been established as an indispensable tool in earth and environmental studies, it is striking how little progress has been made in implementing in them mineral-fluid rate laws that capture the entire set of dynamic processes from nucleation, to crystal growth, to mineral aging and transformation via the Ostwald Step Rule.  Arguably the first such model was developed and implemented in a reactive transport (RT) framework by Steefel and Van Cappellen in 1990, who based their approach at the time on the calculation of a crystal size distribution (CSD) that transformed as a function of time and environmental conditions.  An alternative approach is explored in this talk wherein discrete mineral “pools” are included in the RT framework that may represent either distinct mineral phases (as in the classical Ostwald Step Rule), or may be separate pools of the same mineral with different properties (e.g., specific surface area, intrinsic reactivity, or solubility).  Transformation rates from one mineral pool to another are governed by semi-empirical rate constants based on the thermodynamic driving force and the mass or surface area of the reacting phase.  Nucleation is treated within the continuum framework with largely classical exponential rate laws that depend on the supersaturation and interfacial free energy, but also non-classical nucleation pathways.  The Transition State Theory or TST rate laws used Steefel and Van Cappellen (1990) are replaced with process-based rate laws that incorporate ion by ion attachment and detachment reactions based on step densities and velocities.  The new treatment of the continuum from nucleation to crystal growth and aging is applied to an experimental dataset in which magnesite nucleates and grows as a result of forsterite dissolution in CO<sub>2</sub>-saturated aqueous solution.

New approach to modeling nucleation, crystal growth, and the Ostwald step rule

251st National Meeting (2016)

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