Modeling the effect of mineral armoring on the rates of coupled dissolution-precipitation reactions: Implications for chemical weathering

One of the mechanisms influencing weathering rates is mineral armoring, which is the build-up of insoluble coatings on primary mineral phases. Such armoring is often the result of coupled dissolution-precipitation reactions that occur during weathering. While the role of armoring in reducing dissolution rates is widely recognized, a quantitative mechanistic description of the way it impacts mineral weathering rates has not yet been developed. This paper demonstrates how a diffusion boundary layer model can simulate the effect of a porous secondary mineral layer on the rate of primary mineral dissolution under acidic conditions. In the model, the rates are affected by 3 parameters: layer thickness, the ratio of porosity to tortuosity, and the Biot number, which defines the ratio of dissolution to diffusion. Numerical solutions to the equations show that when the porosity/tortuosity ratio is low (~0.01), dissolution rates can be reduced by several orders of magnitude relative to armor-free minerals. Biot numbers greater than 1 were also found to lead to a significant reduction in the dissolution rate. Moreover, comparison with reported experimental data shows that the model can accurately simulate the change in thickness of the secondary mineral coating with time. The dissolution rates of pristine coating-free minerals in laboratory experiments can be orders of magnitude higher than those observed under field conditions, and the results suggest that mineral armoring can account for much of this discrepancy.