Abstract
Quantitative prediction of mineral reaction rates in the subsurface remains a
daunting task partly because a key parameter for macroscopic models, the reactive site
density, is poorly constrained. Here we report atomic force microscopy (AFM)
measurements on the calcite surface of monomolecular step densities, treated as
equivalent to the reactive site density, as a function of aqueous calcium-to-carbonate ratio
and saturation index. Data for the obtuse step orientation are combined with existing step
velocity measurements to generate a model that predicts overall macroscopic calcite
growth rates. The model is quantitatively consistent with several published macroscopic
rates under a range of alkaline solution conditions, particularly for two of the most
comprehensive data sets without the need for additional fit parameters. The model
reproduces peak growth rates and its functional form is simple enough to be incorporated
into reactive transport or other macroscopic models designed for predictions in porous
media. However, it currently cannot model equilibrium, pH effects, and may
overestimate rates at high aqueous calcium-to-carbonate ratios. The discrepancies in
rates at high calcium-to-carbonate ratios may be due to differences in pre-treatment, such
as exposing the seed material to SI ≥ 1.0 to generate/develop growth hillocks, or other
factors.
