Mechanisms influencing micron and nanometer-scale reaction rate patterns during dolostone dissolution

- Atomic force microscope study of dolostone dissolution.
- Secondary Mg-rich phase comprising nano-scale particles forms on surface.
- Reaction rate spectra are typically asymmetric and skewed towards higher values.
- Asymmetric spectra are due to high dissolution rates at grain boundaries and etch pits.
- General extreme value model captures asymmetric behavior of rate spectra.

Water-rock interactions often involve the dissolution of a primary mineral and the precipitation of a new secondary phase. Many of the mechanisms governing such coupled reactions can only be observed directly using high resolution imaging techniques. In this study, atomic force microscopy was used to examine dolostone dissolution at the nanometer and micron scale at different pH conditions in the range 3.5-4.5. During the experiments, a secondary Mg-rich phase comprising nano-scale particles forms on the surface. Importantly, the precipitate does not significantly inhibit dissolution of the dolostone, and although the overall rates of surface retreat are highly dependent on pH, similar mechanisms are found to govern the evolution of the surface. At all pH values, high dissolution rates are observed at etch pits and along grain boundaries, resulting in rate spectra (probability density functions of reaction rates) that are often highly asymmetric and skewed towards higher values. A model based on extreme value theory performs well at capturing the long tails characteristic of the asymmetric distributions, indicating a possible route towards predicting rate spectra in dissolving rocks.