Abstract
The central control of mineral weathering rates on biogeochemical
systems has motivated studies of dissolution for more than 50
years. A complete physical picture that explains widely observed
variations in dissolution behavior is lacking, and some data show
apparent serious inconsistencies that cannot be explained by the
largely empirical kinetic ‘‘laws.’’ Here, we show that mineral
dissolution can, in fact, be understood through the same mechanistic
theory of nucleation developed for mineral growth. In
principle, this theory should describe dissolution but has never
been tested. By generalizing nucleation rate equations to include
dissolution, we arrive at a model that predicts how quartz dissolution
processes change with undersaturation from step retreat, to
defect-driven and homogeneous etch pit formation. This finding
reveals that the ‘‘salt effect,’’ recognized almost 100 years ago,
arises from a crossover in dominant nucleation mechanism to
greatly increase step density. The theory also explains the dissolution
kinetics of major weathering aluminosilicates, kaolinite and
K-feldspar. In doing so, it provides a sensible origin of discrepancies
reported for the dependence of kaolinite dissolution and growth
rates on saturation state by invoking a temperature-activated
transition in the nucleation process. Although dissolution by nucleation
processes was previously unknown for oxides or silicates,
our mechanism-based findings are consistent with recent observations
of dissolution (i.e., demineralization) in biological minerals.
Nucleation theory may be the missing link to unifying mineral
growth and dissolution into a mechanistic and quantitative framework
across the continuum of driving force.
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