Mechanisms of classical crystal growth theory explain quartz and silicate dissolution behavior

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.silica ͉ kinetics ͉ mineralization O ver long time scales, the geochemistry of earth systems is, in large part, controlled by the kinetics of silicate mineral dissolution. Because waters contain a wide variety of solute types and concentrations, including significant levels of aqueous silica, there is considerable need to understand the dependence of silicate mineral dissolution rates on chemical driving force, as measured by the extent of undersaturation. This need has motivated intense investigations of both mineral weathering and the corrosion behavior of silica-based glasses.Basic thermodynamic principles predict that mineral dissolution rates should increase with increasing driving force or chemical potential; however, experimental studies of major silicate minerals show that this dependence is complex. A further complication is the so-called ''salt effect'' reported for quartz, SiO 2 , whereby the dissolution rate of this oxide end-member to all silicates is increased up to 100 times in the presence of the major cationic solutes found in natural waters (Na ϩ , K ϩ , Ca 2ϩ , Mg 2ϩ ) (1-3). In contrast, dissolution rates of silicate minerals have only a weak sensitivity to the introduction of electrolytes (4). To explain these behaviors, many of the widely used rate models are based on variants of transition state t...

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