Impact of nano-size weathering products on the dissolution rates of primary minerals

The natural weathering rates of primary minerals are often orders of magnitude lower than the rates of mineral dissolution measured in laboratory experiments. Primary dissolution rates are thought to be determined by the rate of secondary mineral precipitation, and in this paper we present a new approach to quantify the role played by interfacial energy, crystal size, and degree of supersaturation on precipitation kinetics in a population of crystals growing in a supersaturated fluid. We demonstrate that net mineral precipitation rates in systems that are close to equilibrium, and which possess a large number of micron and nanometer scale crystals, can be much lower than the rates predicted by standard kinetic equations. Moreover, when crystals are small enough, net dissolution dominates even when the system is supersaturated with respect to large crystals so that the standard reaction rate models used to describe bulk rates will no longer apply. Importantly, secondary minerals that form from the incongruent dissolution of primary phases are often submicron in size and field conditions are often far closer to equilibrium than those typically encountered in laboratory experiments. Thus, we propose that standard kinetic models - which ignore interfacial energy effects in small crystals - may be unsuitable to describe reaction kinetics in weathering systems.