The mechanism of surface coating formation (the so-called surface altered layers [SALs] or leached layers) during weathering of silicate minerals is controversial and hinges on understanding the saturation state of the fluid at the dissolving mineral surface. Here we present in-situ data on the evolution of the interfacial fluid composition during dissolution of wollastonite (CaSiO3), obtained using interferometry and micro pH and ion-selective electrodes. Steep concentration gradients develop at the mineral interface as soon as it makes contact with the solution. This interfacial fluid becomes supersaturated with respect to amorphous silica that forms a surface coating, limiting fluid access to the mineral surface and hence affecting the dissolution rate. The thickness of the supersaturated zone and the precipitated layer depends on the relative rates of mass transport and surface reaction in the system; this effect could contribute to the discrepancy between dissolution rates measured in the field and in the laboratory. As well, our results have implications for predictions of silicate weathering rates and hence climate evolution, as different assumptions on dissolution mechanisms affect calculations on CO2 drawdown during weathering and consequent effects on estimates of global mean temperatures.

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