Clay mineral growth can directly affect the chemistry and permeability of many natural systems, including soils, marine sediments, Earth surface waters, geothermal powerplants and carbon dioxide storage sites. Notably, the sluggish precipitation of clay minerals has been hypothesised to hinder Earth surface weathering rates. To date, there are limited data on the rate of clay mineral growth and on the effect of the aqueous solution saturation states on these rates. In this study we quantify the growth and dissolution rates of sepiolite, a 2:1 layered Mg-phyllosilicate (Mg 4 Si 6 O 15 (OH) 2 *6H 2 O) as a function of aqueous solution saturation state in a series of mixed flow experiments. Results of the both the dissolution and growth experiments are consistent with , where r refers to the surface area normalized growth rate of sepiolite in units of mol/cm 2 /s, denotes Gibbs Free energy of the sepiolite dissolution reaction, which is <0 for undersaturated solutions, 0 at equilibrium and >0 for supersaturated solutions, σ refers to Temkin's stoichiometric number, R stands for the gas constant and T symbolizes absolute temperature. This rate equation suggests that sepiolite dissolution and growth are consistent with transition state theory and the concept of micro-reversibility. The relative decrease in aqueous Mg 2+ and Si concentrations in the outlet aqueous solutions of the experiments, and the X-Ray diffraction patterns of the precipitates collected from the experiments, confirm the growth of crystalline sepiolite. The results of longer-term experiments suggest that sepiolite growth rates decrease over time. Such a decrease in the growth rate has been associated with poisoning or destruction of the
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