Solubility and near-equilibrium dissolution rates of quartz in dilute NaCl solutions at 398-473K under alkaline conditions

The dissolution-precipitation of quartz controls porosity and permeability in many lithologies and may be the best studied mineral-water reaction. However, the rate of quartz-water reaction is relatively well characterized far from equilibrium but relatively unexplored near equilibrium. We present kinetic data for quartz as equilibrium is approached from undersaturation and more limited data on the approach from supersaturated conditions in 0.1molal NaCl+NaOH+NaSiO(OH)₃ solutions with pH 8.2-9.7 at 398, 423, 448, and 473K. We employed a potentiometric technique that allows precise determination of solution speciation within 2kJmol^-1 of equilibrium without the need for to perturb the system through physical sampling and chemical analysis. Slightly higher equilibrium solubilities between 423 and 473K were found than reported in recent compilations. Apparent activation energies of 29 and 37kJmol^-1 are inferred for rates of dissolution at two surface sites with different values of connectedness: dissolution at Q¹ or Q² silicon sites, respectively. The dissolution mechanism varies with ΔG such that reactions at both sites control dissolution up until a critical free energy value above which only reactions at Q¹ sites are important. When our near-equilibrium dissolution rates are extrapolated far from equilibrium, they agree within propagated uncertainty at 398K with a recently published model by Bickmore et al. (2008). However, our extrapolated rates become progressively slower than model predictions with increasing temperature. Furthermore, we see no dependence of the postulated Q¹ reaction rate on pH, and a poorly-constrained pH dependence of the postulated Q² rate. Our slow extrapolated rates are presumably related to the increasing contribution of dissolution at Q³ sites far from equilibrium. The use of the potentiometric technique for rate measurement will yield both rate data and insights into the mechanisms of dissolution over a range of chemical affinity. Such measurements are needed to model the evolution of many natural systems quantitatively.