THE DISSOLUTION KINETICS OF QUARTZ IN SODIUM-CHLORIDE SOLUTIONS AT 25-DEGREES-C TO 300-DEGREES-C

A general expression for the dissolution kinetics of quartz is developed from a compilation of published rate measurements and new hydrothermal data. The rate equation is based upon a surface reaction model that correlates changes in modeled surface complexes with quartz reactivity in aqueous solutions. The model is fitted to 271 independent measurements of dissolution rate and quantifies the reaction kinetics at 25-degrees to 300-degrees-C for solution pH(T) of 2 to 12 and 0 to 0.3 molal sodium by the general equation: rate = exp-10.7 T exp (-66000/RT)(theta>SiOH)1 + exp4.7 T exp (-82700/RT)(theta(>SiO(tot)-)1.1 where quartz dissolution rate (mol m-2 s-1) is determined by T (temperature, degrees-C), theta(>SiOH) (fraction of surface complexes as >SiOH), and theta(>SiO(tot-)) (fraction of >SiO- plus >SiO-Na+). The rate equation gives a good fit to the data over a wide range of reaction rates (10(11)) and has properties suggesting robustness. It describes the pH dependence of dissolution rates, cation-specific effects observed at near-neutral pH, their diminishing effects at higher pH, and has a reaction order near one, implying first order behavior. Results of this study suggest the dissolution mechanism is independent of temperature over 25-degrees to 300-degrees-C and support observations that the temperature dependence of surface association constants varies with the association constant of water. The mechanistic model proposes that solution composition affects quartz dissolution rates by changes in the properties and interactions of water with surface structures. Total reactivity is determined by the relative contributions of water and hydroxide-promoted dissolution as endmember mechanisms. At acidic H, quartz dissolution is limited by slow reaction with weakly nucleophilic molecular water. At high pH, the accumulation of net-negative surface charge facilitates water polarization to favor a hydroxide-promoted mechanism. These mechanisms are quantified in the first and second terms of the rate equation, respectively. At intermediate pH, alkali cations play an indirect role in increasing dissolution rates by affecting solvent properties to promote the mechanism involving hydroxyl ion. The magnitude of rate enhancement by alkali correlates with the free energy of hydration of individual alkali ions. This dissolution rate model can estimate quartz reactivity in diverse natural and engineered earth systems.