An experimental study of the effect of aqueous fluoride on quartz and alkali-feldspar dissolution rates

Far-from-equilibrium, steady-state dissolution rates at pH 2 of quartz at 50 and 100 °C, K-feldspar at 50 and 100 °C, and albite at 25 °C have been determined as a function of aqueous NaF concentration up to 5×10−3 mol/kg from mixed-flow reactor experiments. Albite rates were also measured as a function of aqueous Si, Al and Na activity. Dissolution rates of each mineral increase monotonically with increasing NaF concentration. Measured quartz dissolution rates are found to be consistent with the surface speciation model proposed by Dove [Am. J. Sci. 294 (1994) 665] and are successfully modeled assuming rates stem from the sum of the detachment rates of two distinct surface species: >SiOH0 and >SiOH2F0. Measured alkali-feldspar dissolution rates are found to be consistent with the general multi-oxide dissolution mechanism proposed by Oelkers [Geochim. Cosmochim. Acta 65 (2001) 3703]; these rates are successfully described usingr+=k(a3H+/aAl3+)nwhere r+ represents the surface area normalized far-from-equilibrium dissolution rate, k refers to a rate constant, ai denotes the activity of the subscripted aqueous species and n defines a stoichiometric constant. The effect of aqueous fluoride on feldspar dissolution rates in this mechanism stems from aqueous Al–F complex formation, which lowers aAl3+. Values of the constant n obtained from data regression correspond closely to those reported from fluoride-free experiments reported by Oelkers et al. [Geochim. Cosmochim. Acta 58 (1994) 2011] and Gautier et al. [Geochim. Cosmochim. Acta 58 (1994) 4549]. This close agreement suggests this same mechanism can be used describe accurately the effect on aluminosilicate dissolution rates of aqueous species other than F−.