The dissolution kinetics of quartz as a function of pH and time at 70°C

Abstract

A single-pass, flow-through apparatus was used to determine the dissolution rate of quartz at 70°C as a function of pH and time. Dissolution rate data were obtained over the pH range 1.4 to 11.8 in nine separate experiments each lasting 50 days. The quartz dissolution rates were defined by the silica release rate to solution. Speciation-solubility calculations using the geochemical modeling code EQ3/6 indicate that the fluid was maintained far from equilibrium with respect to quartz and well-undersaturated with respect to all possible secondary minerals in all runs. The dissolution rates were independent of pH at values (10 −15.3 mol/cm 2 · s) consistent with the data of Rimstidt and Barnes (1980) up to approximately pH 6, but at higher pH the rates increased with increasing pH, proportional to a H + −0.5 , being almost four orders of magnitude higher at pH 11.8. The rate constants for quartz dissolution at 70°C were 10 −15.3 mol/cm 2 · s in the pH-independent region extending from acid through neutral solutions, and 10 −17.8 mol/cm 2 · s in more alkaline solutions. Etch pits were strongly developed in the runs with the more alkaline solutions (pH > 8), in which the rates were the highest. This appears consistent with a surface reaction-controlled dissolution mechanism.