DIFFERENCE IN THE MECHANISMS AND RATES OF QUARTZ DISSOLUTIONS AND PRECIPITATION AT 150°C

Abstract

The quartz-water reaction SiO2 + 2H2O = H 4SiO40 is usually described by the following rate law: dm/dt = kd(S/M)(1 - m/meq , where m and meq are the real and equilibrium concentrations of silica in the solution, t is the time, kd is the dissolution rate constant, S is the surface area of quartz, and M is the mass of water. This equation is used in mathematical modeling not only for undersaturated but also for supersaturated solutions, for which experimental data are very scarce. In order to fill this gap, experiments were carried out in the quartz-water system at 150°C. Quartz dissolution is adequately described by the above equation at k d = 9.7 × 10-11 mol m-2 s -1, which is similar to kd values reported by previous researchers. The concentrations of silica in solution approached the equilibrium value in only one month. In contrast, there was practically no quartz precipitation within the time interval studied (seven months). This implies violation of the principle of microscopic reversibility, which is inherent in the equation, i.e., quartz dissolution and precipitation proceed with different rates and by different reaction mechanisms. In particular, quartz precipitation occurs probably via formation of α-cristobalite, a metastable silica modification. The data presented were obtained in experiments with crushed quartz pretreated with fresh portions of water (five times for three days) at 250°C. The precipitation rate of quartz was much higher without such treatment because of the presence of dust particles. These particles were formed as a result of preliminary quartz dissolution rather than during its grinding. Most likely, the dust particles were detached from the surface of larger grains owing to predominant dissolution along surface defects, which were formed when the quartz was crushed.