Disturbance of Thermodynamic Equilibrium of the Quartz-Water System and Silica Separation from the Liquid Phase at a Small Temperature Gradient

In electric furnaces destined for experimental geochemical investigations, a small temperature gradient (TG) always exists, and it is difficult to avoid it. In our electric furnaces (from bottom to top) the temperature rises in the lower part (TG = 0.25°/cm) and falls in the upper part (TG = -0.16°/cm). Long-term runs with quartz and water at 300 °C have shown the attainment of equilibrium at TG equal to -0.16°/cm. At TG equal to 0.25°/cm, the silica re- precipitated from the ampoule bottom onto its walls above the meniscus, and the aqueous silica concentration was reduced and became 300 times lower than quartz solubility. The obtained results were explained by the combined action of slow convection of water and thermal diffusion of silica. The efficient mechanism of fractionation revealed by us can be extended to other substances and has geochemical and technological importance.