Silica Distribution in the System Quartz–Water–Vapor Depending on the Temperature Gradient

Abstract

Careful measurements of temperature on the surface of autoclaves indicate that small temperature gradients (TG) occur in the standard electric furnaces. These gradients can affect the results of long-lasting (up to 775 days) experiments in the system quartz–water–vapor at 300°C. From the bottom of the autoclave to its top, the temperature decreased in the upper parts of the electric furnaces and increased in their lower parts (TG =–0.08 and 0.15°C/cm, respectively). In the upper parts of the electric furnaces, the concentration of dissolved silica (m) was close to the quartz solubility (10 mmol/kg), and no other changes took place, which is consistent with the currently conventionally admitted notion that quartz is stable under these conditions. In the lower parts of the electric furnaces, m decreased to 0.03 mmol/kg, and opal was precipitated on the walls of the capsules above the solution (the opal was transformed into secondary quartz with time). These data suggest that no equilibrium silica distribution between liquid and vapor water phases was reached. We have suggested and analyzed as wide as possible circle of hypotheses conceivably able to explain this unequilibrated state. The most realistic explanation of the phenomenon seems to be that distillation is initiated by preferable evaporation of the solution in its thin (<100 nm) layer at the meniscus edge. A mathematic model of the process is suggested. The model is consistent with experimental data. The phenomenon in question can be detected in various experimental and technological systems and hampers the attainment of complete equilibrium. In natural systems, this phenomenon can lead to the migration of cavities partly filled with solution at an inversion of the geothermal gradient (beneath sills and lava flows).