Dissolution and precipitation of silica at low temperatures

Abstract

The dissolution of amorphous silica as monosilicic acid and the establishment of solubility equilibrium with silica gel and with colloidal silica have been demonstrated in recent chemical papers. The rates of dissolution and precipitation are slow at ordinary temperatures; at temperatures near the boiling-point both solubilities and rates of dissolution are much higher. The solubility is little affected by pH between values of 1 and 9, but rises rapidly at pH's over 9. On the basis of this work and supplementary experiments, the geologic literature on the low-temperature solubility of silica is reviewed and various geologic applications are suggested. Much of the confusion in the literature can be traced to failure to recognize the slowness with which silica dissolves and precipitates or polymerizes. Opal dissolves in hot water to about the same extent as other forms of amorphous silica; probably it has a comparable solubility also at lower temperatures, but the rate of dissolution is so extremely slow that massive opal and diatomite can exist almost indefinitely in the presence of natural waters. In sea water amorphous silica has about the same solubility as in fresh water. Silica in true solution is not precipitated by electrolytes; colloidal silica may be precipitated by electrolytes, the rate and completeness of precipitation depending on pH and on the kind and concentration of the ions. Most of the silica in natural waters is in true solution rather than in colloidal dispersion. This means that silica brought to the sea by streams cannot be coagulated by the electrolytes of sea water. The factors that keep the concentration of silica below its equilibrium solubility (with respect to amorphous silica) in most natural waters are not completely understood; the slowness of dissolution, the use of silica by organisms and slowly-established equilibria with crystalline silica or with authigenic silicates doubtless all play a part. The origin of sedimentary chert may be plausibly ascribed to dissolution of remains of siliceous organisms and reprecipitation of the silica (initially as amorphous silica), but not in general to direct inorganic precipitation.