The Geochemistry of Silica in Sedimentary Environments
Amorphous silica dissolves in either fresh or marine water to the extent of 100- 140 ppm at ordinary temperatures. This is true solution, not colloidal dispersion, most or all of the silica being in the form of monosilicic acid, H4SiO4. Crystalline forms of silica have lower solubilities, quartz least of all. The solubility increases as temperature rises, but is little affected by changes of pH in the range 0-9.
Silica ordinarily does not precipitate at once from supersaturated solutions, but forms a colloid. In concentrated solutions the colloid may eventually set to a gel or precipitate as flocculent masses, but in fairly dilute solutions the dispersed sol is very stable. At equilibrium the sol contains dissolved silica in amounts given by the figures above, in addition to the colloidal particles. Tire reactions of forming and disaggregating tire sol particles are slow, so that supersaturated solutions of molecularly dispersed silica and very dilute solutions of colloidal silica can persist for days or weeks.
Colloidal silica may be precipitated by evaporation, by cooling, or ,by addition of electrolytes, but if the solution is at equilibrium tire dissolved part of the silica does not precipitate. Precipitation of dissolved silica may be brought about by organisms, by adsorption, by reaction with cations to form silicates, and probably by slow approach to equilibrium with a crystalline form of silica.
In natural waters silica is mostly in true solution. Its concentration is less tiran the amount at equilibrium with amorphous silica, both because it is derived largely from materials less soluble than amorphous silica and because it is removed from solution by the processes described above. Most precipitated silica is disseminated widely through sediments. To form chert requires an unusually abundant supply, diagenetic redistribution of freshly precipitated silica, or later replacement.
Figures & Tables
Silica in Sediments
The Symposium on Silica in Sediments was presented in March, 1958 at the meeting in Los Angeles. The subject was selected by the Research Committee and approval of the Council then proceeded to develop the symposium and organized the papers, and later was authorized to edit and prepare the papers for publication. Cards for written questions directed at the authors of the papers were available during the presentation of the papers. The authors had an opportunity to examine the questions and later to answer them as a panel before those attending the discussion. Additional questions and replies developed between panelists and members of the audience and such were admitted to the discussion for as long a period as seemed feasible within the time available. Written answers to most of the questions were prepared later by the panelists and appear in this publication after respective articles.