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Silica occurs as amorphous to crystalline form in many types of igneous, metamorphic, and sedimentary rocks, but in sediments and sedimentary rocks much of the silica is detrital material. The chief forms of silica are hydrous opal, crytocrystalline chalcedony, and crystalline quartz.

The primary aim of this symposium on silica in sediments is chiefly toward the geochemical and organic aspects rather than the study of detrital particles of silica. However, elastic deposits of siliceous organisms, volcanic ash, and other elastics serving as sources of silica are of interest.

Sedimentary silica enters into the development of authigenic quartz and silicates, into clay minerals, and into other secondary compounds as a result of geochemical action. The precipitation of chert, silicification of fossils, formation of skeletal parts and coverings of organisms, cementation of particles, and the relation of silica to diagenesis are matters which warrant research and discussion.

Knowledge of the various aspects of silica in sediments is important economically in regard to mineral deposits, petroleum geology, and ground water. Regeneration of quartz particles, cementation by silica, and differential solution have a great effect on porosity and permeability of aquifers and petroleum reservoirs, thereby reducing or increasing petroleum or water accumulation.

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