Abstract

Petrographic studies show that carbonate replacement of detrital crystalline silicate minerals is an important and widespread process in sedimentary rocks. Investigations of three different rock suites indicate that authigenic siliceous features commonly occur in or near strata that contain partially to completely replaced silicate grains. This association implies a genetic relationship and suggests that silica released by carbonate replacement may be an important source of authigenic silica in some sedimentary rocks.

Such replacement is probably more common in sedimentary rocks than has been suspected because completely replaced grains commonly leave no evidence of their original presence in the sediment, or the replaced grains are preserved as carbonate pseudomorphs that can be mistaken for grains of clastic carbonate. Even in occurrences containing relicts of incompletely replaced silicate grains it generally is not possible to determine to what extent replacement has taken place.

The mechanism of solution, migration, and reprecipitation of silica is uncertain. Earlier writers have suggested that carbonate replacement of opal and subsequent reprecipitation of the released silica occurs in response to pH variations. If so, the replacement and reprecipitation of silica from crystalline silicates might be similarly explained. Recent data, however, cast doubt on this theory.

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