The Behavior of Silica in Diagenesis

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Abstract
Chemical reactions between detrital minerals, the incorporated biota and their tests, and interstitial fluids are considered to tend toward equilibria under three different stages ot diagenesis of sediments. Such stages are: initial or depositional, intermediate or early burial, and late burial or pre-metamorphic. Collectively the effect upon quartzose sandstones is: (1) During deposition, there is distinct pitting and rounding of quartz grains by solution, and instability of detrital chert, i.e., minor but persistent solution of silica. (2) Early burial is characterized by precipitation of quartz as overgrowths, i.e., minor but characteristic precipitation of quartz. (3) Late burial is characterized by addition of carbonate cement and as a replacement of quartz grains; or, where carbonate is absent, interpenetration of quartz grains increasing in degree with depth of burial or application of pressure as indicated by folding. Among subgraywacke sandstones chert tends to be metastable during the early burial stage and reacts with clay to form authigenic clay minerals and micas. Deep burial is characterized by introduction of carbonates corroding quartz and replacing the clay matrix, and authigenesis of mica from interstitial clay. Occurrence of silica in Silurian carbonates with coral reef development shows interrelationships between carbonate minerals and chert which indicate chert is introduced during lithification of the reef flank sediments and precedes dolomitization. Analyses of saline formation waters from Paleozoic carbonate strata buried to depths of several thousands of feet within the Illinois Basin provide data which suggest that solubility of silica is independent of pH below values of 8, and that temperature may be the important control of silica solubility.

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