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Paragenetic history of the cementation observed in many sand-carbonate rocks can best be explained by understanding the complex physical-chemical changes which cause the precipitation and solution of cements in sediments.
Beta quartz and other metastable forms of SiO2 are widely distributed by sedimentary processes and commonly make up a considerable volume of some sediments. Several sources of silica are available: (1) abrasion of siliceous sediments along beaches, (2) siliceous tests, and (3) eolian quartz dust. Silica contributed by these sources forms silica cements in sediments.
Three zones of diagenesis beneath the depositional interface may be recognized, each with differing chemical and physical characteristics. The metastable forms of SiO2 are dissolved in the upper zone because the trapped sea water is universally undersaturated with respect to these phases. In the middle zone, alpha quartz precipitates as silica cement and overgrowths when the conentration of SiO2 of the interstitial fluid rises above 14 ppm. This is due to the higher solubility of the metastable forms of SiO2 (up to 140 ppm.) than that of alpha quartz (14 ppm.).
In the lowest or third zone, carbon dioxide concentration decreases while the temperature and pH increase slightly. These conditions favor precipitation of carbonate rocks and the solution of silica.
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