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Abstract

Silica, a general term used for a variety of crystal forms or morphologies of SiO2, is a widespread diagenetic mineral in carbonate rocks. Silica may occur as cement or it may be found as a replacement of original or diagenetically altered sediment. Silica typically replaces or infills carbonate minerals, evaporites and organic material (e.g., petrified wood).

The major source of silica for diagenesis is biogenic opal; therefore, silica is especially prevalent in deep-marine sediments from active upwelling zones and shallower-water carbonates from nutrient-rich carbonate shelves. Sponge spicules, diatoms and radiolarians are the most common biogenic contributors and are diagenetically unstable when compared to siliciclastic grains. Other, generally less significant sources of silica in carbonate rocks include volcanic ash, by-products of chemical weathering in soil zones (silcretes), and hydrothermal fluids. Some bedded cherts from saline lakes may be related to hydrous sodium silicate precursors (e.g. Eugster, 1967).

Except for silcrete formation or hydrothermal alteration, silica diagenesis is rarely a very early- or a very late-stage diagenetic event in carbonate rocks. Rather, it is most typically a product of burial diagenesis. This is due to the timing of the conversion of biogenic opal-A, first to opal-CT lepispheres, and then to stable microquartz or megaquartz. These silica reactions are dependent on temperature (and/or burial depth) and time. In pelagic deposits (away from hydrothermal input), opal-A to opal-CT conversion begins at 20-30°C and may take 10 million years to go to completion; opal-A is rarely found in sediments older than 20 Ma (Hesse, 1990). The conversion of opal-CT to quartz most likely starts at temperatures of ~ 50°C and depths of 500 m, but continues to higher temperatures. Opal-CT is not found in sediments older 144 Ma and chert is relatively scarce in young Cenozoic deposits.

Amorphous silica— also known as opal; isotropic; high negative relief; colorless to gray or brown; normally contains irregular cracks or fractures; occurs as cements, nodules or replacements (especially wood).

Equigranular quartz — equant crystals; in polarized light, the maximum birefringence should be first-order white to pale straw-yellow (unless the thin section is thicker than normal); larger individual crystals are normally hexagonal and may be doubly terminated; no cleavage; normally colorless, but may contain inclusions. Fabric is termed cryptocrystalline (chert) when crystals are <5 μm, microcrystalline for crystals 5-20 μm, and megaquartz for crystals >20 μm. Quartz may occur as individual crystals or in large nodular masses replacing or displacing sediment.

Fibrous quartz — elongate fibers of quartz; same birefringence as equigranular quartz (but birefringence decreases with increasing water content); colorless to brown; common banding or zoning (bands may consist of alternating forms of chalcedony); commonly forms cements, small to very large nodules, and may pseudomorphs other grains nodules or minerals.

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