Chert: biogenic

MicrocrystaUine Silicas. Various microcrystalline (cryptocrystalline) materials such as flint, chert, and diatomaceous earth are found ia nature (see Diatomite). These may arise from amorphous silica, often of biogenic origin, which undergoes compaction and microcrysta11i2ation over geologic time. 

In addition to the above crystalline phases sdica also exists in a few micro-crystadine forms. Such micro crystalline or cryptocrystalline silicas occur in nature and include diatomaceous earth, flint, and chert. They are mostly of biogenic origin forming from compaction of amorphous silica over geologic time. 

To sum up, the main mode of extraction of dissolved silica coming into the seas and oceans from the weathered layer is biogenic precipitation—a process widespread both in recent and in ancient sedimentary basins. Biogenic precipitation of silica from undersaturated solutions could also have been of definite importance in the formation of the Precambrian iron cherts, but no direct evidence has been obtained as yet. 

Sugitani K., Yamamoto K., Adachi M., Kawabe 1., and Sugisaki R. (1998) Archean cherts derived from chemical, biogenic and clastic sedimentation in a shallow restricted basin examples from the Gorge Creek Group in the Pilbara Block. Sedimentology 45(6), 1045-1063. 

Fig. 37. Lanthanide patterns for radiolarian chert sample and biogenic marine carbonates. Note the negative Ce-anomalies, particularly in the chert samples. (Data are from table 21.)...

Hesse, R.. Origin of Chert Diagenesis of Biogenic Siliceous Sediments. In Diagenesis . Mcllreath, I.A. Morrow, D.W., Eds. Geological Association of Canada, Geoscience Canada Reprint Series 4,1990 pp 227-251. 

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