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Geology is the Way


Sample of red radiolarite (or radiolarian chert). Western Tatras, Poland. Photo by Chrumps.

Radiolarite is a siliceous rock constituted by lithified, radiolarian-rich siliceous ooze. Radiolarians are microfossils with a siliceous shell that are part of the zooplankton and live in the first 200 meters of the water column. After their death, radiolarians accumulate on abyssal plains, forming sequences of radiolarian cherts. In general, the deposition of siliceous ooze occurs at very high depth (4000 – 5000 meters in present-day oceans), because deep waters are rich in CO2 that dissolves carbonatic shells of other organisms, concentrating the siliceous ooze.

Dissolution of carbonate shells of foraminifera and silica shells of radiolarians vs depth. Siliceous shells are highly soluble in shallow waters, but can be preserved at depth. On the other hand, carbonate solubility in marine water increases strongly with depth. Modified after Berger et al. in Hsu & Jenkins, 1974.

Diagenetic processes progressively transform the unstable, amorphous hydrated silica of the shells of radiolarians into opal with cryptocrystalline crystals of cristobalite and trydimite, then into cryptocrystalline quartz. Radiolarian chert sequences frequently occur as rhythmic alternations of bedded chert and shales. Many studies have shown that bedded cherts appears linked to global fluctuations of silica in equatorial waters determined by astronomical cycles that occur on time scales of thousands to millions of years. However, other studies have also proposed that their origin is also related to other processes, such as diagenetic enrichment of silica in some layers.

Cherts can be recognized because they are made of cryptocrystalline quartz that shows a conchoidal fracture and it is harder to scratch than metal (around 7 on the Mohs scale): the recognition of radiolarian cherts requires the analysis of the chert at the microscope.

A radiolarian-rich siliceous ooze seen at the Scanning Electron Microscope. Eocene. Photo by Yasuhiro Hata.
Bedded and faulted radiolarian cherts (thick red layers), separated by thin layers of shale. San Simeon State Park, California, U.S.A. Photo: Peter D. Tillman.
Folded sequences of radiolarite (white) and shale (red) from the Batain Group, Eastern Oman. Photo by Simon Virgo.

Abrajevitch, A. (2020). Diagenetic formation of bedded chert: Implications from a rock magnetic study of siliceous precursor sediments. Earth and Planetary Science Letters533, 116039.
Hori, R. S., Cho, C. F., & Umeda, H. (1993). Origin of cyclicity in Triassic‐Jurassic radiolarian bedded cherts of the Mino accretionary complex from Japan. Island Arc2(3), 170-180.
Ikeda, M., Tada, R., & Sakuma, H. (2010). Astronomical cycle origin of bedded chert: a middle Triassic bedded chert sequence, Inuyama, Japan. Earth and Planetary Science Letters297(3-4), 369-378.
Ikeda, M., Tada, R., & Ozaki, K. (2017). Astronomical pacing of the global silica cycle recorded in Mesozoic bedded cherts. Nature communications8(1), 1-9.
Jones, D. L., & Murchey, B. (1986). Geologic significance of Paleozoic and Mesozoic radiolarian chert. Annual Review of Earth and Planetary Sciences14(1), 455-492.
McBride, E. F., & Folk, R. L. (1979). Features and origin of Italian Jurassic radiolarites deposited on continental crust. Journal of Sedimentary Research49(3), 837-868.


Detrital and Authigenic Minerals
Sedimentary Structures
Sedimentary Rocks


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