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.
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.
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