Chert

Required Geological Setting

Neogene and Quaternary sediments, Paleogene and older sediments, Accretionary complexes

Occurrence

Rocks composed mostly of microcrystalline quartz. There are two types of chert: chert of sedimentary origin and chert of diagenetic origin. The sedimentary chert is a type of sedimentary rock composed of remains of radiolaria having siliceous frames with few other sedimentary particles. The diagenetic chert, flint, is siliceous nodules formed by inorganic diagenetic process in limestone and dolostone. The sedimentary chert is deposited in the middle of ocean where continental clastics are not supplied as it is far enough from lands, and on the seafloors deeper than the carbonate compensation depth (CCD). Calcareous sediments such as remains of planktonic calcareous foraminifera and coccolithophorids do not deposit as they are solved in seawater depper than CCD. CCD is 4,000-5,000m at low latitude locations and is shallower at higher latitude locations. Cherts with graphite impurities show black color, and with hematite impurities show red color. The origin of pale green color of cherts is Fe²⁺ formed by reduction of Fe³⁺ in hematite-bearing red chert.