Chert

A dense, hard, fine grained, siliceous-rock with low porosity.

Cherts are tough, compact rocks with low porosity that are composed mainly of microcrystalline quartz (<20 microns in diameter) and varying amounts of impurities (Folk et al 1952; Smith 1960). The quartz occurs as randomly interlocked, microscopic quartz grains (microquartz) or fibrous chalcedony (Folk et al. 1952; Smith 1960; Oldershaw 1968; Knauth 1994). Coarser quartz (>20 microns) is termed megaquartz).

Cherts occur in a variety of geological settings, for example:

- Bedded cherts may form by compaction and recrystallization of silica-rich biogenic sediments made of opaline tests of single-cell organisms (diatoms, radiolaria) or remains of siliceous sponges, both in marine and in lake environments. During diagenesis, the silica in the sediments undergoes a transformation from opal-A through opal-CT to microcrystalline quartz in the mature chert (Oldershaw 1968; Calvert 1971; Lancelot 1973; Hein et al. 1981; Pisciotto 1981; Riech 1981; Levitan 1983; Jones et al. 1986; Compton 1991). Accordingly, these cherts may contain some opal-CT. Silica mobilized from volcaniclastic sediments, hydrothermal solutions and clay minerals may contribute to the silicification (Calvert 1971; Thurston 1972; Pollock 1987; Hesse 1989).

- Cherts in banded iron formations are thought to have formed from primarily chemically precipitated silica. Often they are colored brightly by co-precipitated iron minerals (Sugitani et al. 1998; Rosière et al. 2000; Maliva et al. 2005; Fisher et al. 2008).

- Many cherts appear to have been formed by silicification of volcaniclastic sediments and other rock types (Knauth 1994).

- Nodules, irregular bodies and discontinuous layers of chert are found in marine calcareous sediments. They typically form during early diagenesis by precipitation of silica mobilized from biogenic sources like radiolarian tests or sponge spicules. (Buurman et al. 1971; Meyers 1977; Bustillo et al. 1987; Maliva et al. 1989; Knauth 1994; Madsen et al. 2010).

- Magadi-type cherts, named after their occurrence at Lake Magadi, Kenya, form by leaching of alkali ions from silicates in silica-rich evaporites (Hay 1968; Eugster 1969).

The term "chert" is occasionally also used for massive rocks made primarily of microcrystalline quartz that deposits around submarine volcanic hydrothermal vents ("black smokers"; Hopkinson et al 1999; Gutzmer et al. 2001), or that formed as siliceous sinter, like Rhynie chert (Hesse 1989).

Siliceous precursor rocks of comparable texture that are primarily made of opal-A or opal-CT are sometimes also called chert, but usually with the addition of the dominant silica phase, for example, "opaline chert" (e.g. Weaver et al. 1972; Pisciotto 1981, Calvert 1983; Cady et al. 1996).