Cherts are tough, compact rocks with low porosity that are composed mainly of microcrystalline quartz 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).
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 silicious 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).
- Some Archean cherts appear to have been formed by silicification of volcaniclastic sediments (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 silicious 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).
Classification of Chert
Sub-divisions of Chert
Mineralogy of Chert
|Opal-A||A petrological term for Opal-A.|
|Silica > Opal-C||SiO2 · nH2O|
|Silica > Opal-CT||SiO2 · nH2O|
|Silica > Quartz||SiO2|
Physical Properties of Chert
Synonyms of Chert
Other Language Names for Chert
Varieties of Chert
Novaculite is a uniform, fine-grained hard rock mostly composed of extremely fine-grained to cryptocrystalline quartz. It may be white, gray, or black due to impurities. (It is not a slate.)
A banded variety of chert.
Taconite (also spelled taconyte)
|Quartz||26 photos of Chert associated with Quartz on mindat.org.|
|Chalcedony||26 photos of Chert associated with Chalcedony on mindat.org.|
|Tripuhyite||23 photos of Chert associated with Tripuhyite on mindat.org.|
|Parapierrotite||23 photos of Chert associated with Parapierrotite on mindat.org.|
|Hematite||20 photos of Chert associated with Hematite on mindat.org.|
|Arseniosiderite||17 photos of Chert associated with Arseniosiderite on mindat.org.|
|Marcasite||10 photos of Chert associated with Marcasite on mindat.org.|
|Calcite||8 photos of Chert associated with Calcite on mindat.org.|
|Stibnite||7 photos of Chert associated with Stibnite on mindat.org.|
|Goethite||7 photos of Chert associated with Goethite on mindat.org.|
References for Chert
Internet Links for Chert
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