Editing Chert (section)

===Bedded chert===

Bedded chert, also known as ribbon chert, takes the form of thinly [[Bedding (geology)|bedded]] layers (a few centimeters to a meter in thickness{{sfn|Blatt|Tracy|1996|p=336}}) of nearly pure chert separated by very thin layers of silica-rich [[shale]].{{sfn|Boggs|2006|p=208}} It is usually black to green in color, and the full sequence of beds may be several hundred meters thick. The shale is typically black shale, sometimes with [[pyrite]], indicating deposition in an [[Hypoxia (environmental)|anoxic]] environment.{{sfn|Blatt|Middleton|Murray|1980|p=571}} Bedded chert is most often found in association with [[turbidites]], deep water [[limestone]], submarine [[volcanic rock]],{{sfn|Boggs|2006|p=208}} [[ophiolite]]s, and [[mélange]]s on [[active margin]]s of [[tectonic plate]]s.{{sfn|Blatt|Tracy|1996|pp=335-336}} [[Sedimentary structures]] are rare in bedded cherts.{{sfn|Boggs|2006|p=208}} The typically high purity of bedded chert, like the high purity of other chemically precipitated rock, points to deposition in areas where there is little influx of [[detrital]] sediments (such as river water laden with silt and clay particles.){{sfn|Blatt|Tracy|1996|p=335}} Such impurities as are present include [[authigenic]] pyrite and hematite, formed in the sediments after they were deposited, in addition to traces of detrital minerals.{{sfn|Boggs|2006|p=207}}

Seawater typically contains between 0.01 and 11 [[parts per million]] (ppm) of silica, with around 1 ppm being typical. This is far below saturation, indicating that silica cannot normally be precipitated from seawater through inorganic processes. The silica is instead extracted from seawater by living organisms, such as diatoms, radiolarians, and glass sponges, which can efficiently extract silica even from very unsaturated water,{{sfn|Boggs|2006|p=208, 211-213}} and which are estimated to presently produce {{convert|12|km3||sp=us}} of opal per year in the world's oceans.{{sfn|Blatt|Tracy|1996|p=338}} Diatoms can double their numbers eight times a day under ideal conditions (though doubling once per day is more typical in normal seawater) and can extract silica from water with as little as 0.1 ppm silica.{{sfn|Blatt|Middleton|Murray|1980|p=578}} The organisms protect their skeletons from dissolution by "armoring" them with metal ions. Once the organisms die, their skeletons will quickly dissolve unless they accumulate on the ocean bottom and are buried, forming siliceous ooze that is 30% to 60% silica. Thus, bedded cherts are typically composed mostly of fossil remains of organisms that secrete silica skeletons, which are usually altered by solution and recrystallization.{{sfn|Boggs|2006|p=208, 211-213}}

The skeletons of these organisms are composed of opal-A, an amorphous form of silica, lacking long-range crystal structure. This is gradually transformed to opal-CT, a microcrystalline form of silica composed mostly of bladed crystals of [[cristobalite]] and [[tridymite]].{{sfn|Boggs|2006|p=214}} Much opal-CT takes the form of ''lepispheres'', which are clusters of bladed crystals about 10 microns in diameter.<ref>{{cite journal |last1=Fröhlich |first1=François |title=The opal-CT nanostructure |journal=Journal of Non-Crystalline Solids |date=April 2020 |volume=533 |pages=119938 |doi=10.1016/j.jnoncrysol.2020.119938|bibcode=2020JNCS..53319938F |s2cid=213728852 |doi-access=free |url=https://hal.archives-ouvertes.fr/hal-02570678v1/document }}</ref> Opal-CT in turn transforms to microquartz. In deep ocean water, the transition to opal-CT occurs at a temperature of about {{convert|45|C||sp=us}} while the transition to microquartz occurs at a temperature of about {{convert|80|C||sp=us}}. However, the transition temperature varies considerably, and the transition is hastened by the presence of [[magnesium hydroxide]], which provides a nucleus for the recrystallization. Megaquartz forms at elevated temperatures typical of [[metamorphism]].{{sfn|Boggs|2006|p=214}}

There is evidence that the variety of chert called [[porcelainite]], which is characterized by a high content of opal-CT, recrystallizes at very shallow depths.{{sfn|Boggs|2006|p=214}} The Caballos Novaculite of Texas also shows signs of very shallow water deposition, including shallow water sedimentary structures and [[evaporite]] [[pseudomorphs]], which are casts of crystals of soluble minerals that could have formed only in near-surface conditions. This [[Novaculite|novaculate]] appears to have formed by replacement of carbonate [[fecal pellets]] by chert.{{sfn|Blatt|Middleton|Murray|1980|p=571}}

====Subvarieties====
Bedded cherts can be further subdivided by the kinds of organisms that produced the silica skeletons.{{sfn|Boggs|2006|p=208}}

''Diatomaceous chert'' consists of beds and lenses of [[diatomite]] which were converted during [[diagenesis]] into dense, hard chert. Beds of marine diatomaceous chert comprising [[strata]] several hundred meters thick have been reported from sedimentary sequences such as the [[Miocene]] [[Monterey Formation]] of California and occur in rocks as old as the [[Cretaceous]]. Diatoms were the dominant siliceous organism responsible for extracting silica from seawater from the [[Jurassic]] and later.{{sfn|Boggs|2006|pp=208-209}}

''[[Radiolarite]]'' consists mostly of remains of radiolarians. When the remains are well-cemented with silica, it is known as ''radiolarian chert''.{{sfn|Boggs|2006|pp=209-210}} Many show evidence of a deep-water origin, but some appear to have formed in water as shallow as {{convert|200|m||sp=us}},{{sfn|Boggs|2006|p=210}} perhaps in [[shelf sea]]s where upwelling of nutrient-rich deep ocean water support high organic productivity.{{sfn|Blatt|Tracy|1996|p=336}} Radiolarians dominated the extraction of silica from seawater prior to the Jurassic.{{sfn|Boggs|2006|p=213}}

''Spicularite'' is chert composed of spicules of glass sponges and other invertebrates. When densely cemented, it is known as ''spicular chert''. They are found in association with [[glauconite]]-rich [[sandstone]], [[black shale]], [[clay]]-rich [[limestone]], [[phosphorite]]s, and other nonvolcanic rocks typical of water a few hundred meters deep.{{sfn|Boggs|2006|p=210}}

Some bedded cherts appear devoid of fossils even under close microscopic examination. Their origin is uncertain, but they may form from fossil remains that are completely dissolved in fluids that then migrate to precipitate their silica load in a nearby bed.<ref>{{cite journal |last1=Murray |first1=Richard W. |last2=Jones |first2=David L. |last3=Brink |first3=Marilyn R. Buchholtz ten |title=Diagenetic formation of bedded chert: Evidence from chemistry of the chert-shale couplet |journal=Geology |date=1 March 1992 |volume=20 |issue=3 |pages=271–274 |doi=10.1130/0091-7613(1992)020<0271:DFOBCE>2.3.CO;2|bibcode=1992Geo....20..271M }}</ref>{{sfn|Boggs|2006|pp=215-216}} [[Aeolian processes|Eolian]] quartz has also been suggested as a source of silica for chert beds.<ref>{{cite journal |last1=Cecil |first1=C. Blaine |last2=Hemingway |first2=Bruce S. |last3=Dulong |first3=Frank T. |title=The Chemistry of Eolian Quartz Dust and the Origin of Chert |journal=Journal of Sedimentary Research |date=26 June 2018 |volume=88 |issue=6 |pages=743–752 |doi=10.2110/jsr.2018.39|bibcode=2018JSedR..88..743C |s2cid=134950494 }}</ref> [[Precambrian]] bedded cherts are common, making up 15% of middle Precambrian sedimentary rock,{{sfn|Blatt|Middleton|Murray|1980|p=571}} and may have been deposited nonbiologically in oceans more saturated in silica than the modern ocean. The high degree of silica saturation was due either to intense volcanic activity or to the lack of modern organisms that remove silica from seawater.{{sfn|Boggs|2006|p=216}}