Editing Chert (section)

== Description ==
[[File:Chert beds EverettPA.jpg|thumb|Chert (dark bands) in the [[Devonian]] Corriganville-New Creek limestone, [[Everett, Pennsylvania]]]]
[[File:Folded layers of bedded chert formation in Busuanga, Palawan 02.jpg|thumb|Folded beds of chert comprising the Late Permian to Jurassic-aged Liminangcong Formation at [[Busuanga, Palawan]], Philippines]]

In [[petrology]], the term "chert" refers generally to all chemically precipitated [[sedimentary rocks]] composed primarily of [[microcrystalline]], [[cryptocrystalline]] and microfibrous [[silica]].<ref name=Folk1980>{{cite book |last1=Folk |first1=R.L. |year=1980 |url=https://repositories.lib.utexas.edu/handle/2152/22930 |title=Petrology of Sedimentary Rocks |location=Austin, Texas |publisher=Hemphill|isbn=9780914696148 }}</ref> Most cherts are nearly pure silica, with less than 5% other minerals (mostly [[calcite]], [[Dolomite (mineral)|dolomite]], [[clay minerals]], [[hematite]], and organic matter.)<ref>{{cite book |last1=Blatt |first1=Harvey |last2=Tracy |first2=Robert J. |title=Petrology : igneous, sedimentary, and metamorphic. |date=1996 |publisher=W.H. Freeman |location=New York |isbn=0716724383 |edition=2nd |page=335}}</ref> However, cherts range from very pure cherts with over 99% silica content to impure nodular cherts with less than 65% silica content. Aluminium is the most abundant minor element, followed by iron and [[manganese]] or [[potassium]], [[sodium]], and [[calcium]].{{sfn|Boggs|2006|p=207}} Extracrystalline water (tiny [[Inclusion (mineral)|inclusions]] of water within and around the quartz grains) make up less than 1% of most cherts.<ref>{{cite book |last1=Blatt |first1=Harvey |last2=Middleton |first2=Gerard |last3=Murray |first3=Raymond |title=Origin of sedimentary rocks |date=1980 |publisher=Prentice-Hall |location=Englewood Cliffs, N.J. |isbn=0136427103 |edition=2d |page=571}}</ref>

The [[Folk classification]] divides chert into three [[Texture (geology)|textural]] categories. Granular microquartz is the component of chert consisting of roughly equidimensional [[quartz]] grains, ranging in size from a fraction of a micron to 20 microns, but most typically 8 to 10 microns. [[Chalcedony]] is a microfibrous variety of quartz, consisting of radiating bundles of very thin crystals about 100 microns long. Megaquartz is composed of equidimensional grains over 20 microns in size.<ref name=Folk1980/>{{sfn|Boggs|2006|pp=206-207}} Most chert is microcrystalline quartz with minor chalcedony and sometimes [[opal]], but cherts range from nearly pure opal to nearly pure quartz chert. However, little opal is over 60 million years old.{{sfn|Boggs|2006|pp=206-207}} Opaline chert often contains visible fossils of [[diatoms]], [[radiolarians]], and [[glass sponge]] [[Sponge spicule|spicules]].{{sfn|Blatt|Middleton|Murray|1980|p=572}}

Chert is found in settings as diverse as [[hot spring]] deposits (''[[siliceous sinter]]''), [[banded iron formation]] (''[[jaspilite]]''),{{sfn|Boggs|2006|p=207}} or [[alkaline lake]]s.{{sfn|Blatt|Tracy|1996||pp=336-338}} However, most chert is found either as ''bedded chert'' or as ''nodular chert''.{{sfn|Boggs|2006|p=207}} Bedded chert is more common in [[Precambrian]] beds, but nodular chert became more common in the [[Phanerozoic]] as the total volume of chert in the rock record diminished.{{sfn|Blatt|Middleton|Murray|1980|p=575}} Bedded chert is rare after the early [[Mesozoic]].{{sfn|Blatt|Tracy|1996|pp=339}} Chert became moderately abundant during the [[Devonian]] and [[Carboniferous]] and again became moderately abundant from the [[Jurassic]] to the present.{{sfn|Blatt|Middleton|Murray|1980|p=571}}

===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}}

===Nodular chert===
[[File:Çört yumrusu, Chert.2.jpg|thumb|Chert nodule within soft limestone at [[Akçakoca]], Turkey]]
Nodular chert is most common in limestone but may also be found in [[shale]]s{{sfn|Boggs|2006|p=216}} and sandstones.{{sfn|Blatt|Tracy|1996|p=335}} It is less common in [[dolomite (rock)|dolomite]].<ref name="Knauth1979"/> Nodular chert in [[carbonate rock]]s is found as oval to irregular [[nodule (geology)|nodules]]. These vary in size from powdery quartz particles to nodules several meters in size. The nodules are most typically along bedding planes or [[stylolite]] (dissolution) surfaces, where fossil organisms tended to accumulate and provided a source of dissolved silica, but they are sometimes found cutting across bedding surfaces, where the chert fills [[fossil burrow]]s, [[fluid escape structures]], or fractures. Nodules under a few centimeters in size tend to be egg-shaped, while larger nodules form irregular bodies with knobby surfaces. The outer few centimeters of large nodules may show [[desiccation]] cracks with secondary chert, which likely formed at the same time as the nodule. Calcareous fossils are occasionally present that have been completely silicified.{{sfn|Blatt|Tracy|1996|p=335}} Where chert occurs in chalk or [[marl]], it is usually called [[flint]].{{sfn|Boggs|2006|p=207}}

[[File:Flint with weathered crust.JPG|thumb|Flint with white weathered crust]]
Nodular chert is often dark in color.{{sfn|Blatt|Tracy|1996|p=335}} It can have a white [[weathering]] rind that is known in [[archaeology]] as ''[[Cortex (archaeology)|cortex]]''.

Most chert nodules have [[Texture (geology)|textures]] suggesting they were formed by diagenetic replacement, where silica was deposited in place of [[calcium carbonate]] or [[clay mineral]]s.{{sfn|Boggs|2006|p=207}} This may have taken place where [[meteoric water]] (water derived from snow or rain) mixed with saltwater in the sediment beds, where carbon dioxide was trapped, producing an environment supersaturated with silica and undersaturated with calcium carbonate.<ref name="Knauth1979"/> Nodular chert is particularly common in continental shelf environments.{{sfn|Boggs|2006|p=216}} In the [[Permian Basin (North America)]], chert nodules and chertified fossils are abundant in basin limestones, but there is little in the carbonate buildup zone itself. This may reflect dissolution of opal where carbonate is being actively deposited, a lack of siliceous organisms in these environments, or removal of siliceous skeletons by strong currents that redeposit the siliceous material in the deep basin.{{sfn|Blatt|Middleton|Murray|1980|p=576}}

The silica in nodular chert likely precipitates as opal-A, based on internal banding in nodules,{{sfn|Blatt|Middleton|Murray|1980|p=576}} and may recrystallize directly to microquartz without first recrystallizing to opal-CT.{{sfn|Boggs|2006|p=216}} Some nodular chert may precipitate directly as microquartz, due to low levels of supersaturation of silica.{{sfn|Blatt|Tracy|1996|p=335}}

===Other occurrences===
The [[banded iron formation]]s of [[Precambrian]] age are composed of alternating layers of chert and [[iron oxide]]s.{{sfn|Boggs|2006|pp=217-218}}{{sfn|Blatt|Tracy|1996|pp=339-343}}

Nonmarine cherts may form in saline alkaline lakes as thin lenses or nodules showing sedimentary structures suggestive of [[evaporite]] origin. Such cherts are forming today in the alkaline lakes of the [[East African Rift Valley]].{{sfn|Blatt|Tracy|1996|pp=336-337}} These lakes are characterized by [[sodium carbonate]] brines with very high [[pH]] that can contain as much as 2700 ppm silica. Episodes of runoff of fresh water into the lakes lowers the pH and precipitates the unusual [[sodium silicate]] minerals [[magadiite]] or [[kenyaite]], After burial and [[diagenesis]], these are altered to Magadi-type chert.{{sfn|Blatt|Middleton|Murray|1980|p=576}} The [[Morrison Formation]] contains Magadi-type chert that may have formed in the alkaline Lake T'oo'dichi'.<ref>{{cite journal |last1=Dunagan |first1=Stan P |last2=Turner |first2=Christine E |title=Regional paleohydrologic and paleoclimatic settings of wetland/lacustrine depositional systems in the Morrison Formation (Upper Jurassic), Western Interior, USA |journal=Sedimentary Geology |date=May 2004 |volume=167 |issue=3–4 |pages=269–296 |doi=10.1016/j.sedgeo.2004.01.007|bibcode=2004SedG..167..269D }}</ref>

Chert may also form from replacement of [[calcrete]] in fossil soils ([[paleosols]]) by silica dissolved from overlying [[volcanic ash]] beds.<ref>{{cite journal |last1=Smith |first1=Gary A. |last2=Huckell |first2=Bruce B. |title=The geological and geoarchaeological significance of Cerro Pedernal, Rio Arriba County, New Mexico |journal=New Mexico Geological Society Field Conference Series |date=2005 |volume=56 |page=427 |url=https://nmgs.nmt.edu/publications/guidebooks/downloads/56/56_p0425_p0431.pdf |access-date=10 July 2021}}</ref>