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Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a chemical precipitate or a diagenetic replacement, as in petrified wood. Chert is typically composed of the petrified remains of siliceous ooze, the biogenic sediment that covers large areas of the deep ocean floor, and which contains the silicon skeletal remains of
diatom A diatom (Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising sev ...
s, silicoflagellates, and radiolarians.
Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the ...
cherts are notable for the presence of fossil
cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...
. In addition to microfossils, chert occasionally contains macrofossils. However, some chert is devoid of any fossils. Chert varies greatly in color (from white to black), but most often manifests as gray, brown, grayish brown and light green to rusty redW.L. Roberts, T.J. Campbell, G.R. Rapp Jr., "Encyclopedia of Mineralogy, Second Edition", 1990. R.S. Mitchell, "Dictionary of Rocks", 1985. (occasionally dark green); its color is an expression of trace elements present in the rock, and both red and green are most often related to traces of iron (in its oxidized and reduced forms, respectively).


Description

In
petrology Petrology () is the branch of geology that studies rocks and the conditions under which they form. Petrology has three subdivisions: igneous, metamorphic, and sedimentary petrology. Igneous and metamorphic petrology are commonly taught together ...
, the term "chert" refers generally to all chemically precipitated
sedimentary rocks Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles t ...
composed primarily of microcrystalline, cryptocrystalline and microfibrous silica. Most cherts are nearly pure silica, with less than 5% other minerals (mostly
calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
,
dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...
,
clay minerals Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2 Si2 O5( OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces. Clay mineral ...
,
hematite Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...
, and organic matter.) 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. Extracrystalline water (tiny inclusions of water within and around the quartz grains) make up less than 1% of most cherts. The Folk classification divides chert into three 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. 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. Opaline chert often contains visible fossils of diatoms, radiolarians, and glass sponge
spicules Spicules are any of various small needle-like anatomical structures occurring in organisms Spicule may also refer to: *Spicule (sponge), small skeletal elements of sea sponges *Spicule (nematode), reproductive structures found in male nematodes ( ...
. Chert is found in settings as diverse as hot spring deposits (''
siliceous sinter Geyserite, or siliceous sinter, is a form of opaline silica that is often found as crusts or layers around hot springs and geysers. Botryoidal geyserite is known as fiorite. Geyserite is porous due to the silica enclosing many small cavities. Si ...
''), banded iron formation (''
jaspilite Jaspillite, or ''jaspilite'' also ''itabirite'', is a chemical rock formed similar to chert, but is generally quite iron rich. It is also known as ''jasper taconite''. Jaspillite is typically a banded mixture of hematite and quartz common in the ...
''), or
alkaline lake A soda lake or alkaline lake is a lake on the strongly alkaline side of neutrality, typically with a pH, pH value between 9 and 12. They are characterized by high concentrations of carbonate salts, typically sodium carbonate (and related salt com ...
s. However, most chert is found either as ''bedded chert'' or as ''nodular chert''. Bedded chert is more common in
Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the ...
beds, but nodular chert became more common in the
Phanerozoic The Phanerozoic Eon is the current geologic eon in the geologic time scale, and the one during which abundant animal and plant life has existed. It covers 538.8 million years to the present, and it began with the Cambrian Period, when anima ...
as the total volume of chert in the rock record diminished. Bedded chert is rare after the early Mesozoic. Chert became moderately abundant during the
Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...
and
Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...
and again became moderately abundant from the Jurassic to the present.


Bedded chert

Bedded chert, also known as ribbon chert, takes the form of thinly bedded layers (a few centimeters to a meter in thickness) of nearly pure chert separated by very thin layers of silica-rich
shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...
. 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 anoxic environment. Bedded chert is most often found in association with turbidites, deep water limestone, submarine volcanic rock, ophiolites, and mélanges on
active margin Active may refer to: Music * ''Active'' (album), a 1992 album by Casiopea * Active Records, a record label Ships * ''Active'' (ship), several commercial ships by that name * HMS ''Active'', the name of various ships of the British Royal ...
s of tectonic plates. Sedimentary structures are rare in bedded cherts. 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.) Such impurities as are present include
authigenic Authigenesis is the process whereby a mineral or sedimentary rock deposit is generated where it is found or observed. Such deposits are described as authigenic. Authigenic sedimentary minerals form during sedimentation by precipitation or recrys ...
pyrite and hematite, formed in the sediments after they were deposited, in addition to traces of detrital minerals. 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, and which are estimated to presently produce of opal per year in the world's oceans. 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. 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. 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. Much opal-CT takes the form of ''lepispheres'', which are clusters of bladed crystals about 10 microns in diameter. Opal-CT in turn transforms to microquartz. In deep ocean water, the transition to opal-CT occurs at a temperature of about while the transition to microquartz occurs at a temperature of about . 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. There is evidence that the variety of chert called
porcelainite Mullite or porcelainite is a rare silicate mineral formed during contact metamorphism of clay minerals. It can form two stoichiometric forms: 3 Al2 O32 SiO2 or 2Al2O3 SiO2. Unusually, mullite has no charge-balancing cations present. As a result, ...
, which is characterized by a high content of opal-CT, recrystallizes at very shallow depths. The Caballos Novaculite of Texas also shows signs of very shallow water deposition, including shallow water sedimentary structures and evaporite
pseudomorphs In mineralogy, a pseudomorph is a mineral or mineral compound that appears in an atypical form (crystal system), resulting from a substitution process in which the appearance and dimensions remain constant, but the original mineral is replaced by ...
, which are casts of crystals of soluble minerals that could only have formed in near-surface conditions. This novaculate appears to have formed by replacement of carbonate
fecal pellets Feces ( or faeces), known colloquially and in slang as poo and poop, are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the large intestine. Feces contain a relati ...
by chert.


Subvarieties

Bedded cherts can be further subdivided by the kinds of organisms that produced the silica skeletons. ''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 In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...
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. '' Radiolarite'' consists mostly of remains of radiolarians. When the remains are well-cemented with silica, it is known as ''radiolarian chert''. Many show evidence of a deep-water origin, but some appear to have formed in water as shallow as , perhaps in
shelf sea A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an island ...
s where upwelling of nutrient-rich deep ocean water support high organic productivity. Radiolarians dominated the extraction of silica from seawater prior to the Jurassic. ''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 Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especiall ...
, clay-rich limestone,
phosphorite Phosphorite, phosphate rock or rock phosphate is a non-detrital sedimentary rock that contains high amounts of phosphate minerals. The phosphate content of phosphorite (or grade of phosphate rock) varies greatly, from 4% to 20% phosphorus pentoxi ...
s, and other nonvolcanic rocks typical of water a few hundred meters deep. 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. Eolian quartz has also been suggested as a source of silica for chert beds.
Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the ...
bedded cherts are common, making up 15% of middle Precambrian sedimentary rock, 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.


Nodular chert

Nodular chert is most common in limestone but may also be found in
shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...
s and sandstones. It is less common in
dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...
. Nodular chert in carbonate rocks is found as oval to irregular
nodules Nodule may refer to: *Nodule (geology), a small rock or mineral cluster *Manganese nodule, a metallic concretion found on the seafloor *Nodule (medicine), a small aggregation of cells *Root nodule Root nodules are found on the roots of plants, p ...
. These vary in size from powdery quartz particles to nodules several meters in size. The nodules are most typically along bedding planes or
stylolite Stylolites (Greek: ''stylos'', pillar; ''lithos'', stone) are serrated surfaces within a rock mass at which mineral material has been removed by pressure dissolution, in a deformation process that decreases the total volume of rock. Minerals whi ...
(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 Burrow fossils are the remains of burrows - holes or tunnels excavated into the ground or seafloor - by animals to create a space suitable for habitation, temporary refuge, or as a byproduct of locomotion preserved in the rock record. Because ...
s,
fluid escape structures Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition. Sediments and sedimentary rocks are characterized by bedding, which occurs when layers of sediment, with different partic ...
, 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 Desiccation () is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic (attracts and holds water) substance that induces or sustains such a state in its local vicinity in a moderately sealed container. ...
cracks with secondary chert, which likely formed at the same time as the nodule. Calcareous fossils are occasionally present that have been completely silicified. Where chert occurs in chalk or
marl Marl is an earthy material rich in carbonate minerals, clays, and silt. When hardened into rock, this becomes marlstone. It is formed in marine or freshwater environments, often through the activities of algae. Marl makes up the lower part o ...
, it is usually called flint. Nodular chert is often dark in color with a white weathering rind. Most chert nodules have textures suggesting they were formed by diagenetic replacement, where silica was deposited in place of calcium carbonate or
clay mineral Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2 Si2 O5( OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces. Clay minerals ...
s. 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. Nodular chert is particularly common in continental shelf environments. 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. The silica in nodular chert likely precipitates as opal-A, based on internal banding in nodules, and may recrystallize directly to microquartz without first recrystallizing to opal-CT. Some nodular chert may precipitate directly as microquartz, due to low levels of supersaturation of silica.


Other occurrences

The banded iron formations of
Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the ...
age are composed of alternating layers of chert and
iron oxide Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of whic ...
s. 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 The East African Rift (EAR) or East African Rift System (EARS) is an active continental rift zone in East Africa. The EAR began developing around the onset of the Miocene, 22–25 million years ago. In the past it was considered to be part of a ...
. These lakes are characterized by
sodium carbonate Sodium carbonate, , (also known as washing soda, soda ash and soda crystals) is the inorganic compound with the formula Na2CO3 and its various hydrates. All forms are white, odourless, water-soluble salts that yield moderately alkaline solutions ...
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 Sodium silicate is a generic name for chemical compounds with the formula or ·, such as sodium metasilicate , sodium orthosilicate , and sodium pyrosilicate . The anions are often polymeric. These compounds are generally colorless transparent ...
minerals magadiite or
kenyaite Kenyaite is a mineral and is a peculiar sodium silicate having a layered structure. It has a chemical formula of Na2Si22O41(OH)8 • 6H2O, which means it is made up of variations of different materials including sodium, silicon, hydrogen, and oxyg ...
, After burial and diagenesis, these are altered to Magadi-type chert. The
Morrison Formation The Morrison Formation is a distinctive sequence of Late Jurassic, Upper Jurassic sedimentary rock found in the western United States which has been the most fertile source of dinosaur fossils in North America. It is composed of mudstone, sandsto ...
contains Magadi-type chert that may have formed in the alkaline Lake T'oo'dichi'. Chert may also form from replacement of
calcrete Caliche () is a sedimentary rock, a hardened natural cement of calcium carbonate that binds other materials—such as gravel, sand, clay, and silt. It occurs worldwide, in aridisol and mollisol soil orders—generally in arid or semiarid regions, ...
in fossil soils ( paleosols) by silica dissolved from overlying volcanic ash beds.


Fossils

The cryptocrystalline nature of chert, combined with its above average ability to resist weathering, recrystallization and metamorphism has made it an ideal rock for preservation of early life forms. For example: * The 3.2 Ga chert of the
Fig Tree Formation The Fig Tree Formation, also called Fig Tree Group, is a stromatolite-containing geological formation in South Africa. The rock contains fossils of microorganisms, microscopic life forms of about 3.26 billion years old. Identified organisms includ ...
in the Barberton Mountains between
Eswatini Eswatini ( ; ss, eSwatini ), officially the Kingdom of Eswatini and formerly named Swaziland ( ; officially renamed in 2018), is a landlocked country in Southern Africa. It is bordered by Mozambique to its northeast and South Africa to its no ...
and South Africa preserved non-colonial unicellular bacteria-like fossils. * The Gunflint Chert of western Ontario (1.9 to 2.3 Ga) preserves not only bacteria and
cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...
but also organisms believed to be ammonia-consuming and some that resemble
green algae The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/Streptophyta. The land plants (Embryophytes) have emerged deep in the Charophyte alga as ...
and fungus-like organisms. * The
Apex Chert The Pilbara Craton is an old and stable part of the continental lithosphere located in the Pilbara region of Western Australia. The Pilbara Craton is one of only two pristine Archaean 3.6–2.7 Ga (billion years ago) crusts identified on the ...
(3.4 Ga) of the Pilbara craton, Australia preserved eleven taxa of
prokaryotes A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Connec ...
. However, these findings have been disputed. * The Bitter Springs Formation of the Amadeus Basin, Central Australia, preserves 850 Ma cyanobacteria and algae. * The Rhynie chert (410 Ma) of Scotland has remains of a
Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...
land flora and fauna with preservation so perfect that it allows cellular studies of the fossils.


Prehistoric and historic uses

Chert is of only modest economic importance today as a source of silica (quartz sand being much more important.) However, chert deposits may be associated with valuable deposits of iron, uranium, manganese,
phosphorite Phosphorite, phosphate rock or rock phosphate is a non-detrital sedimentary rock that contains high amounts of phosphate minerals. The phosphate content of phosphorite (or grade of phosphate rock) varies greatly, from 4% to 20% phosphorus pentoxi ...
, and petroleum.


Tools

In prehistoric times, chert was often used as a raw material for the construction of
stone tool A stone tool is, in the most general sense, any tool made either partially or entirely out of stone. Although stone tool-dependent societies and cultures still exist today, most stone tools are associated with prehistoric (particularly Stone Ag ...
s. Like
obsidian Obsidian () is a naturally occurring volcanic glass formed when lava extrusive rock, extruded from a volcano cools rapidly with minimal crystal growth. It is an igneous rock. Obsidian is produced from felsic lava, rich in the lighter elements s ...
, as well as some
rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...
s, felsites, quartzites, and other tool stones used in lithic reduction, chert fractures in a Hertzian cone when struck with sufficient force. This results in
conchoidal Conchoidal fracture describes the way that brittle materials break or fracture when they do not follow any natural planes of separation. Mindat.org defines conchoidal fracture as follows: "a fracture with smooth, curved surfaces, typically sli ...
fractures, a characteristic of all minerals with no cleavage planes. In this kind of fracture, a cone of force propagates through the material from the point of impact, eventually removing a full or partial cone; this result is familiar to anyone who has seen what happens to a plate-glass window when struck by a small object, such as an
air gun An air gun or airgun is a gun that fires projectiles pneumatically with compressed air or other gases that are mechanically pressurized ''without'' involving any chemical reactions, in contrast to a firearm, which pressurizes gases ''chemica ...
projectile. The partial Hertzian cones produced during lithic reduction are called flakes, and exhibit features characteristic of this sort of breakage, including
striking platform In lithic reduction In archaeology, in particular of the Stone Age, lithic reduction is the process of fashioning stones or rocks from their natural state into tools or weapons by removing some parts. It has been intensely studied and many arch ...
s, bulbs of force, and occasionally eraillures, which are small secondary flakes detached from the flake's bulb of force. When a chert stone is struck against an iron-bearing surface, sparks result. This makes chert an excellent tool for starting fires, and both flint and common chert were used in various types of fire-starting tools, such as tinderboxes, throughout history. A primary historic use of common chert and flint was for flintlock
firearms A firearm is any type of gun designed to be readily carried and used by an individual. The term is legally defined further in different countries (see Legal definitions). The first firearms originated in 10th-century China, when bamboo tubes c ...
, in which the chert striking a metal plate produces a spark that ignites a small reservoir containing
black powder Gunpowder, also commonly known as black powder to distinguish it from modern smokeless powder, is the earliest known chemical explosive. It consists of a mixture of sulfur, carbon (in the form of charcoal) and potassium nitrate (saltpeter). Th ...
, discharging the firearm.


Construction

Cherts are subject to problems when used as concrete aggregates. Deeply weathered chert develops surface pop-outs when used in concrete that undergoes freezing and thawing because of the high porosity of weathered chert. The other concern is that certain cherts undergo an alkali-silica reaction with high-alkali cements. This reaction leads to cracking and expansion of concrete and ultimately to failure of the material.


Varieties

There are numerous varieties of chert, classified based on their visible, microscopic and physical characteristics. Some of the more common varieties are: * Flint is a compact microcrystalline quartz. It was originally the name for chert found in chalk or marly limestone formations formed by a replacement of calcium carbonate with silica. Commonly found as nodules, this variety was often used in past times to make bladed tools. Today, some geologists refer to any dark gray to black chert as flint. The dark color is from inclusions of organic matter. Among non-geologists, the distinction between "flint" and "chert" is often one of quality – chert being lower quality than flint. This usage of the terminology is particularly prevalent in Great Britain where most true flint (found in chalk formations) was of better quality than "common chert" (from limestone formations). * "Common chert" is a variety of chert which forms in limestone formations by replacement of calcium carbonate with silica. This is the most abundantly found variety of chert. It is generally considered to be less attractive for producing gem stones and bladed tools than flint. * Jasper is a variety of chert formed as primary deposits, found in or in connection with magmatic formations which owes its red color to hematite inclusions. Jasper frequently also occurs in black, yellow or even green (depending on the type of iron it contains). Jasper is usually opaque to near opaque. Jasper is also present in banded iron formation, where it is described as jaspilite. * Radiolarite is a variety of chert formed as primary deposits and containing radiolarian microfossils. Many show evidence of a deep-water origin, but some appear to have formed in water as shallow as , * Chalcedony is a microfibrous quartz. * Agate is distinctly banded chalcedony with successive layers differing in color or value. * Onyx is a banded agate with layers in parallel lines, often black and white (''sardonyx''). * Magadi-type chert is a variety that forms from a sodium silicate precursor in highly alkaline lakes such as Lake Magadi in Kenya. *
Novaculite Novaculite, also called Arkansas Stone, is a microcrystalline to cryptocrystalline rock type that consists of silica in the form of chert or flint. It is commonly white to grey or black in color, with a specific gravity that ranges from 2.2 to 2.5 ...
is a very dense, fine-grained, and uniform form of very pure white chert with a high content of extracrystalline water. It is most common in the mid- Paleozoic rocks of Texas,
Oklahoma Oklahoma (; Choctaw language, Choctaw: ; chr, ᎣᎧᎳᎰᎹ, ''Okalahoma'' ) is a U.S. state, state in the South Central United States, South Central region of the United States, bordered by Texas on the south and west, Kansas on the nor ...
, and Arkansas in the south-central United States, where it has undergone some metamorphism. * Porcelanite is a term used for fine-grained siliceous rocks with a texture and a fracture resembling those of unglazed porcelain. It likely forms in shallow water and is composed mostly of opal-CT. * Tripolitic chert (or tripoli) is a light-colored porous friable siliceous (largely chalcedonic) sedimentary rock, which results from the weathering (decalcification) of chert or siliceous limestone. *
Siliceous sinter Geyserite, or siliceous sinter, is a form of opaline silica that is often found as crusts or layers around hot springs and geysers. Botryoidal geyserite is known as fiorite. Geyserite is porous due to the silica enclosing many small cavities. Si ...
is porous, low-density, light-colored siliceous rock deposited by waters of hot springs and geysers. * Mozarkite a varicolored, easily polished Ordovician chert that takes a high polish. It is the state rock of Missouri. Other lesser used archaic terms for chert are firestone and silex.


See also

* ** ** not to be confused with Concretion ** ** ** * ** , archaeological artefacts of the Clovis culture in New Mexico ** , a prehistoric chert mine in Alba County, Romania


References


External links


Photo & note re: Fig Tree Formation


* Schopf, J.W. (1999) ''Cradle of Life: The Discovery of Earth's Earliest Fossils'', Princeton University Press, 336 p. 

{{Authority control Firelighting using percussion Sedimentary rocks Lithics Quartz varieties