Chert

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For the Spanish municipality, see Xert.

Chert
Sedimentary rock
Chert hand sample
A hand sample of chert
Composition
Microcrystalline or cryptocrystalline quartz

Chert (/ˈtʃɜːrt/) is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz,[1] the mineral form of silicon dioxide (SiO2).[2] Chert is characteristically of biological origin, but may also occur inorganically as a chemical precipitate or a diagenetic replacement, as in petrified wood.[3]

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 diatoms, silicoflagellates, and radiolarians.[4] Precambrian cherts are notable for the presence of fossil cyanobacteria.[5] In addition to microfossils,[4] chert occasionally contains macrofossils.[6][7] However, some chert is devoid of any fossils.[8]

Chert varies greatly in color, from white to black, but is most often found as gray, brown, grayish brown and light green to rusty red[9][10] and occasionally as dark green.[11] Its color is an expression of trace elements present in the rock. Both red and green are most often related to traces of iron in its oxidized and reduced forms, respectively.[4][12]

Description

Chert (dark bands) in the Devonian Corriganville-New Creek limestone, Everett, Pennsylvania
Folded beds of chert comprising the Late Permian to Jurassic-aged Liminangcong Formation at Busuanga, Palawan, Philippines

In

clay minerals, hematite, and organic matter.)[14] 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.[8] Extracrystalline water (tiny inclusions of water within and around the quartz grains) make up less than 1% of most cherts.[15]

The

glass sponge spicules.[17]

Chert is found in settings as diverse as

alkaline lakes.[18] However, most chert is found either as bedded chert or as nodular chert.[8] 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.[19] Bedded chert is rare after the early Mesozoic.[20] Chert became moderately abundant during the Devonian and Carboniferous and again became moderately abundant from the Jurassic to the present.[21]

Bedded chert

Bedded chert, also known as ribbon chert, takes the form of thinly

authigenic pyrite and hematite, formed in the sediments after they were deposited, in addition to traces of detrital minerals.[8]

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,[26] and which are estimated to presently produce 12 cubic kilometers (2.9 cu mi) of opal per year in the world's oceans.[27] 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.[28] 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.[26]

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.[29] Much opal-CT takes the form of lepispheres, which are clusters of bladed crystals about 10 microns in diameter.[30] Opal-CT in turn transforms to microquartz. In deep ocean water, the transition to opal-CT occurs at a temperature of about 45 °C (113 °F) while the transition to microquartz occurs at a temperature of about 80 °C (176 °F). 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.[29]

There is evidence that the variety of chert called

fecal pellets by chert.[21]

Subvarieties

Bedded cherts can be further subdivided by the kinds of organisms that produced the silica skeletons.[23]

Diatomaceous chert consists of beds and lenses of

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.[31]

shelf seas where upwelling of nutrient-rich deep ocean water support high organic productivity.[22] Radiolarians dominated the extraction of silica from seawater prior to the Jurassic.[34]

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

black shale, clay-rich limestone, phosphorites, and other nonvolcanic rocks typical of water a few hundred meters deep.[33]

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.[35][36] Eolian quartz has also been suggested as a source of silica for chert beds.[37] Precambrian bedded cherts are common, making up 15% of middle Precambrian sedimentary rock,[21] 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.[38]

Nodular chert

Chert nodule within soft limestone at Akçakoca, Turkey

Nodular chert is most common in limestone but may also be found in

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.[25] Where chert occurs in chalk or marl, it is usually called flint.[8]

Flint with white weathered crust

Nodular chert is often dark in color with a white weathering rind.[25]

Most chert nodules have textures suggesting they were formed by diagenetic replacement, where silica was deposited in place of calcium carbonate or clay minerals.[8] 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.[1] Nodular chert is particularly common in continental shelf environments.[38] 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.[39]

The silica in nodular chert likely precipitates as opal-A, based on internal banding in nodules,[39] and may recrystallize directly to microquartz without first recrystallizing to opal-CT.[38] Some nodular chert may precipitate directly as microquartz, due to low levels of supersaturation of silica.[25]

Other occurrences

The banded iron formations of Precambrian age are composed of alternating layers of chert and iron oxides.[40][41]

Nonmarine cherts may form in saline alkaline lakes as thin lenses or nodules showing sedimentary structures suggestive of

East African Rift Valley.[42] 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.[39] The Morrison Formation contains Magadi-type chert that may have formed in the alkaline Lake T'oo'dichi'.[43]

Chert may also form from replacement of

paleosols) by silica dissolved from overlying volcanic ash beds.[44]

Fossils

An erosion resistant layer of chert in the Eocene Ping Chau Formation, Hong Kong

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.[45]

For example:

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, and petroleum.[57]

Tools

Mill Creek chert from the Parkin Site in Arkansas

In prehistoric times, chert was often used as a raw material for the construction of

bulbs of force, and occasionally eraillures, which are small secondary flakes detached from the flake's bulb of force.[58]

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

black powder, discharging the firearm.[2][59]

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.[60]

Varieties

There are numerous varieties of chert, classified based on their visible, microscopic and physical characteristics.[9][10] Some of the more common varieties are:

  • 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.[61][62][63] The dark color is from inclusions of organic matter.[21] 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).[64]
  • "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.[65] Jasper is also present in banded iron formation, where it is described as jaspilite.[8]
  • Radiolarite is a variety of chert formed as primary deposits and containing
    radiolarian microfossils.[32] Many show evidence of a deep-water origin, but some appear to have formed in water as shallow as 200 meters (660 ft),[32]
  • Agate is distinctly banded chalcedony with successive layers differing in color or value.[66]
  • Onyx is a banded agate with layers in parallel lines, often black and white (sardonyx).[67]
  • 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.[69]
  • 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.[70]
  • Siliceous sinter is porous, low-density, light-colored siliceous rock deposited by waters of hot springs and geysers.[8]
  • Mozarkite a varicolored, easily polished Ordovician chert that takes a high polish. It is the state rock of Missouri.[71]

Other lesser used archaic terms for chert are firestone and silex.[72]

See also

  • Petrology – Branch of geology that studies the formation, composition, distribution and structure of rocks
    • Eolith – Knapped flint nodule
    • Nodule (geology) – Small mass of a mineral with a contrasting composition to the enclosing sediment or rock not to be confused with Concretion
    • Obsidian – Naturally occurring volcanic glass
    • Opal – Hydrated amorphous form of silica
    • Whinstone – Quarrying term for any hard dark-coloured rock
  • Archaeology – Study of human activity via material culture
    • Clovis Points – New World prehistoric projectile, archaeological artefacts of the Clovis culture in New Mexico
    • Piatra Tomii – late Jurassic limestone outcrop, a prehistoric chert mine in Alba County, Romania

References

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