Chert

Chert ( /ˈtʃɜːrt/) is a hard, fine-grained sedimentary rock composed of crystals of quartz (silica) that are very small (microcrystalline or cryptocrystalline).[1] Quartz (silica) is the mineral form of silicon dioxide (SiO2).[2] Chert is often of biological origin (organic) but may also occur inorganically as a chemical precipitate or a diagenetic replacement (e.g., petrified wood).[3] Geologists use chert as a generic name for any type of microcrystalline or cryptocrystalline quartz.

Chert is usually of biological origin, being 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. Depending on its origin, it can contain either microfossils, small macrofossils, or both. It varies greatly in color (from white to black), but most often manifests as gray, brown, grayish brown and light green to rusty red (occasionally dark green too); 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).

Chert
Sedimentary rock
Chert
Chert

Occurrence

Flint with weathered crust
Flint with white weathered crust

Chert occurs in carbonate rocks as oval to irregular nodules in greensand, limestone, chalk, and dolostone formations as a replacement mineral, where it is formed as a result of some type of diagenesis. Where it occurs in chalk or marl, it is usually called flint. It also occurs in thin beds, when it is a primary deposit (such as with many jaspers and radiolarites). Thick beds of chert occur in deep marine deposits. These thickly bedded cherts include the novaculite of the Ouachita Mountains of Arkansas, Oklahoma, and similar occurrences in Texas and South Carolina[4] in the United States. The banded iron formations of Precambrian age are composed of alternating layers of chert and iron oxides.

Chert also occurs in diatomaceous deposits and is known as diatomaceous chert. 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.[5]

Terminology

Chert beds EverettPA
Chert (dark bands) in the Devonian Corriganville-New Creek limestone, Everett, Pennsylvania

In petrology the term "chert" is used to refer generally to all rocks composed primarily of microcrystalline, cryptocrystalline and microfibrous quartz. The term does not include quartzite. Chalcedony is a microfibrous (microcrystalline with a fibrous structure) variety of quartz.

Strictly speaking, the term "flint" is reserved for varieties of chert which occur in chalk and marly limestone formations.[6][7] 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 prevalent in North America and is likely caused by early immigrants who brought the terms from England where most true flint (that found in chalk formations) was indeed of better quality than "common chert" (from limestone formations).

Among petrologists, chalcedony is sometimes considered separately from chert due to its fibrous structure. Since many cherts contain both microcrystalline and microfibrous quartz, it is sometimes difficult to classify a rock as completely chalcedony, thus its general inclusion as a variety of chert.

Fossils

Tung Ping Chau 6
Chert layer (prominent band near top of outcrop) in the Eocene Ping Tau 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.[8]

For example:

Prehistoric and historic uses

Tools

In prehistoric times, chert was often used as a raw material for the construction of stone tools. Like obsidian, as well as some rhyolites, 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 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 projectile. The partial Hertzian cones produced during lithic reduction are called flakes, and exhibit features characteristic of this sort of breakage, including striking platforms, 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, in which the chert striking a metal plate produces a spark that ignites a small reservoir containing black powder, discharging the firearm.[2]

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.[13]

In some areas, chert is ubiquitous as stream gravel and fieldstone and is currently used as construction material and road surfacing. Part of chert's popularity in road surfacing or driveway construction is that rain tends to firm and compact chert while other fill often gets muddy when wet.

Monuments

Chert has been used in late nineteenth-century and early twentieth-century headstones or grave markers in Tennessee and other regions.

Varieties

There are numerous varieties of chert, classified based on their visible, microscopic and physical characteristics.[14][15] 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.[16]
  • "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 iron(III) 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.
  • Radiolarite is a variety of chert formed as primary deposits and containing radiolarian microfossils.
  • 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.
  • Opal is a hydrated silicon dioxide. It is often of a Neogenic origin. In fact it is not a mineral (it is a mineraloid) and it is generally not considered a variety of chert, although some varieties of opal (opal-C and opal-CT) are microcrystalline and contain much less water (sometime none). Often people without petrological training confuse opal with chert due to similar visible and physical characteristics.
  • Magadi-type chert is a variety that forms from a sodium silicate precursor in highly alkaline lakes such as Lake Magadi in Kenya.
  • Porcelanite is a term used for fine-grained siliceous rocks with a texture and a fracture resembling those of unglazed porcelain.
  • Siliceous sinter is porous, low-density, light-colored siliceous rock deposited by waters of hot springs and geysers.
  • Mozarkite has won distinction because of its unique variation of colors and its ability to take a high polish.

Other lesser used terms for chert (most of them archaic) include firestone, silex, silica stone, chat, and flintstone.

See also

Çört yumrusu, Chert.2
Akcakoca chert nodules within soft limestone

References

  1. ^ Knauth, L. Paul. "A model for the origin of chert in limestone." Geology 7, no. 6 (1979): 274-277.
  2. ^ a b "Chert: Sedimentary Rock - Pictures, Definition, Formation". geology.com. Retrieved 12 May 2018.
  3. ^ Bates, R. L.; Jackson, J., eds. (1984). Dictionary of Geological Terms, 3rd Ed. American Geological Institute. Doubleday. p. 85. ISBN 0385181019. OCLC 465393210.
  4. ^ "OpenLearn Live: 19th February 2016: A Week In South Carolina:". OpenLearn. The Open University. Retrieved 20 February 2016.
  5. ^ *Sam Boggs, Jr., "Principles of Sedimentology and Stratigraphy", Prentice Hall, 2006, 4th Ed., ISBN 0-13-154728-3
  6. ^ George R. Rapp, "Archaeomineralogy", 2002. ISBN 3-540-42579-9
  7. ^ Barbara E. Luedtke, "The Identification of Sources of Chert Artifacts", American Antiquity, Vol. 44, No.4 (Oct., 1979), 744–57.
  8. ^ The Earliest Life: Annotated listing Archived 2006-04-26 at the Wayback Machine
  9. ^ Fig Tree Formation of South Africa
  10. ^ Gunflint chert Archived 2005-06-12 at the Wayback Machine
  11. ^ Biogenicity of Microfossils in the Apex Chert
  12. ^ Cyanobacertial fossils of the Bitter Springs Chert, UMCP Berkeley
  13. ^ Terry R. West. "Geology Applied to Engineering," Waveland Press, 1995 ISBN 1577666550
  14. ^ W.L. Roberts, T.J. Campbell, G.R. Rapp Jr., "Encyclopedia of Mineralogy, Second Edition", 1990. ISBN 0-442-27681-8
  15. ^ R.S. Mitchell, "Dictionary of Rocks", 1985. ISBN 0-442-26328-7
  16. ^ Bates,, R. L., and Jackson, J. (eds.) (1984). Dictionary of Geological Terms, 3rd Ed. American Geological Institute. Doubleday. p. 187. ISBN 0385181019 OCLC 465393210

External links

Chytridiomycetes

Chytridiomycetes () is a class of fungi. Members are found in soil, fresh water, and saline estuaries. They are first known from the Rhynie chert. It has recently been redefined to exclude the taxa Neocallimastigomycota and Monoblepharidomycetes, which are now a phylum and a sister-class respectively.Chytridiomycetes is the major class of the phylum Chytridiomycota, which contains a number of parasitic species. At least two species in this class are known to infect a number of amphibian species.

Clear Creek Chert

The Clear Creek Chert is a geologic formation in Indiana.

Corona Heights Park

Corona Heights Park is a park in the Castro and Corona Heights neighborhoods of San Francisco, California, United States. It is situated immediately to the south of Buena Vista Park. Corona Heights is bounded in part by Flint Street on the east, Roosevelt Way to the north, and 16th Street to the south. The base of the hill is at approximately 300 feet (91 m), while the peak extends to 520 feet (158 m) above sea level.Corona Heights Playground and the Randall Museum are located within the Corona Heights Park. The whole area is underlain by Franciscan chert bedrock, and a large percentage of the hill is barren. At the hilltop, the chert bedrock in terra cotta red is clearly visible. The steps leading up to the peak are not supported by handrails. The peak of the hill is windy, but it offers an unobstructed panoramic view of the city of San Francisco from downtown to the Twin Peaks.

Flint

Flint is a hard, sedimentary cryptocrystalline form of the mineral quartz, categorized as a variety of chert. It occurs chiefly as nodules and masses in sedimentary rocks, such as chalks and limestones. Inside the nodule, flint is usually dark grey, black, green, white or brown in colour, and often has a glassy or waxy appearance. A thin layer on the outside of the nodules is usually different in colour, typically white and rough in texture. From a petrological point of view, "flint" refers specifically to the form of chert which occurs in chalk or marly limestone. Similarly, "common chert" (sometimes referred to simply as "chert") occurs in limestone.

Flint is very durable and can be found along streams and beaches. Its use to make stone tools dates back millions of years. Due to some properties of flint it breaks into sharp edged pieces making it useful for knife blades and other sharp tools. During the Stone Age access to flint was so important for survival that people would travel or trade to obtain flint. Flint Ridge in eastern Ohio was an important source of flint and Native Americans extracted the flint from hundreds of quarries along the ridge. This "Ohio Flint" was traded across the eastern United States and has been found as far west as the Rocky Mountains and south around the Gulf of Mexico.

Flint Hills

The Flint Hills, historically known as Bluestem Pastures or Blue Stem Hills, are a region in eastern Kansas and north-central Oklahoma named for the abundant residual flint eroded from the bedrock that lies near or at the surface. It consists of a band of hills stretching from Kansas to Oklahoma, extending from Marshall and Washington Counties in the north to Cowley County, Kansas and Kay and Osage Counties in Oklahoma in the south, to Geary and Shawnee Counties west to east. Oklahomans generally refer to the same geologic formation as the Osage Hills or "the Osage."

The Flint Hills Ecoregion is designated as a distinct region because it has the most dense coverage of intact tallgrass prairie in North America. Due to its rocky soil, the early settlers were unable to plow the area, resulting in the predominance of cattle ranches, which are in turn largely benefited by the tallgrass prairie.

The Flint Hills Discovery Center, a science and history museum focusing on the Flint Hills, opened in Manhattan, Kansas, in April 2012.

Fort Payne Formation

The Fort Payne Formation, or Fort Payne Chert, is a geologic formation found in the southeastern region of the United States. It is a Mississippian Period cherty limestone, that overlies the Chattanooga Shale (or locally the Maury Formation), and underlies the St. Louis Limestone (lower Tuscumbia Limestone in Alabama). To the north, it grades into the siltstone Borden Formation. It preserves fossils dating back to the Carboniferous period.Eugene Allen Smith named the Fort Payne Formation for outcrops at Fort Payne, Alabama.

Grassy Knob Chert

The Grassy Knob Chert is a geologic formation in Indiana.

Huntersville Chert

The Huntersville Chert is a geologic formation in West Virginia. It preserves fossils dating back to the Devonian period.

Knapping

Knapping is the shaping of flint, chert, obsidian or other conchoidal fracturing stone through the process of lithic reduction to manufacture stone tools, strikers for flintlock firearms, or to produce flat-faced stones for building or facing walls, and flushwork decoration. The original Germanic term "knopp" meant strike, shape, or work, so it could theoretically have referred equally well to making a statue or dice. Modern usage is more specific, referring almost exclusively to the hand-tool pressure-flaking process pictured.

Martz Rock Shelters

The Martz Rock Shelters was an archaeological site located near Myersdale, Somerset County, Pennsylvania, US on the farm of Harry Martz. The Somerset County Archaeological Survey began its excavations on June 14, 1938 and was completed six days later. The site was located about 30 miles from Metropolitan Pittsburgh. The site was discovered around 1938 during the Works Projects Administration excavation project, necessary for the construction of state highway 219. It was located at a hill overlooking the Casselman River from which a shale ledge protruded about two hundred and fifty feet above the river. The opening of the caves faced south. The site was destroyed during the construction of the highway .Artifacts found at the site included:

charcoal and ash

flint, chert and quartz spalls

animal bones

projectile points

potsherds

scrapersChert and quartz are not naturally found in Somerset County.

Mill Creek chert

Mill Creek chert is a type of chert found in Southern Illinois and heavily exploited by members of the Mississippian culture (800 to 1600 CE). Artifacts made from this material are found in archaeological sites throughout the American Midwest and Southeast. It is named for a village and stream near the quarries, Mill Creek, Illinois and Mill Creek, a tributary of the Cache River. The chert was used extensively for the production of utilitarian tools such as hoes and spades, and for polished ceremonial objects such as bifaces, spatulate celts and maces.

Nodule (geology)

In sedimentology and geology, a nodule is small, irregularly rounded knot, mass, or lump of a mineral or mineral aggregate that typically has a contrasting composition, such as a pyrite nodule in coal, a chert nodule in limestone, or a phosphorite nodule in marine shale, from the enclosing sediment or sedimentary rock. Normally, a nodule has a warty or knobby surface and exists as a discrete mass within the host strata. In general, they lack any internal structure except for the preserved remnants of original bedding or fossils. Nodules are closely related to concretions and sometimes these terms are used interchangeably. Minerals that typically form nodules include calcite, chert, apatite (phosphorite), anhydrite, and pyrite.In sedimentology and geology, nodular is used to describe a sediment or sedimentary rock composed of scattered to loosely packed nodules in matrix of like or unlike character. It is also used to describe mineral aggregates that occur in the form of nodules, e.g. colloform mineral aggregate with a bulbed surface.Nodule is also used for widely scattered concretionary lumps of manganese, cobalt, iron, and nickel found on the floors of the world's oceans. This is especially true of manganese nodules. Manganese and phosphorite nodules form on the seafloor and are syndepositional in origin. Thus, technically speaking, they are concretions instead of nodules.Chert and flint nodules are often found in beds of limestone and chalk. They form from the redeposition of amorphous silica arising from the dissolution of siliceous spicules of sponges, or debris from radiolaria and the postdepositional replacement of either the enclosing limestone or chalk by this silica.

Onondaga Limestone

The Onondaga Limestone is a group of hard limestones and dolostones of Devonian age that form an important geographic feature in some areas in which it outcrops, in others; especially its Southern Ontario portion, the formation can be less prominent as a local surface feature.In upstate New York and southern Ontario the sedimentary rocks tend to slope slightly southward, and the Onondaga outcrops in a line that usually forms an escarpment (the steep face of a cuesta), because of its resistance to erosion. The outcrop can be traced from the Hudson River valley westward along the southern rim of the Mohawk River valley, passing just south of Syracuse, and along the northern heads of the major Finger Lakes to Buffalo, New York. From Fort Erie, Ontario it runs to Windsor just north of the Lake Erie shoreline, becoming less prominent as one travels westward. It is not distinct west of Windsor, but begins to become noticeable as a steep hill just northwest of Leamington, as it forms a low ridge/escarpment along much of the Lake Erie shoreline.

In several spots it is breached by geologically young streams and spectacular waterfalls are formed, such as at Chittenango Falls just east of Syracuse, Buttermilk Falls at Le Roy, New York and Indian Falls west of Batavia.

A few other breaches occur in older valleys, which likely once had waterfalls, but erosion eventually obliterated them. Such breaches occur at the Tully valley, the Genesee River valley near Avon, New York, and at Port Colborne, Ontario, where the old valley forms a harbor on Lake Erie.

The formation is broken by the only major fault line in western New York, the Linden Fault just east of Batavia, where the eastern side of the fault has dropped down and the ledge moved southward relative to the western side. On the western side of the fault in Genesee County the escarpment achieves its greatest prominence. The New York State Thruway has a rock cut at Batavia which clearly shows the fault and is a popular point for geology class field trips. The fault, which runs from Attica, New York northward to Lake Ontario, is still active and periodically causes minor earthquakes in the area.

The Onondaga Limestone also can be found in other areas where rocks of the same age outcrop, such as in western Pennsylvania and Michigan but they do not form prominent geographic features.

A similar and more prominent outcrop known as the Niagara Escarpment runs parallel and about 25 miles (40 kilometers) to the north through upstate New York, but curves northwestward in southern Ontario toward Lake Huron and eventually into Michigan's Upper Peninsula and Wisconsin's Door Peninsula.

Another smaller outcrop known as the Portage Escarpment lies about 35 miles (56 km) to the south, running along the southern ends of the Finger Lakes and forming Cascadilla, Ithaca and Buttermilk Falls in Ithaca.

The Onondaga Escarpment contains significant outcrops of flint (a type of chert) which bears the escarpment's name. This variety of chert was of great importance to First Nations peoples throughout Southern Ontario, who used it to make stone tools (lithics) such as projectile points and hide scrapers. This variety of chert, which is of reasonably high-quality and which was highly valued by First Nations peoples, is often a common variety of chert recovered archaeologically from sites relatively adjacent to outcrops; for example, Onondaga-variety chert comprises 95% of all of the flint material from some sites in Milton, Ontario. The material has also been found as well at some distance from its original source; Onondaga chert has been recovered at the late archaic Duck Lake archaeological site in northern Michigan, circa 400 kilometers from the nearest outcropping of the material. This wide distribution implies either a very large seasonal migration of ancient peoples or long-distance trade routes, with both likely being the case at different times throughout the prehistory of the Great Lakes region.

Pezizomycotina

Pezizomycotina make up the majority of the Ascomycota fungi and include most lichenized fungi too. Pezizomycotina contains the filamentous ascomycetes and is a subdivision of the Ascomycota (fungi that form their spores in a sac-like ascus). It is more or less synonymous with the older taxon Euascomycota. These fungi reproduce by fission rather than budding and this subdivision includes almost all the ascus fungi that have fruiting bodies visible to the naked eye (exception: genus Neolecta, which belongs to the Taphrinomycotina).

See the taxobox for a list of the classes that make up the Pezizomycotina. The old class Loculoascomycetes (consisting of all the bitunicate Ascomycota) has been replaced by the two classes Eurotiomycetes and Dothideomycetes. The rest of the Pezizomycotina also include the previously defined hymenial groups Discomycetes (now Leotiomycetes) and Pyrenomycetes (Sordariomycetes).

Some important groups in Pezizomycotina include: Pezizomycetes (the operculate discomycetes), Leotiomycetes (the inoperculate discomycetes), Laboulbeniomycetes, Sordariomycetes, Dothideomycetes.

Paleopyrenomycites from the Early Devonian Rhynie Chert is the oldest known fossil member of Pezizomycotina, although its position within this subdivision is unclear.

Pilbara Craton

The Pilbara Craton is an old and stable part of the continental lithosphere located in Pilbara, Western Australia.

The Pilbara Craton is one of only two pristine Archaean 3.6-2.7 Ga (billion years ago) crusts identified on the Earth, along with the Kaapvaal Craton in South Africa. Both locations may have once been part of the Vaalbara Supercontinent or the continent of Ur.

In May 2017, evidence of the earliest known life on land may have been found in 3.48-billion-year-old geyserite and other related mineral deposits (often found around hot springs and geysers) uncovered in the Pilbara Craton.The earliest direct evidence of life on Earth may be fossils of microorganisms permineralized in 3.465-billion-year-old Australian Apex chert rocks.

Radiolarite

Radiolarite is a siliceous, comparatively hard, fine-grained, chert-like, and homogeneous sedimentary rock that is composed predominantly of the microscopic remains of radiolarians. This term is also used for indurated radiolarian oozes and sometimes as a synonym of radiolarian earth. However, radiolarian earth is typically regarded by Earth scientists to be the unconsolidated equivalent of a radiolarite. A radiolarian chert is well-bedded, microcrystalline radiolarite that has a well-developed siliceous cement or groundmass.

Rhynie, Aberdeenshire

Rhynie () (Scottish Gaelic: Roinnidh) is a village in Aberdeenshire, Scotland which the A97 road runs through, situated 14 miles northwest of Alford.

The Rhynie Chert is named after the village as well as the extinct plant genus Rhynia. The Rhynie Chert is a sediment deposited in the Devonian period, is the oldest fossil insect in the world (Rhyniognatha hirsti).The missionary, teacher and chocolateer Alexander Murdoch Mackay was born in Rhynie on 13 October 1849.

Rhynie chert

The Rhynie chert is an Early Devonian sedimentary deposit exhibiting extraordinary fossil detail or completeness (a Lagerstätte). It is exposed near the village of Rhynie, Aberdeenshire, Scotland; a second unit, the Windyfield chert, is located some 700 m away. The Rhynie chert contains exceptionally preserved plant, fungus, lichen and animal material preserved in place by an overlying volcanic deposit. The bulk of the fossil bed consists of primitive plants (which had water-conducting cells and sporangia, but no true leaves), along with arthropods, lichens, algae and fungi.

This fossil bed is remarkable for two reasons. First, the age of the site (Pragian, Early Devonian, formed about 410 million years ago) places it at an early stage in the colonisation of land. Second, these cherts are famous for their exceptional state of ultrastructural preservation, with individual cell walls easily visible in polished specimens. Stomata have been counted and lignin remnants detected in the plant material, and the breathing apparatus of trigonotarbids—of the class Arachnida—(known as book lungs) can be seen in cross-sections. Fungal hyphae can be seen entering plant material, acting as decomposers and mycorrhizal symbionts.

Xert

Xert (Valencian pronunciation: [ˈtʃɛɾt], Spanish: Chert) is a municipality in the comarca of Baix Maestrat in the Valencian Community, Spain.

The mountains known as Moles de Xert rise above the town and are included in its administrative area.

Crystalline
Cryptocrystalline
Amorphous
Miscellaneous
Notable varieties

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