AskDefine | Define chert

Dictionary Definition

chert n : variety of silica containing microcrystalline quartz

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English

Noun

  1. An impure, massive, generally dull colored, flintlike quartz or hornstone. By general usage in mineralogy and geology, a chert does not have a conchoidal fracture. By general North American usage in archaeology (archeology) the term chert has been used until recently for quartzes (including flint) with a conchoidal fracture.

Translations

Extensive Definition

Chert () is a fine-grained silica-rich microcrystalline, cryptocrystalline or microfibrous sedimentary rock that may contain small fossils. It varies greatly in value and in color (from white to black), but most often manifests as gray, brown, grayish brown and light green to rusty red; 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 crops out as oval to irregular nodules in greensand, limestone, chalk, and dolostone formations as a replacement mineral, where it is formed as a result of diagenesis. 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 geosynclinal deposits. These thickly bedded cherts include the novaculite of the Ouachita Mountains of Arkansas, Oklahoma, and similar occurrences in Texas in the United States. The banded iron formations of Precambrian age are composed of alternating layers of chert and iron oxides.

Concerning the terms "chert", "chalcedony" and "flint"

There is much confusion concerning the exact meanings and differences among the terms "chert", "chalcedony" and "flint" (as well as their numerous varieties). In petrology the term "chert" is used to generally refer to all rocks composed primarily of microcrystalline, cryptocrystalline and microfibrous quartz. The term does not include quartzite. Chalcedony is a microfibrous (microcrystaline 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. Among non-geologists (in particular among archaeologists), 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 America and is likely caused by early immigrants who imported 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 microcrystaline and microfibrous quartz, it is sometimes difficult to classify a rock as completely chalcedony, thus its general inclusion as a variety of chert.

Chert and Precambrian fossils

The cryptocrystalline nature of chert, combined with its above average ability to resist weathering, recrystallisation and metamorphism has made it an ideal rock for preservation of early life forms.
For example:

Prehistoric and historic uses

In prehistoric times, chert was often used as a source material for stone tools. Like obsidian, as well as some rhyolites, felsites, quartzites and a few 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 airgun 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 steel, sparks result. This makes it 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.
In some areas chert is ubiquitous as stream gravel and fieldstone and is currently used as construction material and road surfacing.
Chert has been used in late 19th-century and early 20th-century headstones or grave markers in Tennessee and other regions.

Varieties of Chert

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 microcrystaline quartz. It is found in chalk or marly limestone formations and is formed by a replacement of calcium carbonate with silica. It is commonly found as nodules. This variety was often used in past times to make bladed tools.
  • "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 colour 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 is 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 microcrystaline and contain much less water (sometime none). Often people without petrological training confuse opal with chert due to similar visible and physical characteristics.
Other lesser used terms for chert (most of them archaic) include, firestone, silex, silica stone and flintstone.

References

External links

chert in Min Nan: Hoé-chio̍h
chert in Catalan: Sílex
chert in Czech: Buližník
chert in Welsh: Callestr
chert in Danish: Flint
chert in German: Hornstein (Gestein)
chert in Estonian: Ränikivi
chert in Spanish: Sílex
chert in Esperanto: Siliko
chert in French: Silex
chert in Indonesian: Rijang
chert in Italian: Selce
chert in Hebrew: צור (סלע)
chert in Latin: Silex
chert in Latvian: Krams
chert in Lithuanian: Titnagas
chert in Dutch: Vuursteen
chert in Japanese: 燧石
chert in Norwegian: Flint
chert in Norwegian Nynorsk: Flint
chert in Polish: Krzemień
chert in Portuguese: Sílex
chert in Romanian: Silicolit
chert in Russian: Кремень
chert in Simple English: Chert
chert in Slovak: Silicit
chert in Slovenian: Kremen
chert in Finnish: Piikivi
chert in Swedish: Flinta
chert in Thai: หินเชิร์ต
chert in Vietnamese: Đá lửa (lịch sử)
chert in Ukrainian: Кремінь
chert in Chinese: 燧石
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