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Chert is a fine-grained sedimentary rock composed of quartz (SiO2) that is microcrystalline or cryptocrystalline quartz. It is usually organic rock but also occur inorganically as a chemical precipitate or a diagenetic replacement. It occurs as nodules, concretionary masses, and as layered deposits.
Name origin: Term is used to refer generally to all rocks composed primarily of microcrystalline, cryptocrystalline and microfibrous quartz
Texture: Non-clastic sedimentary rock
Grain size: Cryptocrystalline, cannot be seen except under very high magnification.
Colour: All colours, dependent on impurities present when precipitated.
Other features: Smooth to touch, glassy, exhibits conchoidal fracture.
Occurrence of Chert
Chert occurs in carbonate rocks that are greensand, limestone, chalk, and dolostone formations as exchange mineral, where it is formed as a result of some type of diagenesis. if where it occurs in chalk or marl, it is 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. The banded iron formations of Precambrian age are composed of alternating layers of chert and iron oxides.
It 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.
Chert Classification and Types
There are many varieties of chert, that classified visible, microscopic and physical characteristics
Flint is a high 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.
Known Common chert is a variety of chert which forms in limestone formations by replacement of calcium carbonate with silica. This chert type is most abundant.
Jasper is a variety of this rock 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 this rock 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.
Chert is in most cases a biogenic rock, it is made of siliceous tests of diatoms, radiolarians, siliceous sponge spicules, etc. Sometimes microscopic fossilized remains of these sea creatures may be preserved in these rocks. Their siliceous tests are not made of quartz initially, but after burial, compaction, and diagenesis, opaline siliceous sediments transform to quartz. Although the material it is made of ultimately came from siliceous tests of marine species, the rock itself is often not deposited in situ. It may move as a silica-rich liquid and form nodules in rocks by replacing the original (usually carbonate) material. So It is also sometimes said to be a rock of chemogenic origin. Bedded variety seems to be often associated with turbidity currents.
Chert may occur as the microcrystals of silicon dioxide grow in soft sediments that will become limestone or chalk. In these precipitates, when the dissolved silica is transported to the formation zone by the movement of groundwater, a large number of silicon dioxide microcrystals are transformed into irregularly shaped nodules or concretes.
If the nodules or concretes are numerous, they can grow enough to be joined together to form a nearly continuous notch layer in the sedimentary mass. it formed in this way is a chemical sedimentary rock.
Part of the silicon dioxide in the container is thought to have a biological origin. In some parts of the ocean and in shallow seas, many diatoms and radios live in the water. These organisms have a glassy silica skeleton. Some sponges also produce “spicule” of silica.
When these organisms die, the silica skeletons fall to the bottom, dissolve, re-crystallize, and the notch may be part of a nodule. In some regions, the sedimentation rate of these materials is high enough to produce thick and later rock layers. It formed in this way can be considered as biological sedimentary rock.
Where is it found?
Bedded cherts may form by compaction and recrystallization of silica-rich biogenic sediments made of opaline tests of single-cell organisms (diatoms, radiolaria) or remains of silicious sponges, both in marine and in lake environments. During diagenesis, the silica in the sediments undergoes a transformation from opal-A through opal-CT to microcrystalline quartz in the mature chert (Oldershaw 1968; Calvert 1971; Lancelot 1973; Hein et al 1981; Pisciotto 1981; Riech 1981; Levitan 1983; Jones et al 1986; Compton 1991). Accordingly, these cherts may contain some opal-CT. Silica mobilized from volcaniclastic sediments, hydrothermal solutions and clay minerals may contribute to the silicification (Calvert 1971; Thurston 1972; Pollock 1987; Hesse 1989).
– Cherts in banded iron formations are thought to have formed from primarily chemically precipitated silica. Often they are colored brightly by co-precipitated iron minerals (Sugitani et al 1998; Rosière et al 2000; Maliva et al 2005; Fisher et al 2008).
– Some Archean cherts appear to have been formed by silicification of volcaniclastic sediments (Knauth 1994).
– Nodules, irregular bodies and discontinuous layers of chert are found in marine calcareous sediments. They typically form during early diagenesis by precipitation of silica mobilized from biogenic sources like radiolarian tests or sponge spicules. (Buurman et al 1971; Meyers 1977; Bustillo et al 1987; Maliva et al 1989; Knauth 1994; Madsen et al 2010).
– Magadi-type cherts, named after their occurrence at Lake Magadi, Kenya, form by leaching of alkali ions from silicates in silica-rich evaporites (Hay 1968; Eugster 1969).
Chert Characteristics and Properties
Chert is as hard as crystalline quartz with a hardness rating of seven in the Mohs scale — maybe a bit softer, 6.5, if it still has some hydrated silica in it. Beyond simply being hard, chert is a tough rock. It stands above the landscape in outcrops that resist erosion. Oil drillers dread it because it’s so hard to penetrate.
It has a curvy conchoidal fracture that is smoother and less splintery than the conchoidal fracture of pure quartz; ancient toolmakers favored it, and high-quality rock was a trade item between tribes.
Unlike quartz, it is never transparent and not always translucent. It has a waxy or resinous luster unlike the glassy luster of quartz.
The colors of chert range from white through red and brown to black, depending on how much clay or organic matter it contains. It often has some sign of its sedimentary origin, such as bedding and other sedimentary structures or microfossils. They may be abundant enough for a chert to get a special name, as in the red radiolarian chert carried to land by plate tectonics from the central ocean floor.
In prehistoric times, it was often used as a raw material for the construction of stone tools.
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.
In some areas, it 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. However, where cherty gravel ends up as fill in concrete, the slick surface can cause localized failure.
It has been used in late nineteenth-century and early twentieth-century headstones or grave markers in Tennessee and other regions.
- In today’s world, chert has very few uses, but many ancient cultures used it to make tools for cutting and scraping and also used it to make weapons like arrowheads and ax heads. It is very hard and durable and the edges of chert are very sharp.
- Chert is found in many colors. Most common colors are blue, green, red and yellow. White coloration usually indicates it contains carbonate impurities, while black indicates organic matter.
- Darker color chert is often referred to as flint. It can be found in chalk or marly limestone formations and formed by a replacement of calcium carbonate with silica. It’s commonly found as nodules.
- Red to brown chert receive their color when it contains iron oxide and are then referred to as jasper. It is usually opaque to near opaque.
- The most abundantly found variety of chert is “common chert”. It is a variety of chert which forms in limestone formations by replacement of calcium carbonate with silica. It is considered to be less attractive for producing gem stones than flint.
- When struck against steel, it produces a spark which results in heat. It makes an excellent tool for starting fires.
- A primary historic use of chert and flint was to make a “flintlock gun”. The firearm had a metal plate that produced a spark when struck with chert. It ignited a small reservoir containing black powder that discharged the firearm.
- It was used in the late 1800’s and early 1900’s as grave markers or headstones.
- Marble Bar Chert in Western Australia is considered one of the earliest and best preserved sedimentary successions on Earth.
- Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
- Chert. (2017, February 9). New World Encyclopedia, . Retrieved 22:36, April 11, 2019 from //www.newworldencyclopedia.org/p/index.php?title=Chert&oldid=1003201.
- Alden, Andrew. (2018, June 22). Learn More About Chert Rock. Retrieved from https://www.thoughtco.com/what-is-chert-1441025
- Wikipedia contributors. (2019, March 31). Chert. In Wikipedia, The Free Encyclopedia. Retrieved 22:37, April 11, 2019, from https://en.wikipedia.org/w/index.php?title=Chert&oldid=890301003