Chert Rock Type, Composition, Formation, Occurrence & Uses

Chert Rock
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Chert is a fine-grained and hard sedimentary rock that usually has a biological origin. Chert can also exist inorganically as a chemical precipitate or diagenetic replacement in petrified wood.

Chert can be of various colors due to trace elements in the rock. The most usual colors a chert can take are gray, grayish brown, brown, light green, and rusty red.

Chert breaks with a conchoidal fracture. Chert is commonly known as flint. Both chert and flint are varieties of chalcedony.  Sometimes chert is also referred to as firestone and silex.

 Chert Sample
 Chert Sample

Rock Composition

Chert is mostly composed of microcrystalline (cryptocrystalline) quartz. Quartz is the mineral form of silicon dioxide. These rocks usually consist of petrified (fossilized) remains of siliceous ooze. Siliceous ooze is the biological remains of marine organisms like diatoms, silicoflagellates, and radiolarians. Chert may also contain macrofossils. An interesting fact about chert’s composition is that some specimens exist without any fossils.

Precambrian cherts (cherts from the earliest parts of history) are known for the presence of cyanobacteria fossils.

Chert grows as nodules within sediment mass and then engulfs surrounding sediment deposits. The rock’s size starts to grow it can spread to large areas, usually in square kilometers.

Chert is a broad term for chemically precipitated sedimentary rocks made up of microcrystalline silica. Most charts are nearly pure silica with up to 5% other minerals like calcite, dolomite, clay minerals, hematite, and organic matter. Some cherts may have up to 99% silica, while others have 65% silica content. These specimens usually have abundant aluminum, followed by iron, manganese, potassium, sodium, and calcium.

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

Chert Formation

Chert forms when silicon dioxide microcrystals grow within soft sediment that becomes limestone or chalk. The silicon dioxide crystals start growing into irregularly-shaped nodules or concretions if dissolved silica is transported to the formation site. The silica is transported by underground water movement.

The nodules or chert lumps grow large enough to merge into a single continuous chert layer within the sediment mass. The chert formed due to this process is known as chemical or inorganic chert.

Chert usually exists organically. This chart forms in marine environments or shallow seas with a huge population of diatoms and radiolarians with a glassy silica skeleton. The silica skeleton falls to the bottom when these organisms die. It may dissolve, recrystallize or become part of a chert nodule. The nodules grow in size over time and produce rock layers that are extensive and thick. Chert formed as a result of this process is known as biological sedimentary chert.

Chert Varieties

Chert has numerous varieties. These varieties are classified based on visible, physical, and microscopic qualities. Some of the most famous varieties are listed below.

Flint: It is a compact microcrystalline quartz. Chert is often known as flint because the chert found in chalk was originally named flint. Flint is often found as nodules, used to make blade tools in historical times.

Common Chert: It forms in limestone formations when calcium carbonate is replaced with silica.

Jasper: A variety of chert is found in connection with magmatic formations. It is usually red due to hematite but can also be black, yellow, or green.

Radiolarite: Chert forms as primary deposits containing radiolarian microfossils.

Where is Chert Found?

Chert is found as bedded chert in the form of thinly bedded layers ranging from a few centimeters to a meter in thickness. It is also known as ribbon chert. This type of chert is usually pure and is separated by thin layers of silica-rich shale.

Chert is also found as nodular chert. It is more common in limestone but may also exist in shales and sandstone. Nodular chert is common in continental shelf environments.

Its reserves are located all over the world. Chert reserves in Asia are located in India, Japan, China, Iran, Oman, Russia, Taiwan, and Thailand. African countries (Kenya, Morocco, South Africa, and Tanzania) also have significant chert reserves.

European chert reserves are in Poland, Portugal, Serbia, Spain, Sweden, the United Kingdom, Austria, France, Greece, Italy, and Malta. USA and Australia also have huge chert reserves.

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

Chert Uses

  • Chert’s use in construction projects is avoided because it pop-outs when exposed to harsh weather. Certain chert varieties undergo an alkali-silica reaction when mixed with high-alkali cement. This reaction can crack and expand concrete.
  • Chert has limited use today. It is mined mostly as a source of silica. Some chert deposits have valuable iron, uranium, manganese, petroleum, and phosphorite.
  • Chert was used as a raw material to make stone tools in prehistoric times.

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