Chert

Cherts ( in German often under the name of chert ) sedimentary and diagenetic and resulting pebble rocks.

Demarcation

The English term Chert represents the entire group of sedimentary and diagenetic entstandenenen pebble rocks, and has established itself fachsprachlich in the German geological literature. As a German translation of, Chert 'is often, chert ' is used, although this term is ambiguous.

Rock

The structure of cherts is very fine, so they are only under the microscope can resolve ( microcrystalline) or even there is little or no ( cryptocrystalline ). The rock may contain microfossils. His color is different and varies between white and black, but usually it is gray, brown, gray-brown or light green to rusty red. The color is due to traces of additional elements or minerals. The red and green colors are generally due to additions of iron in oxiderter or reduced form.

Education

Cherts formed as a result of displacement of the original minerals by silica in the rock -forming processes ( diagenesis ) as oval or irregularly shaped tubers in green sand, limestone, chalk and dolomite. Moreover, they occur as sedimentary rocks, often in thin-film deposits that occur as continuous layers, such as in many radiolarite occurrence. Even with these play conversion processes such as Opal in quartz and the formation of a continuous dense rock an essential role.

The Chert - family

The distinction of cherts as chert and flint, and their varieties is a constant source of confusion. There are many rocks and minerals, which are made of microporous or cryptocrystalline quartz or microscopic quartz fibers.

• Flint is a dense, microcrystalline rock. It is found in chalk or marly limestone and is formed by the displacement of calcium carbonate by silica. It occurs as nodules or in more or less extended plates. The term flint ' in the narrower sense is by Rapp ( 2002) reserved for Hornstein species found in chalk, limestone and marls. Outside of geology, especially in archeology, the distinction between flint and chert occurs due to the quality of the rock in relation to the use as a stone tool.
• Usually chert formed in limestone also due to the displacement of calcium carbonate by silica. He is due to impurities but not so easy to split as flint
• Porcellanite is a fine- grained rock with similarities to unglazed porcelain.
• Radiolarite consists of radiolarian diatomite from diatoms

Other chert types are created, among other things Novaculite or sponge spicules Spiculite.

Many a time to be counted among the Chert rocks, that have also been finely crystalline, but no rocks are, but minerals. Example of this are:

• Jasper is colored very differently and often occurs in conjunction with the occurrence of igneous rocks on
• Chalcedony is a finely fibrous quartz variety and in the narrower sense does not rock, but a mineral. Petrographically chalcedony stones are not counted as chert due to the fine-fiber structure of the mineral. The distinction is blurred, however, as chalcedony usually consists of a mixtures of microcrystalline quartz and feinfasrigem.
• Agate is a banded chalcedony with alternating clear, differently colored layers
• Onyx is a parallel banded agate, often black and white

Quartzite also does not belong in this group of rocks, he is just as hornfels a metamorphic rock. Also opal, a hydrated silica, does not belong to the family of sedimentary siliceous rocks.

Occurrence

As a deep-sea sediment cherts powerful layers can form, for example in the Kulm -pebble slates of the Rhenish Massif and the resin. Other examples are the Ouachita Mountains in Arkansas the Novaculite, Oklahoma and similar deposits in Texas. The Banded Iron Formations of the Precambrian consist of alternating layers of chert and iron oxides such as magnetite (Fe3O4 ) and hematite (Fe2O3 ).

Cherts are beyond present as diatomaceous deposits such as diatomaceous earth. Layers of such diatomaceous rocks have been described, for example from the Miocene Monterey Formation of California.

Chert and Precambrian fossils

The fine-grained, crypto- crystalline nature of cherts in conjunction with the resistance to weathering, recrystallisation and metamorphism has favored the tradition of traces of early life on Earth.

Examples are:

• The 3.416 Ga ( billion years) old cherts in the Buck Reef Chert of the Fig Tree Group in the Barberton greenstone belt on the border of Swaziland and South Africa contain unicellular fossils that resemble bacteria.
• The Gunflint Chert in Western Ontario ( 1.9 to 2.3 Ga) includes not only bacteria, such as cyanobacteria, but also those that are similar to green algae and sponges, or other, of which it is assumed that their metabolism is based on ammonia.
• The Apex Chert ( 3.4 Ga ) of the Pilbara Craton in Australia contains eleven species of prokaryotes.
• The Bitter Springs Formation of the Amadeus Basin, Central Australia, contains 850 Ma ( million years) old cyanobacteria and algae.
• The Devonian Rhynie Chert (400 Ma) in Scotland recover the oldest remains of land plants, the preservation is so perfect that the cell structure of the fossils can be studied.

Prehistoric use

In prehistoric times cherts were used as raw material for the manufacture of stone tools. Just as obsidian, rhyolite, felsite, quartzite and other stones cleave tool cherts with the typical quartz conchoidal fracture. Due to the sharp edges as well as the different sizes, the discounts and kernels have cherts, the rock was often used, especially in the variety flint. This also applies to the use as a cigarette lighter.