Chert (rock)

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Cherts (dark layers) in the Corriganville-New Creek Devonian limestone, Everett , Pennsylvania

Cherts (in German often under the name Hornstein ) are sedimentary and diagenetically formed pebbles .

Demarcation

The English technical term Chert stands for the entire group of sedimentary and diagenetically formed pebbles and has established itself in the German-speaking geological literature. 'Hornstein' is often used as the German translation for 'Chert', even if this term is ambiguous.

rock

The structure of Cherts is very fine-grained , so that it can only be resolved under the microscope (microcrystalline) or even there hardly or not at all (cryptocrystalline). The rock can contain microfossils . Its color is different and varies between white and black, but mostly it is gray, brown, gray-brown or light green to rust-red. The color is due to traces of additional elements or minerals . The red and green colors are usually due to the addition of iron in oxidized or reduced form.

education

Cherts are formed as oval to irregularly shaped tubers in green sand , limestone , chalk and dolomite as a result of the displacement of original minerals by silicon dioxide during the rock formation processes ( diagenesis ) . In addition, they occur as sedimentary rocks in often thin-layered deposits that appear as continuous layers , such as in many radiolarite deposits. In these, too, transformation processes such as that of opal into quartz and the formation of a consistently dense rock play an essential role.

The Chert family

The exact scope of the term chert is a constant source of confusion. In the real sense, the term exclusively includes sedimentary-diagenetic formations, which predominantly consist of micro- / cryptocrystalline silicon dioxide .

  • Flint is a dense, micro- / cryptocrystalline rock. It is found in chalk or marly limestone and is formed by the displacement of calcium carbonate by silicon dioxide. It occurs as tubers or in more or less extensive plates. According to Rapp (2002), the term 'flint' in the narrower sense is reserved fortypes of chert foundin chalk, limestone and marl . Outside of geology , especially in archeology , the distinction between flint and chert is based on the quality of the rock in relation to its use as a stone tool .
  • Common chert also forms in limestone due to the displacement of calcium carbonate by silicon dioxide. However, it cannot be split as easily as flint due to impurities.
  • Porcelainite is a fine-grained stone with similarities to unglazed porcelain .
  • Radiolarite (also Lydite or Kieselschiefer ) is a sedimentary rock that emerged from radiolarian mud
  • Diatomite is a sedimentary rock that emerged from diatomaceous mud

Also Spiculite (sedimentary rocks with a high content of Schwammskleren) may be at a very high SiO 2 fall into the category of total chert.

Often, however, micro- or cryptocrystalline aggregates, which are commonly considered to be varieties of the mineral quartz, are given the name Chert. So Chalcedony a fibrillar quartz variety, petrography is not counted as chert due to this construction. The demarcation is, however, fuzzy, as chalcedony usually consists of a mixture of fine-grained and grainy-directionless microcrystalline quartz. Different forms of chalcedony are:

  • Jasper is colored very diverse and often occurs in conjunction with the occurrence of igneous rocks on
  • Agate is a clearly banded chalcedony with alternating, differently colored layers.
  • Onyx is a parallel banded agate, often black and white.

Quartzite , on the other hand, is generally not counted among the Cherts. Like Hornfels, it is a metamorphic rock . Also Opal , a hydrated amorphous silicon dioxide, does not belong to the family of sedimentary rocks pebbles.

Occurrence

Cherts can form thick layers as deep-sea sediments , for example in the Kulm silica slates of the Rhenish Slate Mountains and the Harz Mountains . Other examples are the novaculites of the Ouachita Mountains in Arkansas , Oklahoma, and similar occurrences in Texas . The Banded Iron Formations of the Precambrian consist of alternating layers of chert and iron oxides such as magnetite (Fe 3 O 4 ) and hematite (Fe 2 O 3 ).

Cherts also occur as diatom deposits such as kieselguhr . Layers of such diatomaceous rocks have been described, for example, from the Miocene Monterey Formation in California .

Cherts and Precambrian fossils

Handpiece of the Precambrian Banded Iron Formation from Upper Michigan with red chert layers

The fine-grained, cryptocrystalline nature of Cherts in combination with the resistance to weathering , recrystallization and metamorphosis has favored the transmission of traces of early life on earth.

Examples are:

Prehistoric use

Hoe and other stone tools from Mill Creek Chert , Parkin Archeological State Park , Arkansas

In the Paleolithic and later, cherts were used as raw material for the manufacture of stone tools. Like obsidian , rhyolite , rockite , quartzite and other tool stones, cherts split with the shell-like fracture typical of quartz . Due to the sharp edges as well as the different sizes of the chert's chips and cores , the stone was often used, especially in the flint variety.

Chert was also often used as a striking stone against a pyrite or marcasite to start a fire by striking a spark.

Industrial use

Chert has been used for grinding calcined flint, which is used as a bleach in the manufacture of ceramics, since the late 18th century. The potter Josiah Wedgwood recognized in 1772 that Chert from Derbyshire was much better than granite for millstones, as the granite abrasion left annoying black spots in the white ceramic. Porcelain factories have since used Chert as millstones for their grinders, with the large blocks spinning and on a floor paved with smaller blocks.

literature

  • JW Schopf: Cradle of Life: The Discovery of Earth's Earliest Fossils . Princeton University Press, 1999, ISBN 0-691-00230-4 (English).

Web links

Commons : Chert  - collection of images, videos and audio files

Individual evidence

  1. ^ Wolfgang Reichel, Jan-Michael Lange: Cherts (chert stones) from the Döhlen basin near Dresden . In: Geologica Saxonica . tape 52/53 , 2007, pp. 117–128 ( online version [PDF; 1.9 MB ]).
  2. ↑ George R. Rapp: Archaeomineralogy . 2002, ISBN 3-540-42579-9 , pp. 79 ( p. 79 in Google Book search).
  3. ↑ Michael S. Clark: Sequence stratigraphy of an interbedded biogenic-clastic reservoir, Belridge Diatomite at Lost Hills Field, San Joaquin Basin, California. 2001, accessed February 24, 2010 .
  4. ^ The earliest life: Annotated listing. ( Memento of the original from August 8, 2012 on WebCite ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. Paleobotanical working group of the University of Münster @1@ 2Template: Webachiv / IABot / www.uni-muenster.de
  5. ↑ A. Hofmann: Archaean hydrothermal system in the Barberton Greenstone Belt and Their significance as a habitat for early life. In: S. Golding, M. Glikson (Eds.): Earliest Life on Earth: Habitats, Environments and Methods of Detection . Springer-Verlag, 2011, ISBN 978-90-481-8793-5 , p. 51-78 .
  6. ↑ Gunflint chert. (No longer available online.) Geological Survey of Canada, archived from the original September 26, 2011 ; Retrieved February 24, 2010 .
  7. ^ BT De Gregorio, TG Sharp: Determining the biogenicity of microfossils in the Apex Chert, Western Australia, using transmission electron microscopy . In: Lunar and Planetary Science . tape XXXIV , 2003 ( online version [PDF; 250 kB ]).
  8. ↑ Localities of the Proterozoic: Bitter Springs Formation, Australia. UCMP - University of California, Berkeley , Museum of Paleontology, Berkeley , accessed February 24, 2010 .
  9. ↑ Julie Bunting: [ Bygone industries of the Peak, Derbyshire. ] In: The Peak Advertiser, Oct. 16, 1995, March 11, 2005 update, p. 7. Published in Illustrated Paperback in 2006, Wildtrack Publishing, Sheffield, ISBN 1-904098-01-0