Black slate
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Black shales are pelitic (clayey) sedimentary rocks of marine origin. It is not always real slate that has undergone mountain formation , but also undeformed claystones that split along the original layer surfaces. In order to take this into account, the use of the more neutral terms black clay stones or black pelite is recommended.
composition
Typical of black slate is its carbon content , which gives the rock its color. In addition to clay minerals as the basic building material, black slate often also contains quartz or silica gel , feldspar and types of mica , all very finely divided, often also pyrite , marcasite and phosphorite . Many black shales contain numerous metals such as iron and other heavy metals .
Emergence
Black shale is formed on the sea floor from sapropel (digested sludge) when there is insufficient oxygen. Such conditions can occur when stagnant waters are not mixed, for example in geological epochs with a calm, balanced climate without large temperature differences. On the one hand, the lack of oxygen leads to the incomplete decomposition of dead organisms and their carbonization in the muddy clay of the seabed, which gives the later rock its typical blackness. In addition, in the event of a lack of oxygen and the presence of sulfate, bacterial reduction of sulfate to hydrogen sulfide (H 2 S) ( desulfurication ) sets in , whereby heavy metals dissolved in seawater as sulfides , for example pyrite (FeS 2 ) and chalcopyrite (FeCuS 2 ), precipitate and in the sediment be stored. In the fine distribution, these sulfides also color the rock black. Black slates are formed from the layered, schisty clays through weak regional metamorphosis at low pressure over long periods of time.
Black shale are often fossil-bearing. Fossils are atypical for real slate, because according to their definition they are sediments that have been changed by rock metamorphosis, their cleavage surfaces are created by high pressure and the originally existing fossils are lost. On the other hand, black slate stands in the transition area from slate to clay slate and is only slightly metamorphic, its cleavage surfaces are the sediment layers. One speaks of slate only in this case because black slate has the good cleavage properties typical of slate.
Age
Many black slates are quite old and come from the Paleozoic , the ancient times , from the Cambrian to Ordovician , Silurian , Devonian and Carboniferous to the Permian . Black slate also occurs in the Mesozoic Era ( Jurassic and Cretaceous ). There are also currently places where digested sludge is deposited, from which black slate could one day form, for example in the deeper basins of the Baltic Sea and the Black Sea . The Black Sea (Greek: pontos euxeinos ) is eponymous for the geological expression euxinic , which is used to describe a low -oxygen , hydrogen-sulphide-containing environment.
Classification
After fossil tour
- The Burgess Shale from the Cambrian of Canada ( British Columbia ) is famous for the good state of preservation and richness of its fossils. With his discovery, the distant world of the first complex ecosystems opened up to paleontologists , he is considered the window to the Cambrian.
- The alum slate of Scandinavia of the middle and upper Cambrian Scandinavia, in which lime tubers (orsten) are embedded.
- Graptolite slates come from the Ordovician and Silurian times and contain their typical key fossils , the graptolites (see picture).
- Bundenbacher slate from the Devonian of the Hunsrück .
- Posidonia slate , for example Holzmadener slate ( Lias ε).
According to economic use
- Copper shale contains copper pyrites ( chalcopyrite ) and other heavy metal sulfides in minable quantities.
- The raw material for nuclear power plants and atomic bombs , uranium , was also extracted from deep black slate layers and the leather slate underneath, especially in East Thuringia . The recultivated mining areas in the Gera and Ronneburg area formed the backdrop for the Federal Garden Show in 2007 .
- Alum and vitriols , later also sulfuric acid , called vitriol oil in the 18th century, were extracted from pyrite-containing alum slate in historical alum plants.
- Oil shale and bitumen shale also served here and there as a source of energy. An example of this is the Estonian kuckersite (Ordovician oil shale).
literature
- L. Bauer, F. Tvrz: Der Kosmos-Mineralienführer , Gondrom Verlag, Bindlach, 1993, ISBN 3-8112-1115-3
- Chris Pellant: Stones and Minerals , 4th edition, Urania Verlag 2002, ISBN 3-332-00998-2
- Rudolf Jubelt, Peter Schreiter: Rock determination book , 8th edition, VEB Verlag for basic industry, Leipzig, 1987, ISBN 3-342-00239-5