Shale is a fine-grained, clastic Clastic rocks are composed of fragments, or clasts, of pre-existing rock. Geologists most commonly, but not always, use the term with reference to sedimentary rocks sedimentary rock Sedimentary rock is a type of rock that is formed by sedimentation of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution. Particles that form a sedimentary rock by accumulating composed of mud Mud is a liquid or semi-liquid mixture of water and some combination of soil, silt, and clay. Ancient mud deposits harden over geological time to form sedimentary rock such as shale or mudstone . When geological deposits of mud are formed in estuaries the resultant layers are termed bay muds. Mud is closely related to slurry and sediment that is a mix of flakes of clay minerals Clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths and other cations. Clays have structures similar to the micas and therefore form flat hexagonal sheets. Clay minerals are common weathering products and low temperature hydrothermal alteration products. Clay and tiny fragments (silt Silt is granular material of a grain size between sand and clay derived from soil or rock. Silt may occur as a soil or as suspended sediment in a surface water body. It may also exist as soil deposited at the bottom of a water body-sized particles) of other minerals, especially quartz Quartz is the second most abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2 and calcite Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (Ca . The ratio of clay to other minerals is variable.[1] Shale is characterized by breaks along thin laminae or parallel layering or bedding less than one centimeter in thickness, called fissility.[1] Mudstones Mudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm (0.0025 in) with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the appearance of fissility or parallel layering. This, on the other hand, are similar in composition but do not show the fissility.

Contents

Historical mining terminology

Before the mid 19th century, the terms slate Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. The result is a foliated rock in which the foliation may not correspond to the original sedimentary layering. Slate is frequently grey in color, especially, shale and schist The schists form a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is produced. By definition, schist contains more than 50% platy were not sharply distinguished.[2] In the context of underground coal mining The goal of coal mining is to economically remove coal from the ground. Coal is valued for its energy content, and since the 1880s is widely used to generate electricity. Steel and cement industries use coal as a fuel for extraction of iron from iron ore and for cement production. In the United States, United Kingdom, and South Africa, a coal mine, shale was frequently referred to as slate Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. The result is a foliated rock in which the foliation may not correspond to the original sedimentary layering. Slate is frequently grey in color, especially well into the 20th century.[3]

Texture

Shale typically exhibits varying degrees of fissility breaking into thin layers, often splintery and usually parallel to the otherwise indistinguishable bedding plane because of parallel orientation of clay mineral flakes.[1] Non-fissile rocks In geology, rock is a naturally occurring solid aggregate of minerals and/or mineraloids of similar composition but made of particles smaller than 0.06 mm are described as mudstones Mudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm (0.0025 in) with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the appearance of fissility or parallel layering. This (1/3 to 2/3 silt particles) or claystone (less than 1/3 silt). Rocks with similar particle sizes but with less clay (greater than 2/3 silt) and therefore grittier are siltstones Siltstone is a sedimentary rock which has a grain size in the silt range, finer than sandstone and coarser than claystones.[1] Shale is the most common sedimentary rock.[4]

Sample of drill cuttings Drill cuttings refers to any material removed from a borehole while drilling petroleum wells. Although sand and shale make up the majority of the cuttings encountered while drilling a well, depending on the location, any number of formations will actually be encountered. These include but are not limited to: anhydrite, calcite, chalk, chert, clay, of shale while drilling an oil well An oil well is a general term for any boring through the earth's surface that is designed to find and acquire petroleum oil hydrocarbons. Usually some natural gas is produced along with the oil. A well that is designed to produce mainly or only gas may be termed a gas well in Louisiana Some Louisiana urban environments have a multicultural, multilingual heritage, being so strongly influenced by an admixture of 18th century French, Spanish and African cultures that they are considered to be somewhat exceptional in the U.S. Before the American influx and statehood at the beginning of the 19th century, the territory of current. Sand grain = 2 mm. in dia.

Composition and color

Shales are typically composed of variable amounts of clay minerals and quartz grains and the typical color is gray. Addition of variable amounts of minor constituents alters the color of the rock. Black shale results from the presence of greater than one percent carbonaceous material and indicates a reducing Redox describes all chemical reactions in which atoms have their oxidation number (oxidation state) changed. This can be either a simple redox process, such as the oxidation of carbon to yield carbon dioxide (CO2) or the reduction of carbon by hydrogen to yield methane (CH4), or a complex process such as the oxidation of sugar(C6H12O6) in the environment.[1] Black shale can also be referred to as black metal.[5] Red, brown and green colors are indicative of ferric oxide (hematite Hematite, also spelled as hæmatite, is the mineral form of iron oxide (Fe2O3), one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum. Hematite and ilmenite form a complete solid solution at temperatures above 950°C - reds), iron hydroxide (goethite Goethite (FeO), (pronounced: "Gertite" /ˈgɝtaɪt/) named after the German polymath Johann Wolfgang von Goethe, is an iron bearing oxide mineral found in soil and other low-temperature environments. Goethite has been well known since prehistoric times for its use as a pigment. Evidence has been found of its use in paint pigment samples - browns and limonite Limonite is an ore consisting in a mixture of hydrated iron oxide-hydroxide of varying composition. The generic formula is frequently written as FeO(OH)·nH2O, although this is not entirely accurate as limonite often contains a varying amount of oxide compared to hydroxide - yellow), or micaceous minerals (chlorite The chlorites are a group of phyllosilicate minerals. Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice; Mg, Fe, Ni, and Mn, biotite Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula K3AlSi3O10(F,OH)2. More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers include siderophyllite. Biotite was and illite Illite is a non-expanding, clay-sized, micaceous mineral. Illite is a phyllosilicate or layered alumino-silicate. Its structure is constituted by the repetition of tetrahedron – octahedron – tetrahedron layers. The interlayer space is mainly occupied by poorly hydrated potassium cations responsible for the absence of swelling. Structurally - greens).[1]

Clays are the major constituent of shales and other mudrocks. The clay minerals represented are largely kaolinite Kaolinite is a clay mineral, part of the group of industrial minerals, with the chemical composition Al2Si2O54. It is a layered silicate mineral, with one tetrahedral sheet linked through oxygen atoms to one octahedral sheet of alumina octahedra. Rocks that are rich in kaolinite are known as china clay, white clay, or kaolin, montmorillonite Montmorillonite is a very soft phyllosilicate group of minerals that typically form in microscopic crystals, forming a clay. It is named after Montmorillon in France. Montmorillonite, a member of the smectite family, is a 2:1 clay, meaning that it has 2 tetrahedral sheets sandwiching a central octahedral sheet. The particles are plate-shaped with and illite. Clay minerals of Late Tertiary The Tertiary is a term for a geologic period 65 million to 1.8 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary. The period began with the demise of the non-avian dinosaurs in the Cretaceous–Tertiary extinction event, at the start of the Cenozoic era, spanning to the beginning of mudstones are expandable smectites Clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths and other cations. Clays have structures similar to the micas and therefore form flat hexagonal sheets. Clay minerals are common weathering products and low temperature hydrothermal alteration products. Clay whereas in older rocks especially in mid to early Paleozoic The Paleozoic or Palaeozoic Era (from the Greek palaios , "old" and zoe (ζωή), "life", meaning "ancient life") is the earliest of three geologic eras of the Phanerozoic Eon. The Paleozoic spanned from roughly 542 to 251 million years ago (ICS, 2004), and is subdivided into six geologic periods; from oldest to shales illites predominate. The transformation of smectite to illite produces silica The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with a chemical formula of Si , sodium, calcium, magnesium, iron and water. These released elements form authigenic An authigenic mineral or sedimentary rock deposit is one that was generated where it is found or observed. Can be used to characterize metamorphic minerals formed in situ during metamorphism, or to characterize sedimentary minerals formed during sedimentation instead of being transported from elsewhere by water or wind. Authigenic sedimentation is quartz Quartz is the second most abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2, chert Chert is a fine-grained silica-rich microcrystalline, cryptocrystalline or microfibrous sedimentary rock that may contain small fossils. It varies greatly in color (from white to black), but most often manifests as gray, brown, grayish brown and light green to rusty red; its color is an expression of trace elements present in the rock, and both, calcite Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (Ca , dolomite Dolomite is the name of a sedimentary carbonate rock and a mineral, both composed of calcium magnesium carbonate Ca , ankerite, hematite Hematite, also spelled as hæmatite, is the mineral form of iron oxide (Fe2O3), one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum. Hematite and ilmenite form a complete solid solution at temperatures above 950°C and albite Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. As such it represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula Na , all trace to minor (except quartz) minerals found in shales and other mudrocks.[1]

Shales and mudrocks contain roughly 95 percent of the organic matter in all sedimentary rocks. However, this amounts to less than one percent by mass in an average shale. Black shales which form in anoxic conditions contain reduced free carbon along with ferrous iron (Fe2+) and sulfur (S2-). Pyrite The mineral pyrite, or iron pyrite, is an iron sulfide with the formula Fe and amorphous iron sulfide along with carbon produce the black coloration.[1]

Formation

Limey shale overlaid by limestone Limestone is a sedimentary rock composed largely of the mineral calcite . Like most other sedimentary rocks, limestones are composed of grains; however, most grains in limestone grains are skeletal fragments of marine organisms such as coral or foraminifera. Other carbonate grains comprising limestones are ooids, peloids, intraclasts, and, Cumberland Plateau The Cumberland Plateau is the southern part of the Appalachian Plateau. It includes much of eastern Kentucky and western West Virginia, part of Tennessee, and a small portion of northern Alabama and northwest Georgia . The terms "Allegheny Plateau" and the "Cumberland Plateau" both refer to the dissected plateau lands lying, Tennessee The State of Tennessee is rooted in the Watauga Association, a 1772 frontier pact generally regarded as the first constitutional government west of the Appalachians. What is now Tennessee was initially part of North Carolina, and later part of the Southwest Territory. Tennessee was admitted to the Union as the 16th state on June 1, 1796. In the

The process in the rock cycle The rock cycle is a fundamental concept in geology that describes the dynamic transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous. As the diagram to the right illustrates, each type of rock is altered or destroyed when it is forced out of its equilibrium conditions. An igneous rock such as which forms shale is compaction Compaction refers to the process by which a sediment progressively loses its porosity due to the effects of loading. This forms part of the process of lithification. When a layer of sediment is originally deposited, it contains an open framework of particles with the pore space being usually filled with water. As more sediment is deposited above. The fine particles that compose shale can remain suspended in water long after the larger and denser particles of sand have deposited. Shales are typically deposited in very slow moving water and are often found in lakes and lagoonal A lagoon is a body of shallow sea water or brackish water separated from the sea by some form of barrier deposits, in river deltas A delta is a landform that is created at the mouth of a river where that river flows into an ocean, sea, estuary, lake, reservoir, flat arid area, or another river. Deltas are formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river. Over long periods of time, this deposition builds the, on floodplains A floodplain, or flood plain, is flat or nearly flat land adjacent to a stream or river that experiences occasional or periodic flooding. It includes the floodway, which consists of the stream channel and adjacent areas that carry flood flows, and the flood fringe, which are areas covered by the flood, but which do not experience a strong current and offshore from beach sands. They can also be deposited on the continental shelf The continental shelf is the extended perimeter of each continent and associated coastal plain, and was part of the continent during the glacial periods, but is undersea during interglacial periods such as the current epoch by relatively shallow seas and gulfs, in relatively deep, quiet water.

'Black shales' are dark, as a result of being especially rich in unoxidized Redox describes all chemical reactions in which atoms have their oxidation number (oxidation state) changed. This can be either a simple redox process, such as the oxidation of carbon to yield carbon dioxide (CO2) or the reduction of carbon by hydrogen to yield methane (CH4), or a complex process such as the oxidation of sugar(C6H12O6) in the carbon Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds. There are three naturally occurring isotopes, with 12C and 13C being stable, while 14C is radioactive, decaying with a half-life of. Common in some Paleozoic and Mesozoic strata, black shales were deposited in anoxic, reducing environments, such as in stagnant water columns. Some black shales contain abundant heavy metals such as molybdenum, uranium, vanadium, and zinc [6][7][8]. The enriched values are of controversial origin, having been alternatively attributed to input from hydrothermal fluids during or after sedimentation or to slow accumulation from sea water over long periods of sedimentation.[7][9][10]

Fossils, animal tracks/burrows and even raindrop impact craters are sometimes preserved on shale bedding surfaces. Shales may also contain concretions consisting of pyrite, apatite, or various carbonate minerals.

Shales that are subject to heat and pressure of metamorphism alter into a hard, fissile, metamorphic rock known as slate. With continued increase in metamorphic grade the sequence is phyllite, then schist and finally to gneiss.

Weathering shale at a road cut in southeastern Kentucky

When shale is hit against other rock it can emit sparks of various colors like blue, green, purple,yellow, and white. dependent on the type of rock it is hit against.

See also

References

  1. ^ a b c d e f g h Blatt, Harvey and Robert J. Tracy, 1996, Petrology: Igneous, Sedimentary and Metamorphic, 2nd ed., Freeman, pp. 281 - 292 ISBN 0-7167-2438-3
  2. ^ R. W. Raymond, Slate, A Glossary of Mining and Metallurigical Terms, American Institute of Mining Engineers, 1881; page 78.
  3. ^ Albert H. Fay, Slate, A Glossary of the Mining and Mineral Industry, United States Bureau of Mines, 1920; page 622.
  4. ^ "Rocks: Materials of the Lithosphere - Summary". http://wps.prenhall.com/esm_lutgens_foundations_3/0,6540,354318-,00.html. Retrieved 2007-07-31.
  5. ^ Herbert, Bucksch (1996). Dictionary geotechnical engineering: English German. Springer. p. 61. ISBN 978-3540581642. http://books.google.no/books?id=te9FMHjOR4oC&pg=PA61&lpg=PA61&dq=%22black+metal%22+%22black+gold%22+%22black+silver+ore%22&source=bl&ots=j2tbrjIj5y&sig=3Ki5iaISVKZf4ImMzwYfYW7LpSE&hl=nn&ei=XidQTI6sNN2XOIz6ib4B&sa=X&oi=book_result&ct=result&resnum=1&ved=0CBQQ6AEwAA#v=onepage&q=%22black%20metal%22&f=false. Retrieved July 28, 2010.
  6. ^ R. Zangerl and E.S. Richardson,1963, The paleoecologic history of two Pennsylvanian shales, Fieldiana Memoirs v 4, Field Museum of Natural History, Chicago, 352 p.
  7. ^ a b J.D. Vine and E.B. Tourtelot, 1970, Geochemistry of black shale deposits - A summary report: Economic Geology 65 pp. 253-273
  8. ^ R.M. Coveney, 1979, Zinc concentrations in mid-continent Pennsylvanian black shales of Missouri and Kansas, Economic Geology 74 pp. 131-140.
  9. ^ R.M. Coveney, 2003, Metalliferous Paleozoic black shales and associated strata: in D.R. Lenz ed., Geochemistry of Sediments and Sedimentary Rocks, Geotext 4, Geological Association of Canada pp. 135-144
  10. ^ H.D. Holland, 1979, Metals in black shales - A reassessment, Economic Geology 70 pp. 1676-1680

Categories: Sedimentary rocks

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