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Shale is a fine-grained,
clastic Clastic rocks are composed of fragments, or clasts, of pre-existing minerals In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specif ...
sedimentary rock Sedimentary rocks are types of rock Rock most often refers to: * Rock (geology) A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compoun ...

sedimentary rock
formed from
mud Mud is soil, loam, silt or clay mixed with water. It usually forms after rainfall or near water sources. Ancient mud deposits harden over geologic time scale, geological time to form sedimentary rock such as shale or mudstone (generally called l ...

mud
that is a mix of flakes of
clay mineral Clay minerals are , sometimes with variable amounts of , , s, s, and other s found on or near some s. Clay minerals form in the presence of water and have been important to life, and many theories of involve them. They are important constitu ...
s and tiny fragments (
silt Silt is granular material A granular material is a conglomeration of discrete solid, macroscopic scale, macroscopic particles characterized by a loss of energy whenever the particles interact (the most common example would be friction when gra ...
-sized particles) of other minerals, especially
quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tetrahedra, ...

quartz
and
calcite Calcite is a carbonate mineral Carbonate minerals are those mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid E ...

calcite
.Blatt, Harvey and Robert J. Tracy (1996) ''Petrology: Igneous, Sedimentary and Metamorphic'', 2nd ed., Freeman, pp. 281–292 Shale is characterized by its tendency to split into thin layers ( laminae) less than one centimeter in thickness. This property is called '' fissility''. Shale is the most common sedimentary rock. The term ''shale'' is sometimes applied more broadly, as essentially a synonym for
mudrock Mudrocks are a class of fine-grained Granularity (also called graininess), the condition of existing in granular material, granules or Grain, grains, refers to the extent to which a material or system is composed of distinction (philosophy), d ...
, rather than in the more narrow sense of clay-rich fissile mudrock.


Texture

Shale typically exhibits varying degrees of fissility. Because of the parallel orientation of
clay mineral Clay minerals are , sometimes with variable amounts of , , s, s, and other s found on or near some s. Clay minerals form in the presence of water and have been important to life, and many theories of involve them. They are important constitu ...
flakes in shale, it breaks into thin layers, often splintery and usually parallel to the otherwise indistinguishable bedding planes. Non-fissile
rocks In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed, and the proc ...
of similar composition and particle size (less than 0.0625 mm) are described as
mudstone Mudstone, a type of mudrock Mudrocks are a class of fine-grained Granularity (also called graininess), the condition of existing in granular material, granules or Grain, grains, refers to the extent to which a material or system is composed ...

mudstone
s (1/3 to 2/3 silt particles) or claystones (less than 1/3 silt). Rocks with similar particle sizes but with less clay (greater than 2/3 silt) and therefore grittier are
siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock of a clast (sand grain), derived from a basalt Basalt (, ) is a fine-grained extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnes ...
s.


Composition and color

Shales are typically gray in color and are composed of clay minerals and quartz grains. The addition of variable amounts of minor constituents alters the color of the rock. Red, brown and green colors are indicative of ferric oxide (
hematite Hematite (), also spelled as haematite, is a common iron oxide Iron oxides are chemical compounds composed of iron and oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member o ...

hematite
– reds), iron hydroxide (
goethite Goethite (, ) is a mineral of the diaspore group, consisting of iron(III) oxide-hydroxide, specifically the "α" Polymorphism (materials science), polymorph. It is found in soil and other low-temperature environments such as sediment. Goethite ha ...

goethite
– browns and
limonite Limonite () is an iron ore Iron ores are rocks A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compound, chemical composition and the w ...

limonite
– yellow), or micaceous minerals (
chlorite The chlorite ion An ion () is a particle In the Outline of physical science, physical sciences, a particle (or corpuscule in older texts) is a small wikt:local, localized physical body, object to which can be ascribed several physical ...
,
biotite Biotite is a common group of phyllosilicate Silicate minerals are rock-forming mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science conce ...

biotite
and
illite Illite is a group of closely related non-expanding minerals. Illite is a secondary mineral precipitate, and an example of a , or layered alumino-silicate. Its structure is a 2:1 sandwich of silica tetrahedron (T) – octahedron (O) – silica ...

illite
– greens). The color shifts from reddish to greenish as iron in the
oxidized (mild reducing agent) are added to powdered potassium permanganate (strong oxidizing agent), a violent redox reaction accompanied by self-ignition starts. Redox (reduction–oxidation, pronunciation: or ) is a type of chemical reaction A ...

oxidized
(
ferric In chemistry, iron(III) refers to the chemical element, element iron in its +3 oxidation number, oxidation state. In salt (chemistry), ionic compounds (salts), such an atom may occur as a separate cation (positive ion) denoted by Fe3+. The adject ...

ferric
) state is converted to iron in the reduced (
ferrous In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds composed of atoms, mo ...
) state. Black shale results from the presence of greater than one percent carbonaceous material and indicates a reducing environment. Pale blue to blue-green shales typically are rich in carbonate minerals. Clays are the major constituent of shales and other mudrocks. The clay minerals represented are largely
kaolinite Kaolinite ( ) is a clay mineral Clay minerals are , sometimes with variable amounts of , , s, s, and other s found on or near some s. Clay minerals form in the presence of water and have been important to life, and many theories of in ...

kaolinite
,
montmorillonite Montmorillonite is a very soft phyllosilicate group of minerals that form when they precipitate from water solution as microscopic crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ...
and illite. Clay minerals of Late
Tertiary Tertiary ( ) is a widely used but obsolete term for the Period (geology), geologic period from 66 million to 2.6 million years ago. The period began with the demise of the non-bird, avian dinosaurs in the Cretaceous–Paleogene extincti ...

Tertiary
mudstones are expandable
smectite Clay minerals are hydrous In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their ch ...
s, whereas in older rocks (especially in mid-to early
Paleozoic The Paleozoic (or Palaeozoic) Era ( ; from the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its popula ...
shales) illites predominate. The transformation of smectite to illite produces
silica Silicon dioxide, also known as silica, is an oxide An oxide () is a chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any su ...

silica
, sodium, calcium, magnesium, iron and water. These released elements form
authigenic Authigenesis is the process whereby a mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), ...
quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tetrahedra, ...

quartz
,
chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a preci ...

chert
,
calcite Calcite is a carbonate mineral Carbonate minerals are those mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid E ...

calcite
,
dolomiteDolomite may refer to: *Dolomite (mineral) Dolomite () is an anhydrous A substance is anhydrous if it contains no water. Many processes in chemistry can be impeded by the presence of water; therefore, it is important that water-free reagents and ...
,
ankerite Ankerite is a calcium Calcium is a chemical element with the Symbol (chemistry), symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical an ...
, hematite and
albite Albite is a . It is the sodium of the plagioclase series. It represents a plagioclase with less than 10% content. The pure albite endmember has the formula 38. It is a . Its color is usually pure white, hence its name from ''albus''. It is ...

albite
, all trace to minor (except quartz) minerals found in shales and other mudrocks. A typical shale is composed of about 58% clay minerals, 28% quartz, 6% feldspar, 5% carbonate minerals, and 2% iron oxides. Most of the quartz is detrital (part of the original sediments that formed the shale) rather than authigenic (crystallized within the shale after deposition). Shales and other 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−). Amorphous
iron sulfide Iron sulfide or Iron sulphide can refer to range of chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass ...
, along with carbon, produce the black coloration. Because amorphous iron sulfide gradually converts to
pyrite The mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. R ...

pyrite
, which is not an important pigment, young shales may be quite dark from their iron sulfide content, in spite of a modest carbon content (less than 1%), while a black color in an ancient shale indicates a high carbon content. Most shales are marine in origin, and the
groundwater Groundwater is the water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known form ...

groundwater
in shale formations is often highly saline. There is evidence that shale acts as a semipermeable medium, allowing water to pass through while retaining dissolved salts.


Formation

The fine particles that compose shale can remain suspended in water long after the larger particles of sand have been deposited. As a result, shales are typically deposited in very slow moving water and are often found in lakes and
lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' and ''atoll lagoons''. They ...

lagoon
al deposits, in
river delta A river delta is a landform A landform is a natural or artificial feature of the solid surface of the Earth or other planetary body A planet is an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the ...

river delta
s, on
floodplain A floodplain or flood plain or bottomlands is an area of land adjacent to a river A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows int ...
s and offshore below the wave base. Thick deposits of shale are found near ancient continental margins and foreland basins. Some of the most widespread shale formations were deposited by epicontinental seas. Black shales are common in
Cretaceous The Cretaceous ( ) is a geological period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These periods form elements of a hierarchy of division ...

Cretaceous
strata on the margins of the Atlantic Ocean, where they were deposited in
fault Fault commonly refers to: *Fault (geology), planar rock fractures showing evidence of relative movement *Fault (law), blameworthiness or responsibility Fault(s) may also refer to: Arts, entertainment, and media * "Fault", a song by Taproot from ...
-bounded silled basins associated with the opening of the Atlantic during the breakup of
Pangea Pangaea or Pangea () was a supercontinent In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology) ...

Pangea
. These basins were anoxic, in part because of restricted circulation in the narrow Atlantic, and in part because the very warm Cretaceous seas lacked the circulation of cold bottom water that oxygenates the deep oceans today. Most clay must be deposited as aggregates and floccules, since the settling rate of individual clay particles is extremely slow.
Flocculation Flocculation, in the field of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the ...

Flocculation
is very rapid once the clay encounters highly saline sea water. Whereas individual clay particles are less than 4 microns in size, the clumps of clay particles produced by flocculation vary in size from a few tens of microns to over 700 microns in diameter. The floccules start out water-rich, but much of the water is expelled from the floccules as the clay minerals bind more tightly together over time (a process called syneresis). Clay pelletization by organisms that filter feed is important where flocculation is inhibited. Filter feeders produce an estimated 12 metric tons of clay pellets per square kilometer per year along the U.S. Gulf Coast. As sediments continue to accumulate, the older, more deeply buried sediments begin to undergo
diagenesis upright=1.35, Permineralization in vertebra from ''Valgipes bucklandi'' Diagenesis () is the process that describes physical and chemical changes in sediments first caused by water-rock interactions, microbial activity, and compaction after the ...
. This mostly consists of
compaction Compaction may refer to: * Soil compaction In geotechnical engineering#REDIRECT geotechnical engineering {{Redirect category shell, 1= {{R from other capitalisation ..., soil compaction is the process in which stress applied to a soil causes ...
and
lithification Lithification (from the Ancient Greek word ''lithos'' meaning 'rock' and the Latin-derived suffix ''-ific'') is the process in which sediments compact under pressure, expel connate fluids, and gradually become solid rock. Essentially, lithificatio ...
of the clay and silt particles. Early stages of diagenesis, described as ''eogenesis'', take place at shallow depths (a few tens of meters) and are characterized by
bioturbation Bioturbation is defined as the reworking of soil Surface-water- gley developed in glacial till, Northern Ireland.">Northern_Ireland.html" ;"title="glacial till, Northern Ireland">glacial till, Northern Ireland. Soil is a mixture of organic m ...
and mineralogical changes in the sediments, with only slight compaction.
Pyrite The mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. R ...

Pyrite
may be formed in anoxic mud at this stage of diagenesis. Deeper burial is accompanied by ''mesogenesis'', during which most of the compaction and lithification takes place. As the sediments come under increasing pressure from overlying sediments, sediment grains move into more compact arrangements, ductile grains (such as
clay mineral Clay minerals are , sometimes with variable amounts of , , s, s, and other s found on or near some s. Clay minerals form in the presence of water and have been important to life, and many theories of involve them. They are important constitu ...
grains) are deformed, and pore space is reduced. In addition to this physical compaction, chemical compaction may take place via
pressure solution 350px, Schematic diagram of pressure solution accommodating compression/compaction in a clastic rock. Left box shows the situation before compaction. Red arrows indicate areas of maximum stress (= grain contacts). Blue arrows indicate the flow of d ...
. Points of contact between grains are under the greatest strain, and the strained mineral is more soluble than the rest of the grain. As a result, the contact points are dissolved away, allowing the grains to come into closer contact. It is during compaction that shale develops its fissility, likely through mechanical compaction of the original open framework of clay particles. The particles become strongly oriented into parallel layers that give the shale its distinctive fabric. Fissility likely develops early in the compaction process, at relatively shallow depth, since fissility does not seem to vary with depth in thick formations. Kaolinite flakes have less tendency to align in parallel layers than other clays, so kaolinite-rich clay is more likely to form nonfissile mudstone than shale. On the other hand, black shales often have very pronounced fissility (''paper shales'') due to binding of hydrocarbon molecules to the faces of the clay particles, which weakens the binding between particles. Lithification follows closely on compaction, as increased temperatures at depth hasten deposition of cement that binds the grains together. Pressure solution contributes to cementing, as the mineral dissolved from strained contact points is redeposited in the unstrained pore spaces. The clay minerals may be altered as well. For example,
smectite Clay minerals are hydrous In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their ch ...
is altered to
illite Illite is a group of closely related non-expanding minerals. Illite is a secondary mineral precipitate, and an example of a , or layered alumino-silicate. Its structure is a 2:1 sandwich of silica tetrahedron (T) – octahedron (O) – silica ...

illite
at temperatures of about , releasing water in the process. Other alteration reactions include the alteration of smectite to
chlorite The chlorite ion An ion () is a particle In the Outline of physical science, physical sciences, a particle (or corpuscule in older texts) is a small wikt:local, localized physical body, object to which can be ascribed several physical ...

chlorite
and of
kaolinite Kaolinite ( ) is a clay mineral Clay minerals are , sometimes with variable amounts of , , s, s, and other s found on or near some s. Clay minerals form in the presence of water and have been important to life, and many theories of in ...

kaolinite
to illite at temperatures between . Because of these reactions, illite composes 80% of
Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic The Phanerozoic Eon is the current geologic eon in the geologic time scale The geologi ...

Precambrian
shales, versus about 25% of young shales. Unroofing of buried shale is accompanied by ''telogenesis'', the third and final stage of diagenesis. As erosion reduces the depth of burial, renewed exposure to
meteoric waterMeteoric water is the water derived from Precipitation (meteorology), precipitation (snow and rain). This includes water from lakes, rivers, and icemelts, which all originate from precipitation indirectly. While the bulk of rainwater or meltwater fro ...
produces additional changes to the shale, such as dissolution of some of the cement to produce secondary porosity. Pyrite may be oxidized to produce
gypsum Gypsum is a soft sulfate mineral The sulfate minerals are a class of mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with ...

gypsum
. 'Black shales' are dark, as a result of being especially rich in
carbon Carbon (from la, carbo "coal") is a with the C and 6. It is lic and —making four s available to form s. It belongs to group 14 of the periodic table. Carbon makes up only about 0.025 percent of Earth's crust. Three occur naturally, ...

carbon
. Common in some Paleozoic and
Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles and the Age of Conifers, is the second-to-last era An era is a span of time defined for the purposes of chronology or historiography, as in the regnal eras in the history of a given monarchy ...
strata In geology and related fields, a stratum (plural: strata) is a layer of sedimentary rock or soil, or igneous rock that was formed at the Earth's surface, with internally consistent characteristics that distinguish it from other layers. The "str ...
, black shales were deposited in
anoxic The term anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts: * Anoxic waters, sea water, fresh water or groundwater that are depleted of disso ...
, reducing environments, such as in stagnant water columns. Some black shales contain abundant heavy metals such as
molybdenum Molybdenum is a with the Mo and 42. The name is from ''molybdaenum'', which is based on ', meaning , since its ores were confused with lead ores. Molybdenum minerals have been known throughout history, but the element was discovered (in the ...

molybdenum
,
uranium Uranium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

uranium
,
vanadium Vanadium is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science tha ...

vanadium
, and
zinc Zinc is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical element ...

zinc
. The enriched values are of controversial origin, having been alternatively attributed to input from
hydrothermal Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC ...
fluids during or after sedimentation or to slow accumulation from
sea water Seawater, or salt water, is water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all ...

sea water
over long periods of sedimentation. File:Shale in Potokgraben.jpg, Shale in Potokgraben, the
Karawanks The Karawanks or Karavankas or Karavanks ( sl, Karavanke; german: Karawanken, ) are a mountain range of the Southern Limestone Alps on the border between Slovenia Slovenia ( ; sl, Slovenija ), officially the Republic of Slovenia (Slovene: , ...
, Austria File:MesselShaleSplitting.JPG, Splitting shale ( Messel oil shale) with a large knife to reveal fossils File:Shale 8040.jpg,
Weathering Weathering is the deterioration of Rock (geology), rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little o ...
shale at a road cut in southeastern
Kentucky Kentucky ( , ), officially the Commonwealth of Kentucky, is a state State may refer to: Arts, entertainment, and media Literature * ''State Magazine'', a monthly magazine published by the U.S. Department of State * The State (newspaper), ...
Fossil A fossil (from Classical Latin Classical Latin is the form of Latin language Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication used by humans, inc ...

Fossil
s, animal
tracks Track or Tracks may refer to: Routes or imprints * Ancient trackway, any track or trail whose origin is lost in antiquity * Animal track, imprints left on surfaces that an animal walks across * Desire path, a line worn by people taking the shortes ...
or burrows and even raindrop impressions are sometimes preserved on shale bedding surfaces. Shales may also contain
concretion Marlstone aggregate concretion, Sault Ste. Marie, Michigan. A concretion is a hard, compact mass of matter formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil. Concretions ...

concretion
s consisting of pyrite, apatite, or various carbonate minerals. Shales that are subject to heat and pressure of
metamorphism Metamorphism is the change of mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs nat ...
alter into a hard, fissile,
metamorphic rock Metamorphic rocks arise from the transformation of existing rock Rock most often refers to: * Rock (geology) A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the mineral ...

metamorphic rock
known as
slate Slate is a fine-grained, foliated, homogeneous metamorphic rock , a type of metamorphic rock Metamorphic rocks arise from the transformation of existing rock (geology), rock to new types of rock, in a process called metamorphism uprigh ...

slate
. With continued increase in
metamorphic grade upright=1.35, Schematic representation of a metamorphic reaction. Abbreviations of minerals: act = actinolite; chl = Chlorite group">chlorite The chlorite ion, or chlorine dioxide anion An ion () is a particle, atom or molecule with a ...
the sequence is
phyllite Photomicrograph of thin section of phyllite (in cross Polarization (waves), polarised light) Phyllite is a type of Foliation (geology), foliated metamorphic rock created from slate that is further metamorphosed so that very fine grained white mic ...

phyllite
, then
schist Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fair ...

schist
and finally
gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. Gneiss is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous Igneous rock (derived from the Latin Latin (, or ...

gneiss
.


As hydrocarbon source rock

Shale is the most common source rock for hydrocarbons (
natural gas Natural gas (also called fossil gas; sometimes just gas) is a naturally occurring hydrocarbon gas mixture consisting of methane and commonly including varying amounts of other higher alkanes, and sometimes a small percentage of carbon dioxid ...

natural gas
and
petroleum Petroleum, also known as crude oil and oil, is a naturally occurring, yellowish-black liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isoc ...

petroleum
). The lack of coarse sediments in most shale beds reflects the absence of strong currents in the waters of the depositional basin. These might have oxygenated the waters and destroyed organic matter before it could accumulate. The absence of carbonate rock in shale beds reflects the absence of organisms that might have secreted carbonate skeletons, also likely due to an anoxic environment. As a result, about 95% of organic matter in sedimentary rocks is found in shales and other mudrocks. Individual shale beds typically have an organic matter content of about 1%, but the richest source rocks may contain as much as 40% organic matter. The organic matter in shale is converted over time from the original proteins, polysaccharides, lipids, and other organic molecules to
kerogen Kerogen is solid, insoluble organic matter Organic matter, organic material, or natural organic matter refers to the large source of Carbon compounds, carbon-based compounds found within natural and engineered, terrestrial, and aquatic environm ...
, which at the higher temperatures found at greater depths of burial is further converted to
graphite Graphite (), archaically referred to as plumbago, is a Crystallinity, crystalline form of the element carbon with its atoms arranged in a Hexagonal crystal system, hexagonal structure. It occurs naturally in this form and is the most stable for ...

graphite
and petroleum.


Historical mining terminology

Before the mid-19th century, the terms
slate Slate is a fine-grained, foliated, homogeneous metamorphic rock , a type of metamorphic rock Metamorphic rocks arise from the transformation of existing rock (geology), rock to new types of rock, in a process called metamorphism uprigh ...

slate
, shale and
schist Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fair ...

schist
were not sharply distinguished. In the context of underground
coal mining Coal mining is the process of extracting coal Coal is a combustible black or brownish-black sedimentary rock, formed as stratum, rock strata called coal seams. Coal is mostly carbon with variable amounts of other Chemical element, ele ...

coal mining
, shale was frequently referred to as slate well into the 20th century. Black shale associated with coal seams is called black metal.


See also

* * * * * * * * * *


References


External links

{{commonscatinline Industrial minerals