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58-465: 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 . It is the finest-grained foliated metamorphic rock. Foliation may not correspond to the original sedimentary layering, but instead is in planes perpendicular to the direction of metamorphic compression. The foliation in slate, called " slaty cleavage ",

116-458: A coal seam , and draw slate referred to shale that fell from the mine roof as the coal was removed. The British Geological Survey recommends that the term "slate" be used in scientific writings only when very little else is known about the rock that would allow a more definite classification. For example, if the characteristics of the rock show definitely that it was formed by metamorphosis of shale, it should be described in scientific writings as

174-414: A hard, fissile, metamorphic rock known as slate . With continued increase in metamorphic grade the sequence is phyllite , then schist and finally gneiss . Shale is the most common source rock for hydrocarbons ( natural gas and 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

232-400: A long-lasting weathertight roof with a lifespan of around 60–125 years. Some mainland European slate suppliers suggest that using hook fixing means that: The metal hooks are, however, visible and may be unsuitable for historic properties. Slate tiles are often used for interior and exterior flooring, stairs, walkways and wall cladding. Tiles are installed and set on mortar and grouted along

290-530: A metashale. If its origin is uncertain, but the rock is known to be rich in mica, it should be described as a pelite . Slate can be made into roofing slate, a type of roof tile which are installed by a slater . Slate has two lines of breakability—cleavage and grain—which make it possible to split the stone into thin sheets. When broken, slate retains a natural appearance while remaining relatively flat and easy to stack. A series of "slate booms" occurred in Europe from

348-408: A methodology allows eventual correlations in style, metamorphic grade, and intensity throughout a region, relationship to faults , shears , structures and mineral assemblages. In geotechnical engineering , a foliation plane may introduce anisotropy of stress, which is a vital consideration for geotechnical engineers. At some point, this foliation may form a discontinuity that may greatly influence

406-421: A rock in which the individual mineral crystals remain microscopic in size, producing a characteristic slaty cleavage in which fresh cleavage surfaces appear dull. This is in contrast to the silky cleaved surfaces of phyllite , which is the next-higher grade of metamorphic rock derived from mudstone. The direction of cleavage is independent of any sedimentary structures in the original mudrock, reflecting instead

464-411: A rock may not be directly perpendicular to the principal stress direction due to rotation, mass transport, and shortening. Foliation may be formed by realignment of micas and clays via physical rotation of the minerals within the rock. Often this foliation is associated with diagenetic metamorphism and low-grade burial metamorphism. Foliation may parallel original sedimentary bedding, but more often

522-569: A rudimentary damp-proof membrane . Small offcuts are used as shims to level floor joists. In areas where slate is plentiful it is also used in pieces of various sizes for building walls and hedges, sometimes combined with other kinds of stone. Because it is a good electrical insulator and fireproof, it was used to construct early-20th-century electric switchboards and relay controls for large electric motors . Because of its thermal stability and chemical inertness, slate has been used for laboratory bench tops and for billiard table tops. Slate

580-423: A variety of colors even from a single locality; for example, slate from North Wales can be found in many shades of grey, from pale to dark, and may also be purple, green, or cyan. Slate is not to be confused with shale, from which it may be formed, or schist . The word "slate" is also used for certain types of object made from slate rock. It may mean a single roofing tile made of slate, or a writing slate , which

638-1201: Is accompanied by telogenesis , the third and final stage of diagenesis. As erosion reduces the depth of burial, renewed exposure to meteoric water 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 . Black shales are dark, as a result of being especially rich in unoxidized carbon . 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 . 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. Fossils , animal tracks or burrows and even raindrop impressions 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

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696-492: Is caused by strong compression in which fine-grained clay forms flakes to regrow in planes perpendicular to the compression. When expertly "cut" by striking parallel to the foliation with a specialized tool in the quarry, many slates display a property called fissility , forming smooth, flat sheets of stone which have long been used for roofing , floor tiles, and other purposes. Slate is frequently grey in color, especially when seen en masse covering roofs. However, slate occurs in

754-420: 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 , rather than in the narrower sense of clay-rich fissile mudrock. Shale typically exhibits varying degrees of fissility. Because of

812-638: 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 (Fe ) and sulfur (S ). Amorphous iron sulfide , along with carbon, produce

870-430: Is evidence that shale acts as a semipermeable medium, allowing water to pass through while retaining dissolved salts. 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 lagoonal deposits, in river deltas , on floodplains and offshore below

928-455: 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

986-409: Is oriented at some angle to it. The growth of platy minerals, typically of the mica group, is usually a result of prograde metamorphic reactions during deformation. Often, retrograde metamorphism will not form a foliation because the unroofing of a metamorphic belt is not accompanied by significant compressive stress. Thermal metamorphism in the aureole of a granite is also unlikely to result in

1044-452: Is reduced. In addition to this physical compaction, chemical compaction may take place via pressure solution . 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

1102-711: Is used by building professionals as a result of its beauty and durability. Slate is incredibly durable and can last several hundred years, often with little or no maintenance. Natural slate is also fire resistant and energy efficient. Slate roof tiles are usually fixed (fastened) either with nails or with hooks (as is common with Spanish slate). In the UK, fixing is typically with double nails onto timber battens (England and Wales) or nailed directly onto timber sarking boards (Scotland and Northern Ireland). Nails were traditionally of copper, although there are modern alloy and stainless steel alternatives. Both these methods, if used properly, provide

1160-665: The Mid North at Mintaro and Spalding . Slate is abundant in Brazil , the world's second-largest producer of slate, around Papagaios in Minas Gerais , which extracts 95 percent of Brazil's slate. However, not all "slate" products from Brazil are entitled to bear the CE mark . Most slate in Europe today comes from Spain , the world's largest producer and exporter of natural slate, and 90 percent of Europe's natural slate used for roofing originates from

1218-399: The U.S. Gulf Coast . As sediments continue to accumulate, the older, more deeply buried sediments begin to undergo diagenesis . This mostly consists of compaction and lithification 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 and mineralogical changes in

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1276-651: 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 strata on the margins of the Atlantic Ocean , where they were deposited in fault -bounded silled basins associated with the opening of the Atlantic during the breakup of Pangaea . These basins were anoxic, in part because of restricted circulation in

1334-490: The 1870s until the First World War following improvements in railway, road and waterway transportation systems. Slate is particularly suitable as a roofing material as it has an extremely low water absorption index of less than 0.4%, making the material resistant to frost damage. Natural slate, which requires only minimal processing, has an embodied energy that compares favorably with other roofing materials. Natural slate

1392-454: The bands in gneiss (gneissic banding), a preferred orientation of planar large mica flakes in schist (schistosity), the preferred orientation of small mica flakes in phyllite (with its planes having a silky sheen, called phylitic luster – the Greek word, phyllon , also means "leaf"), the extremely fine grained preferred orientation of clay flakes in slate (called " slaty cleavage "), and

1450-421: The black coloration. Because amorphous iron sulfide gradually converts to 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 in shale formations is often highly saline . There

1508-645: The calls of all four species of wild turkey in North America: eastern, Rio Grande, Osceola and Merriam's. Slate is found in the Arctic and was used by Inuit to make the blades for ulus . China has vast slate deposits; in recent years its export of finished and unfinished slate has increased. Deposits of slate exist throughout Australia, with large reserves quarried in the Adelaide Hills in Willunga , Kanmantoo , and

1566-458: The cleavage direction, so caution is required in using reduction ellipsoids to estimate deformation. Before the mid-19th century, the terms "slate", " shale ", and " schist " were not sharply distinguished. In the context of underground coal mining in the United States, the term slate was commonly used to refer to shale well into the 20th century. For example, roof slate referred to shale above

1624-506: 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

1682-408: The color of the rock. Red, brown and green colors are indicative of ferric oxide ( hematite – reds), iron hydroxide ( goethite – browns and limonite – yellow), or micaceous minerals ( chlorite , biotite and illite – greens). The color shifts from reddish to greenish as iron in the oxidized ( ferric ) state is converted to iron in the reduced ( ferrous ) state. Black shale results from

1740-590: The direction of compaction, which begin to impart cleavage to the rock. Slaty cleavage is fully developed as the clay minerals begin to be converted to chlorite and mica. Organic carbon in the rock is converted to graphite . Slate is mainly composed of the minerals quartz, illite , and chlorite, which account for up to 95% of its composition. The most important accessory minerals are iron oxides (such as hematite and magnetite ), iron sulfides (such as pyrite ), and carbonate minerals. Feldspar may be present as albite or, less commonly, orthoclase . Occasionally, as in

1798-473: The direction of principal stress, records the direction of shortening. This is related to the axis of folds, which generally form an axial-planar foliation within their axial regions. Measurement of the intersection between a fold's axial plane and a surface on the fold will provide the fold plunge . If a foliation does not match the observed plunge of a fold, it is likely associated with a different deformation event. Foliation in areas of shearing, and within

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1856-476: The direction of regional compression. Slaty cleavage is continuous, meaning that the individual cleavage planes are too closely spaced to be discernible in hand samples. The texture of the slate is totally dominated by these pervasive cleavage planes. Under a microscope, the slate is found to consist of very thin lenses of quartz and feldspar (QF-domains) separated by layers of mica (M-domains). These are typically less than 100 μm (micron) thick. Because slate

1914-631: The edges. Chemical sealants are often used on tiles to improve durability and appearance, increase stain resistance, reduce efflorescence , and increase or reduce surface smoothness. Tiles are often sold gauged, meaning that the back surface is ground for ease of installation. Slate flooring can be slippery when used in external locations subject to rain. Slate tiles were used in 19th century UK building construction (apart from roofs) and in slate quarrying areas such as Blaenau Ffestiniog and Bethesda , Wales there are still many buildings wholly constructed of slate. Slates can also be set into walls to provide

1972-658: The growth of mica in a foliation, although the growth of new minerals may overprint existing foliation(s). Alignment of tabular minerals in metamorphic rocks , igneous rocks and intrusive rocks may form a foliation. Typical examples of metamorphic rocks include porphyroblastic schists where large, oblate minerals form an alignment either due to growth or rotation in the groundmass. Igneous rocks can become foliated by alignment of cumulate crystals during convection in large magma chambers , especially ultramafic intrusions, and typically plagioclase laths . Granite may form foliation due to frictional drag on viscous magma by

2030-607: The late 19th and early 20th centuries, where the slate is usually dark purple to blackish, and many local structures are roofed with slate tiles. The roof of St. Patrick's Cathedral in New York City and the headstone of John F. Kennedy 's gravesite in Arlington National Cemetery are both made of Monson slate. Foliation (geology) More technically, foliation is any penetrative planar fabric present in metamorphic rocks. Rocks exhibiting foliation include

2088-422: The layers of flattened, smeared, pancake-like clasts in metaconglomerate . Foliation is usually formed by the preferred orientation of minerals within a rock. Usually, this is the result of some physical force and its effect on the growth of minerals. The planar fabric of a foliation typically forms at right angles to the maximum principal stress direction. In sheared zones , however, planar fabric within

2146-468: The mechanical behavior (strength, deformation, etc.) of rock masses in, for example, tunnel , foundation , or slope construction. Shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g., kaolin , Al 2 Si 2 O 5 ( OH ) 4 ) and tiny fragments ( silt -sized particles) of other minerals, especially quartz and calcite . Shale

2204-592: 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 is altered to illite at temperatures of about 55 to 200 °C (130 to 390 °F), releasing water in the process. Other alteration reactions include the alteration of smectite to chlorite and of kaolinite to illite at temperatures between 120 and 150 °C (250 and 300 °F). Because of these reactions, illite composes 80% of Precambrian shales, versus about 25% of young shales. Unroofing of buried shale

2262-433: 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 is very rapid once the clay encounters highly saline sea water. Whereas individual clay particles are less than 4 microns in size,

2320-415: 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

2378-631: The parallel orientation of clay mineral flakes in shale, it breaks into thin layers, often splintery and usually parallel to the otherwise indistinguishable bedding planes . Non-fissile rocks of similar composition and particle size (less than 0.0625 mm) are described as mudstones (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 siltstones . Shales are typically gray in color and are composed of clay minerals and quartz grains. The addition of variable amounts of minor constituents alters

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2436-473: The plane of thrust faults , can provide information on the transport direction or sense of movement on the thrust or shear. Generally, the acute intersection angle shows the direction of transport. Foliations typically bend or curve into a shear, which provides the same information, if it is of a scale which can be observed. Foliations, in a regional sense, will tend to curve around rigid, incompressible bodies such as granite. Thus, they are not always 'planar' in

2494-817: 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 , montmorillonite and illite. Clay minerals of Late Tertiary mudstones are expandable smectites , whereas in older rocks (especially in mid-to early Paleozoic shales) illites predominate. The transformation of smectite to illite produces silica , sodium , calcium , magnesium , iron and water. These released elements form authigenic quartz , chert , calcite , dolomite , ankerite , hematite and albite , all trace to minor (except quartz) minerals found in shales and other mudrocks. A typical shale

2552-449: The purple slates of North Wales , ferrous ( iron(II) ) reduction spheres form around iron nuclei, leaving a light-green, spotted texture. These spheres are sometimes deformed by a subsequent applied stress field into ovoids, which appear as ellipses when viewed on a cleavage plane of the specimen. However, some evidence shows that reduced spots may also form after deformation and acquire an elliptical shape from preferential infiltration along

2610-429: The result of nucleation processes which cause chemical and mineralogical differentiation into bands. This typically follows the same principle as mica growth, perpendicular to the principal stress. Metamorphic differentiation can be present at angles to protolith compositional banding. Crenulation cleavage and oblique foliation are particular types of foliation. Foliation, as it forms generally perpendicular to

2668-444: 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 , which at the higher temperatures found at greater depths of burial is further converted to graphite and petroleum. Before the mid-19th century, the terms slate , shale and schist were not sharply distinguished. In

2726-433: The sediments, with only slight compaction. 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 grains) are deformed, and pore space

2784-768: The slate from Wales and Cumbria is colored slate (non-blue): purple and formerly green in Wales and green in Cumbria. In North America, slate is produced in Newfoundland , eastern Pennsylvania , Buckingham County, Virginia , and the Slate Valley region in Vermont and New York , where colored slate is mined in the Granville, New York , area. A major slating operation existed in Monson, Maine , during

2842-732: The slate industry there. Lesser slate-producing regions in present-day Europe include Wales (with UNESCO landscape status and a museum at Llanberis ), Cornwall (famously the village of Delabole ), Cumbria (see Burlington Slate Quarries , Honister Slate Mine and Skiddaw Slate ) and, formerly in the West Highlands of Scotland, around Ballachulish and the Slate Islands in the United Kingdom . Parts of France ( Anjou , Loire Valley , Ardennes , Brittany , Savoie ) and Belgium (Ardennes), Liguria in northern Italy , especially between

2900-447: The standard sequence formed by the prograde metamorphism of mudrocks ; slate , phyllite , schist and gneiss . The slatey cleavage typical of slate is due to the preferred orientation of microscopic phyllosilicate crystals . In gneiss, the foliation is more typically represented by compositional banding due to segregation of mineral phases. Foliated rock is also known as S-tectonite in sheared rock masses. Examples include

2958-408: The strictest sense and may violate the rule of being perpendicular to the regional stress field, due to local influences. This is a megascopic version of what may occur around porphyroblasts. Often, fine observation of foliations on outcrop, hand specimen and on the microscopic scale complements observations on a map or regional scale. When describing a foliation it is useful to note Following such

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3016-451: The town of Lavagna (whose name is inherited as the term for chalkboard in Italian ) and Fontanabuona valley; Portugal especially around Valongo in the north of the country. Germany 's Moselle River region, Hunsrück (with a former mine open as a museum at Fell ), Eifel , Westerwald , Thuringia and north Bavaria ; and Alta , Norway (actually schist , not a true slate). Some of

3074-460: The wall rocks. Lavas may preserve a flow foliation, or even compressed eutaxitic texture, typically in highly viscous felsic agglomerate , welded tuff and pyroclastic surge deposits. Metamorphic differentiation, typical of gneisses , is caused by chemical and compositional banding within the metamorphic rock mass. Usually, this represents the protolith chemistry, which forms distinct mineral assemblages. However, compositional banding can be

3132-414: 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

3190-426: Was formed in low heat and pressure, compared to most other metamorphic rocks, some fossils can be found in slate; sometimes even microscopic remains of delicate organisms can be found in slate. The process of conversion of mudrock to slate involves a loss of up to 50% of the volume of the mudrock as it is compacted. Grains of platy minerals, such as clay minerals , are rotated to form parallel layers perpendicular to

3248-498: Was traditionally a small, smooth piece of the rock, often framed in wood, used with chalk as a notepad or notice board, and especially for recording charges in pubs and inns. The phrases "clean slate" and " blank slate " come from this usage. Slate is a fine-grained, metamorphic rock that shows no obvious compositional layering but can easily be split into thin slabs and plates. It is usually formed by low-grade regional metamorphism of mudrock . This mild degree of metamorphism produces

3306-455: Was used by earlier cultures as whetstone to hone knives, but whetstones are nowadays more typically made of quartz. In 18th- and 19th-century schools, slate was extensively used for blackboards and individual writing slates , for which slate or chalk pencils were used. In modern homes slate is often used as table coasters. In areas where it is available, high-quality slate is used for tombstones and commemorative tablets. In some cases slate

3364-476: Was used by the ancient Maya civilization to fashion stelae . Slate was the traditional material of choice for black Go stones in Japan, alongside clamshell for white stones. It is now considered to be a luxury. Pennsylvania slate is widely used in the manufacture of turkey calls used for hunting turkeys. The tones produced from the slate, when scratched with various species of wood striker, imitates almost exactly

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