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98-613: Bauxite ( / ˈ b ɔː k s aɪ t / ) is a sedimentary rock with a relatively high aluminium content. It is the world's main source of aluminium and gallium . Bauxite consists mostly of the aluminium minerals gibbsite ( Al(OH) 3 ), boehmite (γ-AlO(OH)) and diaspore (α-AlO(OH)), mixed with the two iron oxides goethite (FeO(OH)) and haematite ( Fe 2 O 3 ), the aluminium clay mineral kaolinite ( Al 2 Si 2 O 5 (OH) 4 ) and small amounts of anatase ( TiO 2 ) and ilmenite ( FeTiO 3 or FeO·TiO 2 ). Bauxite appears dull in luster and

196-457: A molten aluminium salt could be used instead, aluminium oxide has a melting point of 2072 °C (3762°F) so electrolysing it is impractical. In the Hall–Héroult process, alumina, Al 2 O 3 , is dissolved in molten synthetic cryolite , Na 3 AlF 6 , to lower its melting point for easier electrolysis. The carbon source is generally a coke (fossil fuel) . In the Hall–Héroult process

294-400: A (usually small) angle. Sometimes multiple sets of layers with different orientations exist in the same rock, a structure called cross-bedding . Cross-bedding is characteristic of deposition by a flowing medium (wind or water). The opposite of cross-bedding is parallel lamination, where all sedimentary layering is parallel. Differences in laminations are generally caused by cyclic changes in

392-424: A diagenetic structure common in carbonate rocks is a stylolite . Stylolites are irregular planes where material was dissolved into the pore fluids in the rock. This can result in the precipitation of a certain chemical species producing colouring and staining of the rock, or the formation of concretions . Concretions are roughly concentric bodies with a different composition from the host rock. Their formation can be

490-463: A particular sedimentary environment. Examples of bed forms include dunes and ripple marks . Sole markings, such as tool marks and flute casts, are grooves eroded on a surface that are preserved by renewed sedimentation. These are often elongated structures and can be used to establish the direction of the flow during deposition. Ripple marks also form in flowing water. There can be symmetric or asymmetric. Asymmetric ripples form in environments where

588-459: A petition against the mining of bauxite that was signed by 135 accomplished and well known "Intellectuals". This petition helped unite the scattered anti-bauxite movement into a unified opposition against the state. These acts of governmental defiance were met with repressive state actions. Many domestic online reporters were arrested, and legislative action was taken to repress scientific research. Most of India's bauxite ore reserves, which are among

686-465: A red colour does not necessarily mean the rock formed in a continental environment or arid climate. The presence of organic material can colour a rock black or grey. Organic material is formed from dead organisms, mostly plants. Normally, such material eventually decays by oxidation or bacterial activity. Under anoxic circumstances, however, organic material cannot decay and leaves a dark sediment, rich in organic material. This can, for example, occur at

784-484: A rock is usually expressed with the Wentworth scale, though alternative scales are sometimes used. The grain size can be expressed as a diameter or a volume, and is always an average value, since a rock is composed of clasts with different sizes. The statistical distribution of grain sizes is different for different rock types and is described in a property called the sorting of the rock. When all clasts are more or less of

882-465: A sediment after its initial deposition. This includes compaction and lithification of the sediments. Early stages of diagenesis, described as eogenesis , take place at shallow depths (a few tens of meters) and is characterized by bioturbation and mineralogical changes in the sediments, with only slight compaction. The red hematite that gives red bed sandstones their color is likely formed during eogenesis. Some biochemical processes, like

980-431: A sedimentary rock may have been present in the original sediments or may formed by precipitation during diagenesis. In the second case, a mineral precipitate may have grown over an older generation of cement. A complex diagenetic history can be established by optical mineralogy , using a petrographic microscope . Carbonate rocks predominantly consist of carbonate minerals such as calcite, aragonite or dolomite . Both

1078-516: A small-scale property of a rock, but determines many of its large-scale properties, such as the density , porosity or permeability . The 3D orientation of the clasts is called the fabric of the rock. The size and form of clasts can be used to determine the velocity and direction of current in the sedimentary environment that moved the clasts from their origin; fine, calcareous mud only settles in quiet water while gravel and larger clasts are moved only by rapidly moving water. The grain size of

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1176-575: A smaller percentage of pitch, as they need to be more solid than Söderberg anodes. The remains of prebaked anodes are used to make more new prebaked anodes. Prebaked anodes are either made in the same factory where electrolysis happens, or are brought there from elsewhere. The inside of the cell's bath is lined with cathode made from coke and pitch. Cathodes also degrade during electrolysis, but much more slowly than anodes do, and thus they need neither be as high in purity, nor be maintained as often. Cathodes are typically replaced every 2–6 years. This requires

1274-538: A structure called bedding . Sedimentary rocks are often deposited in large structures called sedimentary basins . Sedimentary rocks have also been found on Mars . The study of sedimentary rocks and rock strata provides information about the subsurface that is useful for civil engineering , for example in the construction of roads , houses , tunnels , canals or other structures. Sedimentary rocks are also important sources of natural resources including coal , fossil fuels , drinking water and ores . The study of

1372-458: A temperature in excess of 1,000 °C (1,830 °F). This aluminium oxide is dissolved at a temperature of about 960 °C (1,760 °F) in molten cryolite . Next, this molten substance can yield metallic aluminium by passing an electric current through it in the process of electrolysis, which is called the Hall–Héroult process , named after its American and French discoverers. Prior to

1470-459: A temperature of 150 to 200 °C (300 to 390 °F). At these temperatures, the aluminium is dissolved as sodium aluminate (the Bayer process ). The aluminium compounds in the bauxite may be present as gibbsite (Al(OH) 3 ), boehmite (AlOOH) or diaspore (AlOOH); the different forms of the aluminium component will dictate the extraction conditions. The undissolved waste, bauxite tailings , after

1568-952: A term for a fissile mudrock (regardless of grain size) although some older literature uses the term "shale" as a synonym for mudrock. Biochemical sedimentary rocks are created when organisms use materials dissolved in air or water to build their tissue. Examples include: Chemical sedimentary rock forms when mineral constituents in solution become supersaturated and inorganically precipitate . Common chemical sedimentary rocks include oolitic limestone and rocks composed of evaporite minerals, such as halite (rock salt), sylvite , baryte and gypsum . This fourth miscellaneous category includes volcanic tuff and volcanic breccias formed by deposition and later cementation of lava fragments erupted by volcanoes, and impact breccias formed after impact events . Alternatively, sedimentary rocks can be subdivided into compositional groups based on their mineralogy: Sedimentary rocks are formed when sediment

1666-417: A texture, only the average size of the crystals and the fabric are necessary. Most sedimentary rocks contain either quartz ( siliciclastic rocks) or calcite ( carbonate rocks ). In contrast to igneous and metamorphic rocks, a sedimentary rock usually contains very few different major minerals. However, the origin of the minerals in a sedimentary rock is often more complex than in an igneous rock. Minerals in

1764-425: A valuable indicator of the biological and ecological environment that existed after the sediment was deposited. On the other hand, the burrowing activity of organisms can destroy other (primary) structures in the sediment, making a reconstruction more difficult. Secondary structures can also form by diagenesis or the formation of a soil ( pedogenesis ) when a sediment is exposed above the water level. An example of

1862-495: A variety of methods. The most recent is the use of ion-exchange resin . Achievable extraction efficiencies critically depend on the original concentration in the feed bauxite. At a typical feed concentration of 50 ppm, about 15 percent of the contained gallium is extractable. The remainder reports to the red mud and aluminium hydroxide streams. Bauxite is also a potential source for vanadium . The social and environmental impacts of bauxite extraction are well documented. Most of

1960-647: A youth march. In 2020, A Rocha Ghana also sued the government over the drilling in the reserve after they failed to provide a statement explaining their actions. In early 2009, the Vietnamese Government proposed a plan to mine remote regions of the central highlands. This proposal was highly controversial and sparked a nationwide debate and the most significant domestic conflict since the Vietnam War . Government scientists, journalists, religious leaders, retired high level state officials, and General Võ Nguyên Giáp ,

2058-503: Is deposited out of air, ice, wind, gravity, or water flows carrying the particles in suspension . This sediment is often formed when weathering and erosion break down a rock into loose material in a source area. The material is then transported from the source area to the deposition area. The type of sediment transported depends on the geology of the hinterland (the source area of the sediment). However, some sedimentary rocks, such as evaporites , are composed of material that form at

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2156-421: Is a structure where beds with a smaller grain size occur on top of beds with larger grains. This structure forms when fast flowing water stops flowing. Larger, heavier clasts in suspension settle first, then smaller clasts. Although graded bedding can form in many different environments, it is a characteristic of turbidity currents . The surface of a particular bed, called the bedform , can also be indicative of

2254-438: Is added so that the cryolite ratio is 2–3 to further reduce the melting point, so that the electrolysis can happen at temperatures between 940 and 980 °C (1700 to 1800°F). The density of liquid aluminum is 2.3 g/ml at temperatures between 950 and 1000 °C (1750° to 1830°F). The density of the electrolyte should be less than 2.1 g/ml, so that the molten aluminum separates from the electrolyte and settles properly to

2352-409: Is called bedding . Single beds can be a couple of centimetres to several meters thick. Finer, less pronounced layers are called laminae, and the structure a lamina forms in a rock is called lamination . Laminae are usually less than a few centimetres thick. Though bedding and lamination are often originally horizontal in nature, this is not always the case. In some environments, beds are deposited at

2450-429: Is higher when the sedimentation rate is high (so that a carcass is quickly buried), in anoxic environments (where little bacterial activity occurs) or when the organism had a particularly hard skeleton. Larger, well-preserved fossils are relatively rare. Fossils can be both the direct remains or imprints of organisms and their skeletons. Most commonly preserved are the harder parts of organisms such as bones, shells, and

2548-409: Is lowered vertically and individually by a computer, as the bottom surfaces of the anodes are eaten away during the electrolysis. Compared to Söderberg anodes, computer-controlled prebaked anodes can be brought closer to the molten aluminium layer at the bottom of the cell without any of them touching the layer and interfering with the electrolysis. This smaller distance decreases the resistance caused by

2646-491: Is mirrored by the broad categories of rudites , arenites , and lutites , respectively, in older literature. The subdivision of these three broad categories is based on differences in clast shape (conglomerates and breccias), composition (sandstones), or grain size or texture (mudrocks). Conglomerates are dominantly composed of rounded gravel, while breccias are composed of dominantly angular gravel. Sandstone classification schemes vary widely, but most geologists have adopted

2744-433: Is not treated in this way. Because Hall–Héroult processing consumes copious electrical energy and its electrolysis stage creates much carbon dioxide and fluorocarbon compounds as byproducts , it contributes to air pollution as well as climate change . Elemental aluminium cannot be produced by the electrolysis of an aqueous aluminium salt , because hydronium ions readily oxidize elemental aluminium. Although

2842-448: Is one of the only two upland evergreen forests in Ghana, and makes up a significant portion of the remaining 20% of forested habitat left in Ghana. The Atewa range falls under the jurisdiction of Akyem Abuakwa Traditional Area and is overseen by the king known as Okyenhene. In 2013, an NGO called A Rocha Ghana held a summit with the forestry and water resource commission, the minister of lands,

2940-500: Is reddish-brown, white, or tan. In 1821, the French geologist Pierre Berthier discovered bauxite near the village of Les Baux in Provence , southern France . Numerous classification schemes have been proposed for bauxite but, as of 1982, there was no consensus. Vadász (1951) distinguished lateritic bauxites (silicate bauxites) from karst bauxite ores (carbonate bauxites): In

3038-401: Is reduced. Sediments are typically saturated with groundwater or seawater when originally deposited, and as pore space is reduced, much of these connate fluids are expelled. 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

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3136-411: Is removed from the cell via a siphon every 1 to 3 days in order to avoid having to use extremely high temperature valves and pumps. Alumina is added to the cells as the aluminum is removed. Collected aluminium from different cells in a factory is finally melted together to ensure uniform product and made into metal sheets. The electrolytic mixture is sprinkled with coke to prevent the anode's oxidation by

3234-493: Is the major industrial process for smelting aluminium . It involves dissolving aluminium oxide (alumina) (obtained most often from bauxite , aluminium 's chief ore, through the Bayer process ) in molten cryolite and electrolyzing the molten salt bath, typically in a purpose-built cell. The Hall–Héroult process applied at industrial scale happens at 940–980 °C (1700 to 1800°F) and produces 99.5–99.8% pure aluminium . Recycling aluminum requires no electrolysis, thus it

3332-493: Is the most stable, followed by feldspar , micas , and finally other less stable minerals that are only present when little weathering has occurred. The amount of weathering depends mainly on the distance to the source area, the local climate and the time it took for the sediment to be transported to the point where it is deposited. In most sedimentary rocks, mica, feldspar and less stable minerals have been weathered to clay minerals like kaolinite , illite or smectite . Among

3430-450: Is used as an electrolyte because, among other things, it also dissolves alumina well, conducts electricity, dissociates electrolytically at higher voltage than alumina, and also has a lower density than aluminum at the temperatures required by the electrolysis. Aluminium fluoride (AlF 3 ) is usually added to the electrolyte. The ratio NaF/AlF 3 is called the cryolite ratio and it is 3 in pure cryolite. In industrial production, AlF 3

3528-428: Is usually vented into the atmosphere. Agitation of the molten material in the cell increases its production rate at the expense of an increase in cryolite impurities in the product. Properly designed cells can leverage magnetohydrodynamic forces induced by the electrolysing current to agitate the electrolyte. In non-agitating static pool cells, the impurities either rise to the top of the metallic aluminium, or sink to

3626-538: The Alcoa corporation. In 1997, the Hall–Héroult process was designated a National Historic Chemical Landmark by the American Chemical Society in recognition of the importance of the process in the commercialization of aluminum. Aluminium produced via the Hall–Héroult process, in combination with cheaper electric power , helped make aluminium (and incidentally magnesium ) an inexpensive commodity rather than

3724-526: The Boké Region of Guinea have caused in 35 conflicts which include movements of revolts and road blockades. These conflicts have resulted in the loss of human life, the destruction of heavy machinery, and damage to government buildings. The Atewa range in Ghana, classified as an ecologically important forest reserve with an area of 17,400 hectares (43,000 acres), has been is a recent site of conflict and controversy surrounding baxuite mining. The forest reserve

3822-575: The Boké Region of Guinea, there has been a significant increase in bauxite mining pressure on the local population. This has resulted in potable water issues, air pollution, food contamination, and land expropriation disputes due to improper compensation. Bauxite mining has led to protests, civil unrest, and violent conflicts in Guinea, Ghana, Vietnam, and India. Guinea has a long history of mining related conflicts between communities and mining companies. Between 2015 and 2018, new bauxite mining operations in

3920-838: The Earth sciences , such as pedology , geomorphology , geochemistry and structural geology . Sedimentary rocks can be subdivided into four groups based on the processes responsible for their formation: clastic sedimentary rocks, biochemical (biogenic) sedimentary rocks, chemical sedimentary rocks, and a fourth category for "other" sedimentary rocks formed by impacts, volcanism , and other minor processes. Clastic sedimentary rocks are composed of rock fragments ( clasts ) that have been cemented together. The clasts are commonly individual grains of quartz , feldspar , clay minerals , or mica . However, any type of mineral may be present. Clasts may also be lithic fragments composed of more than one mineral. Clastic sedimentary rocks are subdivided according to

4018-509: The Ghana Integrated Bauxite and Aluminium Development Authority Act that would create the legal framework required to develop and establish an integrated bauxite industry. In may of that year, the government began drilling deep holes in the reserve. These actions sparked several protests, including a 95-kilometre (59 mi) march from the reserve to the presidential palace, an informational billboard campaign led by A Rocha Ghana, and

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4116-538: The Rayagada district of Odisha . Sedimentary rock The sedimentary rock cover of the continents of the Earth's crust is extensive (73% of the Earth's current land surface), but sedimentary rock is estimated to be only 8% of the volume of the crust. Sedimentary rocks are only a thin veneer over a crust consisting mainly of igneous and metamorphic rocks . Sedimentary rocks are deposited in layers as strata , forming

4214-480: The free surface effect can cause the cargo to shift rapidly inside the hold and make the ship unstable, potentially sinking the ship. One vessel suspected to have been sunk in this way was the MS Bulk Jupiter in 2015. One method which can demonstrate this effect is the "can test", in which a sample of the material is placed in a cylindrical can and struck against a surface many times. If a moist slurry forms in

4312-682: The organic material of a dead organism undergoes chemical reactions in which volatiles such as water and carbon dioxide are expulsed. The fossil, in the end, consists of a thin layer of pure carbon or its mineralized form, graphite . This form of fossilisation is called carbonisation . It is particularly important for plant fossils. The same process is responsible for the formation of fossil fuels like lignite or coal. Structures in sedimentary rocks can be divided into primary structures (formed during deposition) and secondary structures (formed after deposition). Unlike textures, structures are always large-scale features that can easily be studied in

4410-488: The ozone layer , are still potent greenhouse gases . The anode effect is mainly a problem in Söderberg technology cells, not in prebaked. Aluminium is the most abundant metallic element in the Earth's crust, but it is rarely found in its elemental state . It occurs in many minerals, but its primary commercial source is bauxite , a mixture of hydrated aluminium oxides and compounds of other elements such as iron. Prior to

4508-497: The Dott scheme, which uses the relative abundance of quartz, feldspar, and lithic framework grains and the abundance of a muddy matrix between the larger grains. Six sandstone names are possible using the descriptors for grain composition (quartz-, feldspathic-, and lithic-) and the amount of matrix (wacke or arenite). For example, a quartz arenite would be composed of mostly (>90%) quartz grains and have little or no clayey matrix between

4606-637: The French crown jewels at the Exposition Universelle of 1855 , and Emperor Napoleon III of France was said to have reserved his few sets of aluminium dinner plates and eating utensils for his most honored guests. Production costs using older methods did come down, but when aluminium was selected as the material for the cap/lightning rod to sit atop the Washington Monument in Washington, D.C. , it

4704-422: The Hall–Héroult process, elemental aluminium was made by heating ore along with elemental sodium or potassium in a vacuum . The method was complicated and consumed materials that were in themselves expensive at that time. This meant that the cost to produce the small amount of aluminium made in the early 19th century was very high, higher than for gold or platinum . Bars of aluminium were exhibited alongside

4802-402: The Hall–Héroult process: Söderberg technology and prebaked technology. In cells using Söderberg or self-baking anodes, there is a single anode per electrolysis cell. The anode is contained within a frame and, as the bottom of the anode turns mainly into CO 2 during the electrolysis, the anode loses mass and, being amorphous , it slowly sinks within its frame. More material to the top of

4900-470: The activity of bacteria , can affect minerals in a rock and are therefore seen as part of diagenesis. Deeper burial is accompanied by mesogenesis , during which most of the compaction and lithification takes place. Compaction takes place as the sediments come under increasing overburden (lithostatic) pressure from overlying sediments. Sediment grains move into more compact arrangements, grains of ductile minerals (such as mica ) are deformed, and pore space

4998-410: The aluminium compounds are extracted contains iron oxides , silica , calcia , titania and some un-reacted alumina . After separation of the residue by filtering, pure gibbsite is precipitated when the liquid is cooled, and then seeded with fine-grained aluminium hydroxide . The gibbsite is usually converted into aluminium oxide , Al 2 O 3 , by heating in rotary kilns or fluid flash calciners to

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5096-411: The aluminium industry. More alumina is added to the electrolyte from the sides of the Söderberg anode after the crust on top of the electrolyte mixture is broken. Prebaked anodes are baked in very large gas-fired ovens at high temperature before being lowered by various heavy industrial lifting systems into the electrolytic solution. There are usually 24 prebaked anodes in two rows per cell. Each anode

5194-458: The anode is continuously added in the form of briquettes made from coke and pitch. The lost heat from the smelting operation is used to bake the briquettes into the carbon form required for the reaction with alumina. The baking process in Söderberg anodes during electrolysis releases more carcinogenic PAHs and other pollutants than electrolysis with prebaked anodes and, partially for this reason, prebaked anode-using cells have become more common in

5292-399: The bottom of deep seas and lakes. There is little water mixing in such environments; as a result, oxygen from surface water is not brought down, and the deposited sediment is normally a fine dark clay. Dark rocks, rich in organic material, are therefore often shales. The size , form and orientation of clasts (the original pieces of rock) in a sediment is called its texture . The texture is

5390-400: The bottom of the electrolysis cell. In addition to AlF 3 , other additives like lithium fluoride may be added to alter different properties (melting point, density, conductivity etc.) of the electrolyte. The mixture is electrolysed by passing a low voltage (under 5 V) direct current at 100–300 kA through it. This causes liquid aluminium to be deposited at the cathode , while

5488-459: The bottom, leaving high-purity aluminium in the middle area. Electrodes in cells are mostly coke which has been purified at high temperatures. Pitch resin or tar is used as a binder. The materials most often used in anodes, coke and pitch resin, are mainly residues from the petroleum industry and need to be of high enough purity so no impurities end up into the molten aluminum or the electrolyte. There are two primary anode technologies using

5586-423: The can, then there is a likelihood for the cargo to liquefy; although conversely, even if the sample remains dry it does not conclusively prove that it will remain that way, or that it is safe for loading. Bauxite is the main source of the rare metal gallium . During the processing of bauxite to alumina in the Bayer process , gallium accumulates in the sodium hydroxide liquor. From this it can be extracted by

5684-426: The carbon anode increases the electrical efficiency at a cost of consuming the carbon electrodes and producing carbon dioxide. While solid cryolite is denser than solid aluminium at room temperature, liquid aluminium is denser than molten cryolite at temperatures around 1,000 °C (1,830 °F). The aluminium sinks to the bottom of the electrolytic cell, where it is periodically collected. The liquid aluminium

5782-629: The case of Jamaica , recent analysis of the soils showed elevated levels of cadmium , suggesting that the bauxite originates from Miocene volcanic ash deposits from episodes of significant volcanism in Central America. Australia is the largest producer of bauxite, followed by Guinea and China . Bauxite is usually strip mined because it is almost always found near the surface of the terrain , with little or no overburden . Increased aluminium recycling , which requires less electric power than producing aluminium from ores, may considerably extend

5880-499: The cement and the clasts (including fossils and ooids ) of a carbonate sedimentary rock usually consist of carbonate minerals. The mineralogy of a clastic rock is determined by the material supplied by the source area, the manner of its transport to the place of deposition and the stability of that particular mineral. The resistance of rock-forming minerals to weathering is expressed by the Goldich dissolution series . In this series, quartz

5978-421: The cement to produce secondary porosity . At sufficiently high temperature and pressure, the realm of diagenesis makes way for metamorphism , the process that forms metamorphic rock . The color of a sedimentary rock is often mostly determined by iron , an element with two major oxides: iron(II) oxide and iron(III) oxide . Iron(II) oxide (FeO) only forms under low oxygen ( anoxic ) circumstances and gives

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6076-572: The current is in one direction, such as rivers. The longer flank of such ripples is on the upstream side of the current. Symmetric wave ripples occur in environments where currents reverse directions, such as tidal flats. Mudcracks are a bed form caused by the dehydration of sediment that occasionally comes above the water surface. Such structures are commonly found at tidal flats or point bars along rivers. Secondary sedimentary structures are those which formed after deposition. Such structures form by chemical, physical and biological processes within

6174-656: The dominant particle size. Most geologists use the Udden-Wentworth grain size scale and divide unconsolidated sediment into three fractions: gravel (>2 mm diameter), sand (1/16 to 2 mm diameter), and mud (<1/16 mm diameter). Mud is further divided into silt (1/16 to 1/256 mm diameter) and clay (<1/256 mm diameter). The classification of clastic sedimentary rocks parallels this scheme; conglomerates and breccias are made mostly of gravel, sandstones are made mostly of sand , and mudrocks are made mostly of mud. This tripartite subdivision

6272-440: The electrolyte mixture and increases the efficiency of prebaked anodes over Söderberg anodes. Prebake technology also has much lower risk of the anode effect (see below), but cells using it are more expensive to build and labor-intensive to use, as each prebaked anode in a cell needs to be removed and replaced once it has been used. Alumina is added to the electrolyte from between the anodes in prebake cells. Prebaked anodes contain

6370-489: The field. Sedimentary structures can indicate something about the sedimentary environment or can serve to tell which side originally faced up where tectonics have tilted or overturned sedimentary layers. Sedimentary rocks are laid down in layers called beds or strata . A bed is defined as a layer of rock that has a uniform lithology and texture. Beds form by the deposition of layers of sediment on top of each other. The sequence of beds that characterizes sedimentary rocks

6468-404: The flow calms and the particles settle out of suspension . Most authors presently use the term "mudrock" to refer to all rocks composed dominantly of mud. Mudrocks can be divided into siltstones, composed dominantly of silt-sized particles; mudstones with subequal mixture of silt- and clay-sized particles; and claystones, composed mostly of clay-sized particles. Most authors use " shale " as

6566-406: The following simplified reactions take place at the carbon electrodes: Cathode : Anode : Overall: In reality, much more CO 2 is formed at the anode than CO: Pure cryolite has a melting point of 1009 ± 1 °C (1848°F). With a small percentage of alumina dissolved in it, its melting point drops to about 1000 °C (1832°F). Besides having a relatively low melting point, cryolite

6664-508: The gas layer and causes it to expand, thus further reducing the surface area where electrolyte and anode are in contact with each other. The anode effect decreases the energy-efficiency and the aluminium production of the cell. It also induces the formation of tetrafluoromethane (CF 4 ) in significant quantities, increases formation of CO and, to a lesser extent, also causes the formation of hexafluoroethane (C 2 F 6 ). CF 4 and C 2 F 6 are not CFCs , and, although not detrimental to

6762-485: The grain. As a result, the contact points are dissolved away, allowing the grains to come into closer contact. The increased pressure and temperature stimulate further chemical reactions, such as the reactions by which organic material becomes lignite or coal. Lithification follows closely on compaction, as increased temperatures at depth hasten the precipitation of cement that binds the grains together. Pressure solution contributes to this process of cementation , as

6860-510: The grains, a lithic wacke would have abundant lithic grains and abundant muddy matrix, etc. Although the Dott classification scheme is widely used by sedimentologists, common names like greywacke , arkose , and quartz sandstone are still widely used by non-specialists and in popular literature. Mudrocks are sedimentary rocks composed of at least 50% silt- and clay-sized particles. These relatively fine-grained particles are commonly transported by turbulent flow in water or air, and deposited as

6958-587: The host rock. For example, a shell consisting of calcite can dissolve while a cement of silica then fills the cavity. In the same way, precipitating minerals can fill cavities formerly occupied by blood vessels , vascular tissue or other soft tissues. This preserves the form of the organism but changes the chemical composition, a process called permineralization . The most common minerals involved in permineralization are various forms of amorphous silica ( chalcedony , flint , chert ), carbonates (especially calcite), and pyrite . At high pressure and temperature,

7056-481: The invention of this process, and prior to the Deville process , aluminium ore was refined by heating ore along with elemental sodium or potassium in a vacuum . The method was complicated and consumed materials that were themselves expensive at that time. This made early elemental aluminium more expensive than gold . As a bulk cargo , bauxite is a Group A cargo that may liquefy if excessively moist. Liquefaction and

7154-435: The lithologies dehydrates. Clay can be easily compressed as a result of dehydration, while sand retains the same volume and becomes relatively less dense. On the other hand, when the pore fluid pressure in a sand layer surpasses a critical point, the sand can break through overlying clay layers and flow through, forming discordant bodies of sedimentary rock called sedimentary dykes . The same process can form mud volcanoes on

7252-461: The military leader of anti-colonial revolution, were among the many people across Vietnamese society who opposed the governments plans. In an attempt to stop the spread of information across the globe, the government banned domestic reporters from reporting on bauxite mining. However, reporters turned to Vietnamese language websites and blogs where the reporting and discussion continued. On April 12, 2009, several well-respected Vietnamese scholars started

7350-450: The mineral dissolved from strained contact points is redeposited in the unstrained pore spaces. This further reduces porosity and makes the rock more compact and competent . Unroofing of buried sedimentary rock 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 sedimentary rock, such as leaching of some of

7448-400: The minister of the environment, and other important stakeholders. They came to the conclusion that no future government should mine bauxite in the region because the reserve is environmentally and culturally significant. In 2016, the government along with NGO's began the process of upgrading the reserved to a national park. However, that year an election took place, and before it became official,

7546-482: The newly elected National Patriotic Party (NPP) rejected the plan. In 2017, the government of Ghana signed a Memorandum of Understanding with China to develop new bauxite mining infrastructure in Ghana. Although there was no official plan to mine the Atewa Forest Reserve, tensions between local communities, NGO and the government began to rise. In 2019, tensions began to reach a peak when the government presented

7644-467: The oxygen evolved. The cell produces gases at the anode. The exhaust is primarily CO 2 produced from the anode consumption and hydrogen fluoride (HF) from the cryolite and flux (AlF 3 ). In modern facilities, fluorides are almost completely recycled to the cells and therefore used again in the electrolysis. Escaped HF can be neutralized to its sodium salt, sodium fluoride . Particulates are captured using electrostatic or bag filters. The CO 2

7742-420: The oxygen from the alumina combines with carbon from the anode to produce mostly carbon dioxide. The theoretical minimum energy requirement for this process is 6.23 kWh/(kg of Al), but the process commonly requires 15.37 kWh. Cells in factories are operated 24 hours per day so that the molten material in them will not solidify. Temperature within the cell is maintained via electrical resistance. Oxidation of

7840-438: The place of deposition. The nature of a sedimentary rock, therefore, not only depends on the sediment supply, but also on the sedimentary depositional environment in which it formed. As sediments accumulate in a depositional environment, older sediments are buried by younger sediments, and they undergo diagenesis. Diagenesis includes all the chemical, physical, and biological changes, exclusive of surface weathering, undergone by

7938-480: The result of localized precipitation due to small differences in composition or porosity of the host rock, such as around fossils, inside burrows or around plant roots. In carbonate rocks such as limestone or chalk , chert or flint concretions are common, while terrestrial sandstones sometimes contain iron concretions. Calcite concretions in clay containing angular cavities or cracks are called septarian concretions . After deposition, physical processes can deform

8036-454: The rock a grey or greenish colour. Iron(III) oxide (Fe 2 O 3 ) in a richer oxygen environment is often found in the form of the mineral hematite and gives the rock a reddish to brownish colour. In arid continental climates rocks are in direct contact with the atmosphere, and oxidation is an important process, giving the rock a red or orange colour. Thick sequences of red sedimentary rocks formed in arid climates are called red beds . However,

8134-469: The same size, the rock is called 'well-sorted', and when there is a large spread in grain size, the rock is called 'poorly sorted'. The form of the clasts can reflect the origin of the rock. For example, coquina , a rock composed of clasts of broken shells, can only form in energetic water. The form of a clast can be described by using four parameters: Chemical sedimentary rocks have a non-clastic texture, consisting entirely of crystals. To describe such

8232-433: The sediment supply, caused, for example, by seasonal changes in rainfall, temperature or biochemical activity. Laminae that represent seasonal changes (similar to tree rings ) are called varves . Any sedimentary rock composed of millimeter or finer scale layers can be named with the general term laminite . When sedimentary rocks have no lamination at all, their structural character is called massive bedding. Graded bedding

8330-402: The sediment, producing a third class of secondary structures. Density contrasts between different sedimentary layers, such as between sand and clay, can result in flame structures or load casts , formed by inverted diapirism . While the clastic bed is still fluid, diapirism can cause a denser upper layer to sink into a lower layer. Sometimes, density contrasts occur or are enhanced when one of

8428-443: The sediment. They can be indicators of circumstances after deposition. Some can be used as way up criteria . Organic materials in a sediment can leave more traces than just fossils. Preserved tracks and burrows are examples of trace fossils (also called ichnofossils). Such traces are relatively rare. Most trace fossils are burrows of molluscs or arthropods . This burrowing is called bioturbation by sedimentologists. It can be

8526-406: The sequence of sedimentary rock strata is the main source for an understanding of the Earth's history , including palaeogeography , paleoclimatology and the history of life . The scientific discipline that studies the properties and origin of sedimentary rocks is called sedimentology . Sedimentology is part of both geology and physical geography and overlaps partly with other disciplines in

8624-703: The strip mining process, the biodiversity and habitat once present in the area is completely lost and the hydrological and soil characteristics in the region are permanently altered. Other environmental impacts of bauxite mining include soil degradation , air pollution , and water pollution . Red mud is a highly alkaline sludge, with a high pH around 13, that is a byproduct of the Bayer process . It contains several elements such as sodium aluminoscilicate , calcium titanate , monohydrate aluminium, and trihydrate aluminium that do not break down in nature. When improperly stored, red mud can contaminate soil and water, which can result in local extinction of all life. Red mud

8722-408: The surface where they broke through upper layers. Sedimentary dykes can also be formed in a cold climate where the soil is permanently frozen during a large part of the year. Frost weathering can form cracks in the soil that fill with rubble from above. Such structures can be used as climate indicators as well as way up structures. Hall%E2%80%93H%C3%A9roult process The Hall–Héroult process

8820-459: The terrestrial and aquatic ecosystems. Historically, the Indigenous peoples living on these lands have shown resistance to development, and oppose any new bauxite mining projects in the area. This has led to violent conflicts between Indigenous communities and police. On December 16, 2000, police killed three Indigenous protestors and wounded over a dozen more during a protest over a bauxite project in

8918-527: The three major types of rock, fossils are most commonly found in sedimentary rock. Unlike most igneous and metamorphic rocks, sedimentary rocks form at temperatures and pressures that do not destroy fossil remnants. Often these fossils may only be visible under magnification . Dead organisms in nature are usually quickly removed by scavengers , bacteria , rotting and erosion, but under exceptional circumstances, these natural processes are unable to take place, leading to fossilisation. The chance of fossilisation

9016-399: The top ten largest in the world, are located on tribal land. These tribal lands are densely populated and home to over 100 million Indigenous Indian peoples.  The mountain summits located on these lands act as a source of water and greatly contribute to the regions fertility. The Indian bauxite industry is interested in developing this land for aluminum production, which poses great risk to

9114-402: The whole cell to be shut down. The anode effect is a situation where too many gas bubbles form at the bottom of the anode and join, forming a layer. This increases the resistance of the cell, because smaller areas of the electrolyte touch the anode. These areas of the electrolyte and anode heat up when the density of the electric current of the cell focuses to go through only them. This heats up

9212-404: The woody tissue of plants. Soft tissue has a much smaller chance of being fossilized, and the preservation of soft tissue of animals older than 40 million years is very rare. Imprints of organisms made while they were still alive are called trace fossils , examples of which are burrows , footprints , etc. As a part of a sedimentary rock, fossils undergo the same diagenetic processes as does

9310-431: The world's bauxite deposits can be found within 1 to 20 metres (3 ft 3 in to 65 ft 7 in) of the earths surface. Strip mining is the most common technique used for extracting shallow bauxite. This process involves removing the vegetation, top soil, and overburden to expose the bauxite ore. The overlying soil is typically stockpiled in order to rehabilitate the mine once operations have finished. During

9408-430: The world's bauxite reserves. As of 2010, approximately 70% to 80% of the world's dry bauxite production is processed first into alumina and then into aluminium by electrolysis . Bauxite rocks are typically classified according to their intended commercial application: metallurgical, abrasive, cement, chemical, and refractory. Bauxite ore is usually heated in a pressure vessel along with a sodium hydroxide solution at

9506-561: Was responsible for killing all life in the Marcal River in Hungary after a spill occurred in 2010. When red mud dries, it turns into dust that can cause lung disease, cancer and birth defects. In the tropical regions of Asia, central Africa, South America and northern Australia, there has been an increase of bauxite mines on traditional and indigenous lands. This has resulted in a number of negative social impacts on local and indigenous peoples. In

9604-693: Was still more expensive than silver . The Hall–Héroult process was invented independently and almost simultaneously in 1886 by the American chemist Charles Martin Hall and by the Frenchman Paul Héroult —both 22 years old. Some authors claim Hall was assisted by his sister Julia Brainerd Hall ; however, the extent to which she was involved has been disputed. In 1888, Hall opened the first large-scale aluminium production plant in Pittsburgh . It later became

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