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The Kaolin deposits of the Charentes Basin in France are clay deposits formed sedimentarily and then confined by other geological structures.
The geological unit called Charentes basin is composed of Eocene and Oligocene deposits, laid above karstic limestone formations of the Campanian, in the north of the Aquitaine Basin. The Charentes basin is named after the departments of Charente and Charente-Maritime. The kaolin clays of Charentes belong to this mainly continental formation often referred as siderolithic, of which the principal outcrop is situated in the South of the Charente-Maritime department, 56 kilometres (35 mi) going north-east from Bordeaux city. The quarries are scattered along a 32 km (20 mi) long, 11 km (6.8 mi) wide, north - south band.
The clay concentrations of economic value are composed of a succession of clays, sands and pebbles. This torrential-stream deposit, close to enlaced rivers, laid to the deposition of sandy-clayey materials, with a variable iron content, coming from a lateritic weathering, of the French “Massif Central” granites. [1] [ citation needed ] [2] [ citation needed ] [3] [ citation needed ] The presence of numerous lignite rich levels indicates that the deposit was performed in the presence of abundant organic matter, leading to important pedogenetic and diagenetic possibilities of evolutions. These chemical and mineralogical evolutions (dissolution–crystallization) allow the neo-formation of kaolin and gibbsite, as well as iron sulfide. [4] [ citation needed ]
At their basement, highly enlaced and with channel shapes, those deposits often fill karstic depressions, leading to the formation of clay wells. The juxtaposition of features are sometimes without explanations using the deposition laws, probably in relation with post-sedimentary strain phenomena, eventually linked to substratum collapse. [5] [ citation needed ] In the upper part of the series, the deposits are more regular, with lateral extensions up to several hundreds of meters.
Those complex geometries, with structures smaller than 20 meters, lead to particularly difficult recognition, estimation and exploitation phases. To this complex geometry, one should notice important lithology variations. The AGS company uses no less than 24 description codes and 8 colour codes, for its samples description. Those classes are subdivided to take into account the grade in organic matter, iron, titanium, potassium, the colour, and the aptitude to flow.[ citation needed ]
The uncertainty in estimating the tonnage of mineral resources or ore reserves depends on a number of factors, and the uncertainty of definition of the deposit boundaries is one of them. [6] [ citation needed ] In deposits with sharp contacts, the geometry may be relatively simple, nevertheless, there is always uncertainty caused by lack of information and large drill hole grid. Generally, these boundaries are determined by mineral grade rather than geological properties: deposit boundaries are chosen based on the cut-off grade. Changing the important factor of cut-off grade, the boundaries of the deposit can be extended or contracted. For this reason, even for the deposits with sharp boundaries, a clear definition of the cut-off grade and distinction between ore and gangue due to dilution during mining, the presence of intermediate layer and the limitation of mining in a selective way are essential. However, in the case of the exploitation of soft materials, extraction can be done more selectively and it would be easier to take into account the geological and geometrical limits. On the other hand, sometimes the uncertainty on the estimation of grades is bigger than the uncertainty on the boundaries definition. Estimation is then performed inside predefined boundaries. One can imagine that the anisotropy and structural complexity of the deposit are due to its geometrical form, while the geometrical dimension of the deposit help us to guess about its economical value.[ citation needed ]
Geometrical features can appear in variographic studies and usually they affect, or hide, grade distribution structures. The presence of a series of nearly homogeneous kaolin areas, linked together in zones, creates a mosaic effect. This phenomenon is due to the existence of periodical settling regimes of the rivers. The size of these zones can affect the form of the variogram and increase the nugget effect due to high differences of values in the edge of the zones. A hole effect is one of the other known phenomena caused by the presence of two or more separated lenses with low difference in grade and shape. The distance between these lenses can thus be estimated.
Thiry [7] [ citation needed ] has mentioned that the actual geological setting of kaolin depositions cannot be explained with only transportation and sedimentation cycles. He also stated that the mineralogical sequences cannot be interpreted without local geochemical transformations. Kulbicki has proved the existence of vermicular minerals (kaolinite and dickite) incompatible with normal sedimentary sequences.
Lignite formations are relatively frequent in Charentes clay deposits. Their thickness changes between some decimeters in lenses, to metric scale in continuous forms. These organic materials had some influences on kaolin deposited layers. Some of the observed influences are as follow: In gathered samples close to these organic materials, clays generally do not contain mica minerals, and especially in the neighborhood of Cuisian lignite, kaolinite is very well-ordered and the clay does not contain swelling clays with hydrazine. Occurrence of gibbsite is always associated with these well-ordered kaolinites. Normally occurrence of hyper-aluminous clays due to the existence of gibbsite is one of the interesting subjects in the history of these kaolins. This causes many discussions about the origin of this mineral. The existence of gibbsite has been mentioned in the studies of Languine and Halm (1951), Caillere and Jourdain (1956), Kulbickie (1956), Dubreuilh et al. (1984) and Delineau (1994).[ citation needed ]
Generally, kaolin deposits have been covered with colored sequences of sand. In some quarries, we can observe red, green and some times black sands. The black color might be due to the existence of pyrite and organic materials. Sometimes fossil woods (floated branches and trunks of trees) can be found and with the coarse size of pebbles (several millimeters) are evidence of a high energy transportation. This type of sand can have some influences on the leaching by mineral and organic acids produced by pyrite and organic materials, of the lower kaolin deposits. Thiry has found that generally these kaolins contain rather well-ordered kaolinite. Obviously, the level of crystallization can control technical properties of kaolinite as well as the structural impurities. The high energy current can interrupt the continuity of the settled layers of kaolin and reduce the simplicity of the estimation methods.
Gibbsite is not stable in presence of quartz and it will be changed into kaolinite minerals, so gibbsite has formed after the deposition and we can call it neo-formation gibbsite. [8] [ citation needed ] Now, the main question is about gibbsite formation in the middle of kaolin series. Due to the pH of leaching, a dissolution of Al2O3 or SiO2 can occur (podzol or laterite profile) The first theory tries to describe this with podzol profiles: it assumes the leaching of silica from minerals and accordingly the gibbsite formation from leached kaolin. We thus should find the hyper-aluminous materials, containing gibbsite in the lower series of kaolin. On the other hand, a second theory proposes the procedure of aluminium leaching in a very acid medium, in deposited organic materials (lignite) with clay. The organic materials can accelerate the solubilization and transportation of aluminium ions with intervention of organic complex. [9] [ citation needed ] proposed the following scenarios for this dissolved aluminium.
Dissolved aluminium can be transported with complex to a less acidic medium.
This theory alone cannot explain what is observed in-situ in the some samples of the “BD” deposit, where gibbsite was found in sandy layers containing quartz.
Bauxite 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 (Fe2O3), the aluminium clay mineral kaolinite (Al2Si2O5(OH)4) and small amounts of anatase (TiO2) and ilmenite (FeTiO3 or FeO.TiO2). Bauxite appears dull in luster and is reddish-brown, white, or tan in color.
Kaolinite is a clay mineral, with the chemical composition Al2Si2O5(OH)4. It is an important industrial mineral. It is a layered silicate mineral, with one tetrahedral sheet of silica (SiO
4) linked through oxygen atoms to one octahedral sheet of alumina (AlO
6) octahedra. Rocks that are rich in kaolinite are known as kaolin or china clay.
Clay is a type of fine-grained natural soil material containing clay minerals. Clays develop plasticity when wet, due to a molecular film of water surrounding the clay particles, but become hard, brittle and non–plastic upon drying or firing. Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impurities, such as a reddish or brownish colour from small amounts of iron oxide.
Shale is a fine-grained, clastic sedimentary rock, formed from mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. 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.
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at the Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. The geological detritus originated from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.
Weathering is the breaking down of rocks, soils, and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs in situ, and should not be confused with erosion, which involves the transport of rocks and minerals by agents such as water, ice, snow, wind, waves and gravity.
Dickite is a phyllosilicate clay mineral named after the metallurgical chemist Allan Brugh Dick, who first described it. It is chemically composed of 20.90% aluminium, 21.76% silicon, 1.56% hydrogen and 55.78% oxygen. It has the same composition as kaolinite, nacrite, and halloysite, but with a different crystal structure (polymorph). Dickite sometimes contains impurities such as titanium, iron, magnesium, calcium, sodium and potassium.
Mudrocks are a class of fine grained siliciclastic sedimentary rocks. The varying types of mudrocks include: siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone is composed are less than 0.0625 mm and are too small to study readily in the field. At first sight the rock types look quite similar; however, there are important differences in composition and nomenclature. There has been a great deal of disagreement involving the classification of mudrocks. There are a few important hurdles to classification, including:
Zeolite facies describes the mineral assemblage resulting from the pressure and temperature conditions of low-grade metamorphism.
Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus, chunks and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic with reference to sedimentary rocks as well as to particles in sediment transport whether in suspension or as bed load, and in sediment deposits.
Metakaolin is the anhydrous calcined form of the clay mineral kaolinite. Minerals that are rich in kaolinite are known as china clay or kaolin, traditionally used in the manufacture of porcelain. The particle size of metakaolin is smaller than cement particles, but not as fine as silica fume.
Ball clays are kaolinitic sedimentary clays that commonly consist of 20–80% kaolinite, 10–25% mica, 6–65% quartz. Localized seams in the same deposit have variations in composition, including the quantity of the major minerals, accessory minerals and carbonaceous materials such as lignite. They are fine-grained and plastic in nature, and, unlike most earthenware clays, produce a fine quality white-coloured pottery body when fired, which is the key to their popularity with potters.
Uranium ore deposits are economically recoverable concentrations of uranium within the Earth's crust. Uranium is one of the more common elements in the Earth's crust, being 40 times more common than silver and 500 times more common than gold. It can be found almost everywhere in rock, soil, rivers, and oceans. The challenge for commercial uranium extraction is to find those areas where the concentrations are adequate to form an economically viable deposit. The primary use for uranium obtained from mining is in fuel for nuclear reactors.
Channel iron deposits (CID) are iron-rich fluvial sedimentary deposits of possible Miocene age occupying meandering palaeochannels in the Early to Mid-Cenozoic Hamerlsey palaeosurface of Western Australia. Examples are also known from Kazakhstan.
Mineral alteration refers to the various natural processes that alter a mineral's chemical composition or crystallography.
The geology of Eswatini formed beginning 3.6 billion years ago, in the Archean Eon of the Precambrian. Eswatini is the only country entirely underlain by the Kaapvaal Craton, one of the oldest pieces of stable continental crust and the only craton regarded as "pristine" by geologists, other than the Yilgarn Craton in Australia. As such, the country has very ancient granite, gneiss and in some cases sedimentary rocks from the Archean into the Proterozoic, overlain by sedimentary rocks and igneous rocks formed during the last 541 million years of the Phanerozoic as part of the Karoo Supergroup. Intensive weathering has created thick zones of saprolite and heavily weathered soils.
An ephemeral acid saline lake is a lake that is relatively high in dissolved salts and has a low pH, usually within the range of <1 - 5 and does not have standing water year round. These types of lakes are identified by high concentrations of evaporite minerals, notably halite, gypsum, and various iron oxides allowing the lakes to become hypersaline. Low pH and evaporite minerals are positively correlated, allowing lakes with lower pH's to have visible evaporite mineral "crusts". Due to the highly unusual geochemistries present in these lake systems, they are considered an extreme environment in nature.
The Ciechocinek Formation, formerly known in Germany as the Green Series is a Jurassic geologic formation that extends across the Baltic coast, from Grimmen, Germany, to Nida, Lithuania, with its major sequence on Poland and a few boreholes on Kaliningrad. Dinosaur remains are among the fossils that have been recovered from the formation, not yet been allocated to a specific genus. The layers assigned to the formation are a group of sediments that differ from the those of the Posidonia Shale and other Toarcian formations of Europe. Most of the sediments of the Polish realm come from deltaic, fluvial and marine deposits. In the Polish realm there is also an abundance of siderite, attributed to mixed saline and marine action, with the rounding off of some due to transport by freshwater or sea currents. Its main equivalents are the Posidonia Shale, upper part of the Rydeback Member, Rya Formation, the Fjerritslev Formation, the Bagå Formation (Bornholm) or the Lava Formation (Lithuania). There are also coeval abandoned informal units in Poland: Gryfice Beds, Lower Łysiec beds, or the "Estheria series".
The soil matrix is the solid phase of soils, and comprise the solid particles that make up soils. Soil particles can be classified by their chemical composition (mineralogy) as well as their size. The particle size distribution of a soil, its texture, determines many of the properties of that soil, in particular hydraulic conductivity and water potential, but the mineralogy of those particles can strongly modify those properties. The mineralogy of the finest soil particles, clay, is especially important.