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Clay minerals are hydrous
aluminium Aluminium (aluminum in American English, American and Canadian English) is a chemical element with the Symbol (chemistry), symbol Al and atomic number 13. Aluminium has a density lower than those of other common Metal, metals, at ap ...
phyllosilicates Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. In mineralogy, silica (silicon dioxide) SiO2 is usually cons ...
, sometimes with variable amounts of
iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is Abundance ...

iron
,
magnesium Magnesium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...
,
alkali metal The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names fo ...
s, alkaline earths, and other
cation An ion () is a particle, atom or molecule with a net electric charge, electrical charge. The charge of the electron is considered negative by convention. The negative charge of an ion is equal and opposite to charged proton(s) considered po ...
s found on or near some
planetary surface OSIRIS-REx collecting a surface sample from asteroid 101955 Bennu in 2020— ''(:File:OSIRIS-REX SamCam TAGSAM Event 2020-10-20.gif, Full-sized image)'' A planetary surface is where the solid (or liquid) material of the outer crust (geology), cru ...
s. Clay minerals form in the presence of water and have been important to life, and many theories of
abiogenesis In evolutionary biology Evolutionary biology is the subfield of biology that studies the evolution, evolutionary processes (natural selection, common descent, speciation) that produced the Biodiversity, diversity of life on Earth. In th ...
involve them. They are important constituents of soils, and have been useful to humans since ancient times in
agriculture Agriculture is the science, art and practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentism, sedentary human civilization, whereby farming of domestication, domesticated species created food ...
and
manufacturing Manufacturing is the Production (economics), production of goods through the use of Work (human activity), labor, machines, tools, and chemical or biological processing or formulation. It is the essence of secondary sector of the economy. The term ...
.


Properties

Clay Clay is a type of fine-grained natural soil material containing clay minerals. Clays develop plasticity (physics), plasticity when wet, due to a molecular film of water surrounding the clay particles, but become hard, brittle and non–plastic ...

Clay
s form flat hexagonal sheets similar to the
mica Mica is a group of sheet silicate minerals, not to be confused with Micah. Mica or MICA may also refer to: Companies * Mica DIY, a UK Retailer Co-operative, Symbol group Acronyms * Mahone Islands Conservation Association * Ministry of I ...
s. Clay
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), rocks of which it is composed, and the proces ...

mineral
s are common
weathering Weathering is the breaking down of rocks, soil Soil (often stylized as SOiL) is an American rock band that was formed in Chicago (''City in a Garden''); I Will , image_map = , map_caption = Interactive maps of ...
products (including weathering of
feldspar Feldspars () are a group of rock-forming tectosilicate minerals that make up about 41% of the Earth's continental crust by weight. Feldspars crystallize from magma as both Intrusive rock, intrusive and extrusive igneous rocks and are also pres ...
) and low-temperature hydrothermal alteration products. Clay minerals are very common in soils, in fine-grained
sedimentary rock Sedimentary rocks are types of Rock (geology), rock that are formed by the accumulation or deposition of mineral or organic matter, organic particles at the Earth's surface, followed by Cementation (geology), cementation. Sedimentation is the coll ...
s such as shale, mudstone, and siltstone and in fine-grained metamorphic slate and phyllite. Clay minerals are usually (but not necessarily) ultrafine-grained (normally considered to be less than 2 micrometres in size on standard particle size classifications) and so may require special analytical techniques for their identification and study. These include X-ray crystallography, X-ray diffraction, electron diffraction methods, various spectroscopic methods such as Mössbauer spectroscopy, infrared spectroscopy, Raman spectroscopy, and Scanning electron microscope, SEM-Energy-dispersive X-ray spectroscopy, EDS or automated mineralogy processes. These methods can be augmented by petrographic microscope, polarized light microscopy, a traditional technique establishing fundamental occurrences or petrologic relationships.


Occurrence

Given the requirement of water, clay minerals are relatively rare in the Solar System, though they occur extensively on Earth where water has interacted with other minerals and organic matter. Clay minerals have been detected at several locations on Mars, including Echus Chasma, Mawrth Vallis, the Memnonia quadrangle and the Elysium quadrangle. Spectrography has confirmed their presence on asteroids including the dwarf planet Ceres (dwarf planet), Ceres and Tempel 1, as well as Jupiter's moon Europa (moon), Europa.


Classification

Clay minerals can be classified as 1:1 or 2:1, this originates because they are fundamentally built of tetrahedron, tetrahedral silicate sheets and octahedron, octahedral hydroxide sheets, as described in the structure section below. A 1:1 clay would consist of one tetrahedral sheet and one octahedral sheet, and examples would be kaolinite and serpentinite. A 2:1 clay consists of an octahedral sheet sandwiched between two tetrahedral sheets, and examples are talc, vermiculite, and montmorillonite. Clay minerals include the following groups: * Kaolin group which includes the minerals kaolinite, dickite, halloysite, and nacrite (Polymorphism (materials science), polymorphs of ). ** Some sources include the serpentine group, kaolinite-serpentine group due to structural similarities. * Smectite group which includes dioctahedral smectites, such as montmorillonite, nontronite and beidellite, and trioctahedral smectites, such as saponite. In 2013, analytical tests by the Curiosity rover found results consistent with the presence of ''smectite clay minerals'' on the planet Mars. * Illite group which includes the clay-micas. Illite is the only common mineral. * Chlorite group includes a wide variety of similar minerals with considerable chemical variation. * Other 2:1 clay types exist such as palygorskite (also known as attapulgite) and sepiolite, clays with long water channels internal to their structure. Mixed blue layer clay variations exist for most of the above groups. Ordering is described as a random or regular order and is further described by the term reichweite, which is German for range or reach. Literature articles will refer to an R1 ordered illite-smectite, for example. This type would be ordered in an ISIS fashion. R0 on the other hand describes random ordering, and other advanced ordering types are also found (R3, etc.). Mixed layer clay minerals which are perfect R1 types often get their own names. R1 ordered chlorite-smectite is known as corrensite, R1 illite-smectite is rectorite.


History

The crystallographic structure of clay minerals became better understood in the 1930s with advancements in the x-ray diffraction (XRD) technique indispensable to decipher their crystal lattice. Standardization in terminology arose during this period as well with special attention given to similar words that resulted in confusion such as sheet and plane.


Structure

Like all phyllosilicates, clay minerals are characterised by two-dimensional ''sheets'' of corner-sharing tetrahedra and/or octahedra. The sheet units have the chemical composition . Each silica tetrahedron shares 3 of its vertex oxygen atoms with other tetrahedra forming a hexagonal array in two-dimensions. The fourth vertex is not shared with another tetrahedron and all of the tetrahedra "point" in the same direction; i.e. all of the unshared vertices are on the same side of the sheet. In clays, the tetrahedral sheets are always bonded to octahedral sheets formed from small cations, such as aluminum or magnesium, and coordinated by six oxygen atoms. The unshared vertex from the tetrahedral sheet also forms part of one side of the octahedral sheet, but an additional oxygen atom is located above the gap in the tetrahedral sheet at the center of the six tetrahedra. This oxygen atom is bonded to a hydrogen atom forming an OH group in the clay structure. Clays can be categorized depending on the way that tetrahedral and octahedral sheets are packaged into ''layers''. If there is only one tetrahedral and one octahedral group in each layer the clay is known as a 1:1 clay. The alternative, known as a 2:1 clay, has two tetrahedral sheets with the unshared vertex of each sheet pointing towards each other and forming each side of the octahedral sheet. Bonding between the tetrahedral and octahedral sheets requires that the tetrahedral sheet becomes corrugated or twisted, causing ditrigonal distortion to the hexagonal array, and the octahedral sheet is flattened. This minimizes the overall bond-valence distortions of the crystallite. Depending on the composition of the tetrahedral and octahedral sheets, the layer will have no charge or will have a net negative charge. If the layers are charged this charge is balanced by interlayer cations such as Na+ or K+. In each case the interlayer can also contain water. The crystal structure is formed from a stack of layers interspaced with the interlayers.


Biomedical applications of clays

As most of the clays are made from minerals, they are highly biocompatible and have interesting biological properties. Due to disc-shaped and charged surfaces, clay interacts with a range of macromolecules such as drugs, protein, polymers, DNA, etc. Some of the applications of clays include drug delivery, tissue engineering, and bioprinting.


Mortar applications

Clay minerals can be incorporated in lime-metakaolin mortars to improve mechanical properties. Electrochemical separation helps to obtain modified saponite-containing products with high smectite-group minerals concentrations, lower mineral particles size, more compact structure, and greater surface area. These characteristics open possibilities for the manufacture of high-quality ceramics and heavy-metal sorbents from saponite-containing products. Furthermore, tail grinding occurs during the preparation of the raw material for ceramics; this waste reprocessing is of high importance for the use of clay pulp as a neutralizing agent, as fine particles are required for the reaction. Experiments on the Histosol deacidification with the alkaline clay slurry demonstrated that neutralization with the average pH level of 7.1 is reached at 30% of the pulp added and an experimental site with perennial grasses proved the efficacy of the technique. Moreover, the reclamation of disturbed lands is an integral part of the social and environmental responsibility of the mining company and this scenario addresses the community necessities at both local and regional levels.


See also

* * * Mudstone/clays on planet Mars ** ** ** * * *


References

{{Authority control Soil chemistry Clay minerals group, *