Clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths and other cations. Clays have structures similar to the micas and therefore form flat hexagonal sheets. Clay minerals are common weathering products (including weathering of feldspar) and low temperature hydrothermal alteration products. Clay minerals are very common in fine grained sedimentary rocks such as shale, mudstone and siltstone and in fine grained metamorphic slate and phyllite. Aluminum redirects here. ... The silicate minerals make up the largest and most important class of rock-forming minerals. ... Fe redirects here. ... General Name, symbol, number magnesium, Mg, 12 Chemical series alkaline earth metals Group, period, block 2, 3, s Appearance silvery white solid at room temp Standard atomic weight 24. ... The alkali metals are a series of elements comprising Group 1 (IUPAC style) of the periodic table: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). ... This article contains information that has not been verified. ... A cation is an ion with positive charge. ... Rock with mica Mica sheet Mica flakes The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. ... Weathering is the decomposition of rocks, soils and their minerals through direct contact with the Earths atmosphere. ... This article does not cite any references or sources. ... Hydrothermal circulation in the oceans is the passage of the water through mid-ocean Ridge (MOR) systems. ... Two types of sedimentary rock: limey shale overlain by limestone. ... Shale Shale is a fine-grained sedimentary rock whose original constituents were clays or muds. ... Mudstone is a fine-grained sedimentary rock whose original constituents were clays or muds. ... Siltstone Siltstone is a geological term for a sedimentary rock whose composition is intermediate in grain size between the coarser sandstone and the finer mudstone. ... For other uses, see Slate (disambiguation). ... Phyllite Phyllite is a type of foliated metamorphic rock primarily composed of quartz, sericite mica, and chlorite; the rock represents a gradiation in the degree of metamorphism between slate and mica schist. ...

Clay minerals include the following groups:

• Kaolin group which includes the minerals kaolinite, dickite, halloysite and nacrite.^[1]
  • Some sources include the serpentine group due to structural similarities (Bailey 1980).
• Smectite group which includes dioctahedral smectites such as montmorillonite and nontronite and trioctahedral smectites for example saponite.^[1]
• Illite group which includes the clay-micas. Illite is the only common mineral.^[1]
• Chlorite group includes a wide variety of similar minerals with considerable chemical variation.^[1]

Kaolin redirects here. ... Dickite is a phyllosilicate mineral, with a similar composition to kaolin, nacrite and halloysite but having a different crystal structure. ... Nacrite Al2Si2O5(OH)4 is a clay mineral that is a polymorph (or polytype) of kaolinite. ... For other uses, see Serpentine (disambiguation). ... A sample of montmorillonite Montmorillonite is a very soft phyllosilicate mineral that typically forms in microscopic crystals, forming a clay. ... Nontronite is the iron(III) rich member of the smectite group of clay minerals. ... Saponite is a monoclinic mineral of the montmorillonite group. ... Structure of Illite mica - USGS. Illite is a non-expanding, clay-sized, micaceous mineral. ... Chlorite is a group of phyllosilicate minerals often classified as clays. ...

History

Knowledge of the nature of clay became better understood in the 1950s with advancements in microscope technology necessary to analyze the infinitesimal nature of clay particles.^[2] Standardization in terminology arose during this period as well^[2] with special attention given to similar words that resulted in confusion such as sheet and plane.^[2]

Structure

Like all phyllosilicates, clay minerals are characterised by two-dimensional sheets of corner sharing SiO₄ and AlO₄ tetrahedra. These tetrahedral sheets have the chemical composition (Al,Si)₃O₄, and each 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 aluminium or magnesium, coordinated by six oxygen atoms. The unshared vertex from the tetrahedral sheet also form 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 categorised 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 corrogated 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.

See also

• Clay
• Expansive clay

For other uses, see Clay (disambiguation). ... There are very few or no other articles that link to this one. ...

References

1. ^ ^{a b c d} Amethyst Galleries. "The Clay Mineral Group." 2006. February 22, 2007. [1]
2. ^ ^{a b c} Bailey, S. W., 1980, Summary of recommendations of AIPEA nomenclature committee on clay minerals, American Mineralogist Volume 65, pages 1-7. [2]