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.
Clays are ultrafine-grained (normally considered to be less than 2 micrometres in size on standard particle size classifications) and so require special analytical techniques. Standards include x-ray diffraction, electron diffraction methods, various spectroscopic methods such as Mössbauer spectroscopy, infrared spectroscopy, and SEM-EDS or automated mineralogy solutions. These methods can be augment polarized light microscopy, a traditional technique establishing fundamental occurrences or petrologic relationships.
Clays are commonly referred to as 1:1 or 2:1. Clays are fundamentally built of tetrahedral sheets and octahedral 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 serpentine. A 2:1 clay consists of an octahedral sheet sandwiched between two tetrahedral sheets, and examples are illite, smectite, attapulgite, and chlorite (although chlorite has an external octahedral sheet often referred to as "brucite").
Clay minerals include the following groups:
Kaolin group which includes the minerals kaolinite, dickite, halloysite, and nacrite (polymorphs of Al2Si2O5(OH)4).
Some sources include the kaolinite-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.
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 sepiolite or attapulgite, clays with long water channels internal to their structure.
Mixed layer clay variations exist for most of the above groups. Ordering is described as random or regular ordering, and is further described by the term reichweite, which is German for range or reach. Literature articles will refer to a R1 ordered illite-smectite, for example. This type would be ordered in an ISISIS 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.
3 See also
Knowledge of the nature of clay became better understood in the 1930s with advancements in x-ray diffraction technology necessary to analyze the molecular nature of clay particles. 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.
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 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.
 See also
^ a b c d Amethyst Galleries. "The Clay Mineral Group." 2006. February 22, 2007. 
^ Moore, D. and R.C. Reynolds, Jr., 1997, X-Ray Diffraction and the Identification and Analysis of Clay Minerals, 2nd ed.: Oxford University Press, New York
^ a b c Bailey, S. W., 1980, Summary of recommendations of AIPEA nomenclature committee on clay minerals, American Mineralogist Volume 65, pages 1-7. 
The Clay Minerals are a part of a general but important group within the phyllosilicates that contain large percentages of water trapped between their silicate sheets. Most clays are chemically and structurally analogous to other phyllosilicates but contain varying amounts of water and allow more substitution of their cations. There are many important uses and considerations of clay minerals. They are used in manufacturing, drilling, construction and paper production. They have geat importance to crop production as clays are a significant component of soils.
It is the physical characteristics of clays (more so than chemical and structural characteristics) that define this group:
Clay minerals tend to form microscopic to sub microscopic crystals.
They can absorb water or lose water from simple humidity changes.
When mixed with limited amounts of water, clays become plastic and are able to be molded and formed in ways that most people are familiar with as children's clay.
When water is absorbed, clays will often expand as the water fills the spaces between the stacked silicate layers.
Due to the absorption of water, the specific gravity of clays is highly variable and is lowered with increased water content.
The hardness of clays is difficult to determine due to the microscopic nature of the crystals, but actual hardness is usually between 2 - 3 and many clays give a hardness of 1 in field tests.
Clays tend to form from weathering and secondary sedimentary processes with only a few examples of clays forming in primary igneous or metamorphic environments.
Clays are rarely found separately and are usually mixed not only with other clays but with microscopic crystals of carbonates, feldspars, micas and quartz.
Clay minerals are divided into four major groups. These are the important clay mineral groups:
The Kaolinite Group
This group has three members (kaolinite, dickite and nacrite) and a formula of Al2Si2O5(OH)4. The different minerals are polymorphs, meaning that they have the same chemistry but different structures (polymorph = many forms). The general structure of the kaolinite group is composed of silicate sheets (Si2O5) bonded to aluminum oxide/hydroxide layers (Al2(OH)4) called gibbsite layers. The silicate and gibbsite layers are tightly bonded together with only weak bonding existing between the s-g paired layers.
Uses: In ceramics, as a filler for paint, rubber and plastics and the largest use is in the paper industry that uses kaolinite to produce a glossy paper such as is used in most magazines.
The Montmorillonite/Smectite Group
This group is composed of several minerals including pyrophyllite, talc, vermiculite, sauconite, saponite, nontronite and montmorillonite They differ mostly in chemical content. The general formula is (Ca, Na, H)(Al, Mg, Fe, Zn)2(Si, Al)4O10(OH)2 - xH2O, where x represents the variable amount of water that members of this group could contain. Talc's formula, for example, is Mg3Si4O10(OH)2. The gibbsite layers of the kaolinite group can be replaced in this group by a similar layer that is analogous to the oxide brucite, (Mg2(OH)4). The structure of this group is composed of silicate layers sandwiching a gibbsite (or brucite) layer in between, in an s-g-s stacking sequence. The variable amounts of water molecules would lie between the s-g-s sandwiches.
Uses: Are many and include a facial powder (talc), filler for paints and rubbers, an electrical, heat and acid resistant porcelain, in drilling muds and as a plasticizer in molding sands and other materials.
The Illite (or The Clay-mica) Group
This group is basically a hydrated microscopic muscovite. The mineral illite is the only common mineral represented, however it is a significant rock forming mineral being a main component of shales and other argillaceous rocks. The general formula is (K, H)Al2(Si, Al)4O10(OH)2 - xH2O, where x represents the variable amount of water that this group could contain. The structure of this group is similar to the montmorillonite group with silicate layers sandwiching a gibbsite-like layer in between, in an s-g-s stacking sequence. The variable amounts of water molecules would lie between the s-g-s sandwiches as well as the potassium ions.
Uses: A common constituent in shales and is used as a filler and in some drilling muds.
The Chlorite Group
This group is not always considered a part of the clays and is sometimes left alone as a separate group within the phyllosilicates. It is a relatively large and common group although its members are not well known. These are some of the recognized members:
Amesite (Mg, Fe)4Al4Si2O10(OH)8
Baileychlore (Zn, Fe+2, Al, Mg)6(Al, Si)4O10(O, OH)8
Chamosite (Fe, Mg)3Fe3AlSi3O10(OH)8
Clinochlore (kaemmererite) (Fe, Mg)3Fe3AlSi3O10(OH)8
Corundophilite (Mg, Fe, Al)6(Al, Si)4O10(OH)8
Daphnite (Fe, Mg)3(Fe, Al)3(Al, Si)4O10(OH)8
Delessite (Mg, Fe+2, Fe+3, Al)6(Al, Si)4O10(O, OH)8
Gonyerite (Mn, Mg)5(Fe+3)2Si3O10(OH)8
Nimite (Ni, Mg, Fe, Al)6AlSi3O10(OH)8
Odinite (Al, Fe+2, Fe+3, Mg)5(Al, Si)4O10(O, OH)8
Orthochamosite (Fe+2, Mg, Fe+3)5Al2Si3O10(O, OH)8
Penninite (Mg, Fe, Al)6(Al, Si)4O10(OH)8
Pannantite (Mn, Al)6(Al, Si)4O10(OH)8
Rhipidolite (prochlore) (Mg, Fe, Al)6(Al, Si)4O10(OH)8
Sudoite (Mg, Fe, Al)4 - 5(Al, Si)4O10(OH)8
Thuringite (Fe+2, Fe+3, Mg)6(Al, Si)4O10(O, OH)8
The term chlorite is used to denote any member of this group when differentiation between the different members is not possible. The general formula is X4-6Y4O10(OH, O)8. The X represents one or more of aluminum, iron, lithium, magnesium, manganese, nickel, zinc or rarely chromium. The Y represents either aluminum, silicon, boron or iron but mostly aluminum and silicon.
The gibbsite layers of the other clay groups are replaced in the chlorites by a similar layer that is analogous to the oxide brucite. The structure of this group is composed of silicate layers sandwiching a brucite or brucite-like layer in between, in an s-b-s stacking sequence similar to the above groups. However, in the chlorites, there is an extra weakly bonded brucite layer in between the s-b-s sandwiches. This gives the structure an s-b-s b s-b-s b sequence. The variable amounts of water molecules would lie between the s-b-s sandwiches and the brucite layers.
Uses: No industrial uses.
Some minerals listed above (specifically chlorite, pyrophyllite and talc) as belonging to one of the clay groups are often excluded by some minerologists. Usually the reason is that their crystal size and character do not consistently conform to the parameters that define a clay. Such minerals are listed here more for their structural similarities. However, all three minerals are quite often found associated with and do behave like clays occasionally.