Clay minerals

Clay minerals
Oxford Clay (Jurassic) exposed near Weymouth, England.

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).[1]
  • 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]
  • 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.[2]



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.[3] Standardization in terminology arose during this period as well[3] with special attention given to similar words that resulted in confusion such as sheet and plane.[3]


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


  1. ^ a b c d Amethyst Galleries. "The Clay Mineral Group." 2006. February 22, 2007. [1]
  2. ^ 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
  3. ^ 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]

Wikimedia Foundation. 2010.

Look at other dictionaries:

  • clay minerals — molio mineralai statusas T sritis chemija apibrėžtis Al, Mg, Cu, Fe, K, Na hidrosilikatai ir aliumosilikatai. atitikmenys: angl. clay minerals rus. глинистые минералы …   Chemijos terminų aiškinamasis žodynas

  • Plastic and liquid limits of clay minerals — ▪ Table Plastic and liquid limits of clay minerals (water content in percent) plastic limit liquid limit kaolinite Na   26   52 kaolinite Ca   36   73 illite Na   34   61 illite Ca   40   90 montmorillonite Na   97 700 montmorillonite Ca   63 177 …   Universalium

  • Cation-exchange capacities and specific surface areas of clay minerals — ▪ Table Cation exchange capacities and specific surface areas of clay minerals mineral cation exchange capacity at pH 7 (milliequivalents per 100 grams) specific surface area (square metre per gram) kaolinite 3–15 5–40 halloysite (hydrated) 40–50 …   Universalium

  • Clay (disambiguation) — Clay is a material primarily composed of a grouping of clay minerals, and can be used as an art medium. Clay may also refer to: The designation for the smallest particle size above colloid of granular material, regardless of composition. Places… …   Wikipedia

  • Clay-water interaction — is an all inclusive term to describe various progressive interactions between clay minerals and water. In the dry state, clay packets exist in face to face stacks like a deck of playing cards, but clay packets begin to change when exposed to… …   Wikipedia

  • Clay dogs — are naturally occurring clay formations that are sculpted by river currents from glacially deposited blue gray clay and then dried by the sun. They exhibit tremendous variety in shape and size, with some being simple and others having highly… …   Wikipedia

  • Clay cutan — Clay Cutans are a geologic fabric that develop around ancient cavities (such as peds) within paleosols. Contents 1 Formation 2 Recognition 3 Practical Significance 4 References …   Wikipedia

  • clay mineral — any of a group of hydrous aluminum silicate minerals, as kaolinite, illite, and montmorillonite, that constitute the major portion of most clays. [1945 50] * * * Any of a group of important hydrous aluminum silicates with a layered structure and… …   Universalium

  • Clay — For other uses, see Clay (disambiguation). The Gay Head cliffs in Martha s Vineyard consist almost entirely of clay. Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic… …   Wikipedia

  • clay — clay1 claylike, adj. /klay/, n. 1. a natural earthy material that is plastic when wet, consisting essentially of hydrated silicates of aluminum: used for making bricks, pottery, etc. 2. earth; mud. 3. earth, esp. regarded as the material from… …   Universalium

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”