Clay has two definitions

To a clay mineralogist, a clay mineral is a layer silicate mineral (also called a phyllosilicate) or other mineral which imparts plasticity and which hardens upon drying or firing. (Guggenheim and Martin 1995)

The word "clay" is also used to refer to a particle size in a soil or sediment. The term is used in the U.S. and by the International Society of Soil Science for a rock or mineral particle in the soil having a diameter less than 0.002 mm (2 microns), whereas sedimentologists classify particles smaller than 0.004 mm as clay.

In practice, "clay" is used to refer to the fine-grained, mineral fraction of earth material, and can include clay silicates as defined above, oxide-hydroxide minerals, such as goethite, hematite, manganese oxides, and some zeolites.

Most of the descriptions below are from THE REGOLITH GLOSSARY (2Mb PDF), published by CRC LEME (



smectites, vermiculite, kaolins, illite, chlorite, interstratified clays, allophane, hematite, goethite, ferrihydrite, lepidocrocite


These are a family of clays that swell when immersed in water or some organic liquids (those which, like water, have polar molecules). Formerly they were known as the montmorillonite group; that name is now only used for one mineral in the smectite group. All smectites have very high cation exchange capacity (of the order of 1000 meq/kg = 1 mmole of positive charge per gram).


Approximately A0.3Al2[Si3.7Al0.3]O10.(OH)2.xH2O, where A is an exchangeable cation, K+, Na+, or 0.5Ca2+ etc. Beidellite is a common early weathering product of feldspars.


Approximately A0.3(Al1.3Mg0.7)[Si4]O10.(OH)2.xH2O, where A is an exchangeable cation, K+, Na+, or 0.5Ca2+ etc. Montmorillonite is less common in soils and regolith than the other aluminous smectite beidellite. Montmorillonite occurs in some large deposits, where it is often known as bentonite. It is particularly useful industrially in drilling muds, for moulds in foundries, for pelletising iron ores, in civil engineering and as absorbents e.g. in pet litters among other uses.


Approximately A0.3(Fe3+2)[Si3.7Al0.3]O10.(OH)2.xH2O, where A is an exchangeable cation, K+, Na+, or 0.5Ca2+ etc. Nontronite is a common weathering product of iron-bearing silicates.


A0.7Mg3[Si3.7Al0.3]O10.(OH)2.xH2O, where A is an exchangeable cation, K+, Na+, or 0.5Ca2+ etc. . Alteration product of magnesium silicates.


Vermiculite has a trioctahedral 2:1 layer, and an interlayer containing both Mg2+ and Al3+, which are able to hold two water molecules in the interlayer. Vermiculite has a higher layer charge than smectite and a high cation exchange capacity (1000-1500 meq/kg). The layers are more strongly held together than are those of smectite, so the basal spacing is smaller (14 Å, cf 15 Å for smectite). On heating the water can be expelled, collapsing the structure to 12.5 Å and then to 10 Å. If a large vermiculite crystal is rapidly heated, the steam physically blows some of the layers apart and accordian-like "worms" are formed, hence its name. Vermiculite is used as an absorbent for potting soils, kitty litter, and industrial clean up.

Kaolin group


Al2Si2O5(OH)4. Major component of regolith, particularly in the plasmic zone of weathering profiles and of hydrothermally altered felsic rocks. Formed by the weathering or alteration of aluminosilicate minerals, primarily plagioclase and muscovite. Typically forms micron-sized hexagonal platey crystals; cation exchange capacity 20-100 meq/kg. Important as an industrial mineral


Al2Si2O5(OH)4.0-2H2O, similar to kaolinite in chemistry, properties and occurrence, but hydrated (or presumed to have been formerly hydrated), with cylindrical, spherical or rarely platey morphology.


Brown, vitreous, almost amorphous Fe3+ member of the kaolin group: Fe2Si2O5(OH)4. Low temperature alteration product of pyrite, olivine, and other iron-rich minerals.


Clay mica, approximately K0.75(Al1.75Mg0.25)(Si3.5Al0.5)O10(OH)2. In the soil or regolith, illite forms chiefly by the weathering of muscovite and feldspar, but the mineral is better known as a diagenetic mineral in buried marine sediments.


Chlorite {(Mg,Al,Fe)12[(Si,Al)8O20](OH)16} has a talc-like TOT (tetrahedral-octahedral-tetrahedral) structure with the interlayer filled with a brucite-like {(Mg,Fe)6(OH)12} layer.  Chlorites are common in low-grade metamorphic rocks, igneous rocks (generally after hydrothermal alteration) and argillaceous rocks and some sediments.

Interstratified clays

Also known (incorrectly) as "mixed-layer" clays, interstratified clays generally make up only a small percentage of the clay fraction of soils and regolith, but may be a more abundant fraction of marine sediments. Generally, one component layer of the interstratified clay is a smectite, and this gives the clay a high exchange capacity and some "shrink-swell" character.

In plan, all the clay silicates have the same structure: a hexagonal silica-oxygen sheet (or sheets) and a hexagonal octahedral sheet. They therefore have little difficulty stacking different layer types on top of one another. Illite and smectite layers may alternate, building up random sequences (ISISSIIISISSIIS) or regular sequences (ISISISIS, or IISIISIISIIS). Kaolinite and smectite may alternate in soil clays, biotite weathers to vermiculite through an intermediate random interstratification (BBBVBBBBVBVBVBBV), gradually becoming semi-regular (BVBVBVBBVBVBBVBVBVBVVB).



Allophane is a hydrated alumino-silicate, formed as spheres about 50 Å across. It does not have a single composition, but ranges from Al2O3.SiO2 to Al2O3.2SiO2. It is difficult to recognize because it gives very poor X-ray reflections (broad bands centred at about 15 Å, 3.4 Å, and 2.5 Å). It is commonest in soils derived from volcanic ash.

Iron oxyhydroxides


Fe2O3, hexagonal close-packed structure, very common in regolith of warm or arid regions. Colour black or blue-black where massive, red where fine-grained and dispersed. Its intense colour may mask the presence of goethite.


FeO(OH), with common substituents Al, Mn, Ni; polymorphous with lepidocrocite. The most common iron oxide mineral in the regolith. Structure: double chains of Fe-O octahedra linked laterally. Yellow-brown, forming as compact aggregates of sub-micron-sized crystals or as dispersed micro-crystals; common as cutans on ferruginous nodules. Very high surface area, up to 200 m2/g, with a high sorptive capacity for cations (eg Cu, Pb, Zn) and anions, especially phosphate. Substitution of Al for Fe reaches 30 mole % in goethites formed in hydromorphic regolith. Goethite can be useful as a geochemical sampling medium in mineral exploration.


FeO(OH), polymorphous with goethite, but having the oxygens in approximate cubic close packing. Recognizable by its orange colour, lepidocrocite is a relatively uncommon mineral, forming in preference to goethite as a direct oxidation product of ferrous iron and in preference to ferrihydrite if oxidation is slow. It also seems to be precipitated rather than goethite in the presence of Cl-.


Approximate composition 5Fe2O3.9H2O. Ferrihydrite is the brown rusty scum visible at springs, where water seeps from cracks in rocks, or as an "oil slick" on some swamp water. Ferrihydrite crystals range from about 2-8 nm in diameter. The degree of organization of these particles is low, and the X-ray pattern is very simple and weak and the lines are broad. The surface area of ferrihydrite crystals ranges from 200-800 m2/g and they are strong adsorbers of phosphate, silica, organic molecules, and heavy metals. Ferrihydrite transforms to a more stable oxyhydroxide (goethite, usually) over a period of a few years. Most ferrihydrite is associated with bacteria (Gallionella and Leptothrix) which gain their energy from the oxidation reaction Fe2+ = Fe3+ + e-.


Try these links for information about industrial clay uses and sources in Australasia.

New South Wales Department of Mineral Resources

Chamber of Minerals and Energy, Western Australia

Queensland Department of Mines and Energy

South Australian DMITRE Minerals

South Australian Resources Information Geoserver

Geos Mining

Stratum Resources

Unimin Australia (Kaolin bentonite)

Arumpo Bentonite Pty Ltd

WA Hudson Resources (Attapulgite palygorskite)

English China Clay (ECC International/Imerys)

New Zealand China Clays Inc.

Indmins: Industrial minerals news



Most of these descriptions of clay are from THE REGOLITH GLOSSARY (2Mb PDF), published by