Clay mineral is referred to by the International Society of Soil Science as a rock, or mineral particle, found in soil, that has a diameter less than two microns (0.002mm). Sedimentologists classify clay as having particles less than 0.004mm. Clay mineralogists look on clay mineral as being a layer of silicate (phyllosilicate), or something similar, that normally exhibits plasticity but hardens when dried. In general clay is looked on as being a fine grained mineral fraction of earth material that includes clay silicates or oxide-hydroxide minerals such as some zeolites, manganese oxides, hematite and/or geothite.
The Many Varieties of Clay Minerals
Iron Oxyphydroxides:
Lepidocrocite. This clay mineral is rather uncommon and can be recognised by its orange colour. It is formed by direct oxidation from ferrous iron when oxidation is slow.
Ferrihydrite. A rusty brown scum found at springs where water seeps into cracks in rocks. Often seen as an 'oil slick' on the water surface. Ferrihydrite is associated with bacteria and grows as a result of oxidation reaction.
Goethite. This is the most common iron oxide clay. It is useful for geochemical sampling during mineral exploration.
Hermatite. Common in warm arid localities. When massive it is a blue-black to black colour and when fine grained and dispersed it is a red colour.
Allophane:
Allophane is commonly found in soils made from volcanic ash. It is rather difficult to recognise but it is a hydrated alumino-silicate.
Interstratified clays:
Interstratified clays are often incorrectly called mixed-layer clays and they make up a very small percentage of clay found in soils. One layer of this clay mineral is smectite (expansive with permanent changes) that gives it its 'shrink-swell' characteristic quality. Most clay silicates have the same structure which facilitates it stacking propensity.
Illite:
Illite is a clay mica associated with the weathering of feldspar and muscovite. It is best known as being a diagenetic mineral buried below marine environments.
Kaolin group:
Hisingerite. A brown clay mineral almost amorphous and quite vitreous. It is formed as a result of low temperature alteration to iron rich minerals such as olivine, pyrite and others.
Halloysite. A similar chemistry, occurrence and property make up as that of kaolinite but is hydrated containing platey, spherical, or cylindrical morphology.
Kaolinite. This clay mineral is formed by the weathering, or alteration, of aluminosilicate minerals. It has an important industrial use.
Vermiculite:
Vermiculite is used as an absorbent material and finds uses in industry for cleaning purposes, kitty litter and for potting soils. It is able to hold water molecules with its inter- layers.
Smectites
This particular family of clay minerals swell when placed in water or other organic liquids that have polar molecules:
Beidelite. A common weathering product that originates in feldspar.
Saponite. A magnesium silicate alteration product.
Nontronite. An iron bearing silicate weathering product.
Montmorillonite. Often called bentonite and usually found in large deposits. It is used in industry in drilling mud, foundry moulds and in pelletising iron ore. It is used in civil engineering as an absorbent.
