Clay

A term used to refer to the finest-grain particles in a sediment, soil, or rock. According to the Wentworth scale (see table), clay is finer than silt, characterized by a grain size of less than approximately 4 micrometers. However, the term clay can also refer to a rock or a deposit containing a large component of clay-size material. Thus clay can be composed of any inorganic materials, such as clay minerals, allophane, quartz, feldspar, zeolites, and iron hydroxides, that possess a sufficiently fine grain size. Most clays, however, are composed primarily of clay minerals. See also: Clay minerals; Feldspar; Quartz; Zeolite

Although the composition of clays can vary, clays can share several properties that result from their fine particle size. These properties include plasticity when wet, the ability to form colloidal suspensions when dispersed in water, and the tendency to flocculate (clump together) and settle out in saline water. See also: Colloid

 

Origin and occurrence

Clays occur most abundantly in nature in soils, sediments, sedimentary rocks, and hydrothermal deposits.

Clays, together with organic matter, water, and air, are one of the four main components of soil. Clays can form directly in a soil by precipitation from solution (neoformed clays); they can form from the partial alteration of clays already present in the soil (transformed clays); or they can be inherited from the underlying bedrock or from sediments transported into the soil by wind, water, or ice (inherited clays). See also: Soil

The type of clays neoformed in a soil depends on the composition of the soil solution, which in turn is a function of climate, drainage, original rock type, vegetation, and time. Generally, neoformed clays that have undergone intense leaching, such as soils formed under wet, tropical climates, are composed of the least soluble elements, such as ferric iron, aluminum, and silicon. These soils contain clays such as gibbsite, kaolinite, goethite, and amorphous oxides and hydroxides of aluminum and iron. Clays formed in soils that are found in dry climates or in soils that are poorly drained can contain more soluble elements, such as sodium, potassium, calcium, and magnesium, in addition to the least soluble elements. These soils contain clays such as smectite, chlorite, and illite, and generally are more fertile than those formed under intense leaching conditions. The importance of the composition of soil solution to the type of clay neoformed at equilibrium can be shown quantitatively in mineral stability diagrams, such as that given in the illustration. See also: Chlorite; Goethite; Illite; Kaolinite

Example of a stability diagram for the system Na2O-K2O-Al2O3-SiO2-H2O drawn for log ([K+]/[H+]) = 4. The soluble species in brackets are in activity units. (After J. I. Drever, The Geochemistry of Natural Waters, Prentice-Hall, 1982)

Examples of clays formed by the transformation of other clays in a soil include soil chlorite and soil vermiculite, the first formed by the precipitation of aluminum hydroxide in smectite interlayers, and the second formed by the leaching of interlayer potassium from illite. Examples of inherited clays in a soil are illite and chlorite-containing soils formed on shales composed of these minerals. See also: Shale

Clays also occur abundantly in sediments and sedimentary rocks. For example, clays are a major component of many marine sediments. These clays generally are inherited from adjacent continents, and are carried to the ocean by rivers and wind, although some clays (such as smectite and glauconite) are neoformed abundantly in the ocean. Clays also are a chief constituent of shale, are found in pore spaces in sandstones, and are found dispersed in limestones, dolomites, and coals. The ashes that remain after coal is burned, for example, are the residue of clays and silts washed into coal swamps and later incorporated into the coal. Hydrothermal clays can form abundantly where rock has been in contact with hot water or steam. Illite and chlorite, for example, form during the deep burial of sediments, and smectite and chlorite form by the reaction of hot, circulating waters at ocean ridges. Kaolinite can form hydrothermally where hot springs invade volcanic ash, given proper solution chemistries. See also: Coal; Dolomite; Glauconite; Limestone; Marine sediments; Sandstone; Sedimentary rocks

Properties and uses

Various clays possess special properties which make them important industrially. Bentonite, a smectite formed primarily from the alteration of volcanic ash, swells; is readily dispersible in water; and possesses strong absorptive powers, including a high cation exchange capacity. These properties lead to uses in drilling muds, as catalysts and ion exchangers, as fillers and absorbents in food and cosmetics, and as binders for taxonite and fertilizers. Swelling clays also must be taken into account in building design. In agriculture, swelling clays in a soil may lead to undesirable tilth and water runoff characteristics, and to unexpected reactions with soil additives, such as the catalytic conversion of pesticides into other compounds, or the fixation of ammonia fertilizer. See also: Bentonite; Catalysis; Ion exchange; Soil chemistry

Kaolinite-rich clays are used as paper-coating materials because they are white and show desirable rheological properties; as additives to rubber because they can bind with organic compounds; and as refractory materials due to their high melting point. Other important uses for clays include the manufacture of brick, ceramics, molding sands, decolorizers, detergents and soaps, medicines, adhesives, liners for ponds and landfills, lightweight aggregate, desiccants, molecular sieves, pigments, greases, paints, plasticizing agents, emulsifying, suspending, and stabilizing agents, and many other products. Ore deposits sometimes are composed of clays (for example, lateritic nickel and bauxites), and clay in shale may play an important role in the generation of petroleum. See also: Clay, commercial; Refractory

 

Bibliography

 

R. Bennett and M. H. Hulbert, Clay Microstructure, 1986

A. Meunier, Clays, 2005

Ali Fazeli = egeology.blogfa.com

Ali Fazeli = Springerlink.blogfa.com

B. Velde (ed.), Origin and Mineralogy of Clays: Clays and the Environment, vol. 1, 1995

Ali Fazeli = egeology.blogfa.com

Ali Fazeli = Springerlink.blogfa.com