Clay (soil type)

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Clay is a naturally occurring material that consists mainly of clay mineral particles, is generally plastically deformable with sufficient water content and becomes brittle when dried or burned. Although clay usually contains layered silicates , it can contain other materials that give it plasticity and harden it when dried or fired. As associated phases, clay can contain materials that do not give it plasticity, e.g. B. quartz , calcite , dolomite , feldspar and organic substances.

In contrast to previous definitions, this definition by the AIPEA (Association Internationale Pour L'Etudes Des Argiles) and the CMS (Clay Minerals Society) does not specify the exact grain size of the clay components, as different disciplines have made their own specifications. In the geosciences , in accordance with the EN ISO 14688 standard, clay particles are considered to be particles <2 µm (sometimes also <4 µm) and in colloid chemistry <1 µm.

The clays do not include man-made materials with clay properties or materials with predominantly organic components such as peat , and some soils , even if these, like clay, have plastic properties and are of natural origin.

Properties and composition

With a higher water content, clay is by definition plastic , i.e. malleable. When drying or firing, clay becomes brittle , which means that it breaks when exposed to stress. Burnt clay is known as ceramic and is significantly more resilient than dried clay due to mineralogical-textural transformations. Only unfired clay can swell , i.e. its volume increases with increasing water content and decreases with decreasing water content.

The clay minerals that give moist clay its plastic properties are generally layered silicates , e.g. B. illite , montmorillonite or kaolinite . In addition, clays can contain admixtures of other minerals that do not contribute to the plastic properties, such as B. quartz , calcite , dolomite , feldspars , metal oxides and hydroxides or colloidal silica and iron hydroxide gel. In addition, clay typically also contains organic, i.e. H. high carbon, ingredients, predominantly in colloidal form.

By Ferrolyse a natural decomposition of the clay particles can take place.


Pottery and ceramics

Illustration from the chapter Argilla (“
Pottery earth”) in the Hortus sanitatis (one of the “Mainz herb books”) from 1491

The use of clay as a raw material for pottery and ceramics is documented up to the Upper Palaeolithic . Already around 24,000 years BC BC mammoth hunters made clay figures such as the Venus of Dolní Věstonice , which was found in the Czech Republic along with numerous animal figures .

Building material

Loam consists of a mixture of clay, silt and sand . For around 10,000 years, clay has been used as building material in the form of air-dried clay bricks , rammed earth and clay plaster . From the 3rd millennium BC BC clay was first burned to bricks on a large scale . Along with wood and stone, clay has always been one of the most important building materials of mankind.

Layers of clay are used to seal canals , ponds , dikes and landfills from the subsoil. Mighty formations of high-density clay are being discussed as repositories for radioactive waste .

Clay has been used as a raw material for cement production since the 20th century .


In addition to cement production, clay is also an important raw material for the production of chamottes , which are used for the interior lining of stoves, for example. B. are required in the steel and glass industry.

In the manufacture of paper , clay is used as a filler to make the paper softer, more pliable and to give it a smooth surface.


Clays of various compositions have been used for therapeutic purposes since prehistoric times. The mechanisms of action are often poorly understood in detail. First and foremost, the high adsorption capacity of the very fine-grain layered silicates is cited as an explanation for the observed healing effects. On the one hand, nutrients bound to the mineral surface can be released; on the other hand, toxins can be absorbed by the clay minerals and thus neutralized.

Current studies show that some deposits of iron-rich clays have a bactericidal effect. The clay minerals themselves ( Fe smectite , 1M d illite ) are less effective here than the high pH value (> 9) of the clay suspensions in combination with dissolved trace elements (Na, Mn, As, Ag, Mo, U).


Web links

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Individual evidence

  1. ^ A b S. Guggenheim, RT Martin: Definition of Clay and Clay Mineral: Joint Report of the AIPEA Nomenclature and CMS Nomenclature Committees. (PDF; 168 kB). In: Clays and Clay Minerals. Volume 43, No. 2, 1995
  2. see e.g. B. Paul R. Pinet: Invitation to Oceanography. 5th edition. Jones and Bartlett, 2008, ISBN 978-0-7637-5993-3 , p. 94, tab. 4-1.
  3. Shelley E. Haydel, Christine M. Remenih, Lynda B. Williams: Broad-spectrum 'in vitro' antibacterial activities of clay minerals against antibiotic-susceptible and antibiotic-resistant bacterial pathogens. In: Journal of Antimicrobial Chemotherapy. Volume 61, No. 2, 2008, pp. 353-361. doi: 10.1093 / jac / dkm468
  4. ^ Lynda B. Williams, Shelley E. Haydel, Rossman F. Giese Jr., Dennis D. Eberl: Chemical and Mineralogical Characteristics of French Green Clays Used for Healing. In: Clays and Clay Minerals. Volume 56, No. 4, 2008, pp. 437-452. PMC 2600539 (free full text)