Living Reference Work Entry

Encyclopedia of Engineering Geology

Part of the series Encyclopedia of Earth Sciences Series pp 1-2

Date: Latest Version


  • Arpita NandiAffiliated withDepartment of Geosciences, East Tennessee State University Email author 


The term “clay” is applied both to earth materials with a particle size of equal or less than 0.005 mm and to the minerals that are microcrystalline, layered, hydrous aluminum phyllosilicates, occasionally with variable amounts of iron, magnesium, and alkali metals (Gillott 1968; West 2010). Therefore, cohesive soils may be composed of mixtures of clay minerals and clay-sized materials like quartz, feldspar, and carbonate. Both clay minerals and clay-sized particles are the product of weathering from preexisting rocks and found on or near the earth surface.


Globally, clay-bearing sediments, also referred to as argillaceous sediments, make up about 60% of the earth surface, with clay minerals comprising up to two-thirds of the components. The atomic structure of clay minerals involves two basic units, tetrahedral silicate sheets (Si+4 cation occurs in fourfold and tetrahedral coordination w ...

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