Clay minerals

Article By:

Eberl, Dennis D. U.S. Geological Survey, Denver Federal Center, Denver, Colorado.

Last reviewed:March 2021

DOI:https://doi.org/10.1036/1097-8542.139900

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• Clay minerals, published November 2019:Download PDF Get Adobe Acrobat Reader

Fine-grained, hydrous, layer silicates that belong to the larger class of sheet silicates known as phyllosilicates. Clay minerals form on Earth in many different environments, including the weathering environment, the sedimentary environment, and the diagenetic-hydrothermal environment. Clay minerals composed of the more soluble elements (for example, smectite and sepiolite) are formed in environments in which these ions can accumulate (for example, in a dry climate, in a poorly drained soil, in the ocean, or in saline lakes), whereas clay minerals composed of less soluble elements (for example, kaolinite (Fig. 1) and halloysite) form in more dilute water such as that found in environments that undergo severe leaching (for example, a hilltop in the wet tropics), where only sparingly soluble elements such as aluminum and silicon can remain. Illite and chlorite are known to form abundantly in the diagenetic-hydrothermal environment by reaction from smectite.

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