Grim, Ralph E. Department of Geology, University of Illinois, Urbana-Champaign, Illinois.
Wahl, Floyd M. Department of Geology, University of Florida, Gainesville, Florida.
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The term first applied to a particular, highly colloidal plastic clay found near Fort Benton in the Cretaceous beds of Wyoming. This clay swells to several times its original volume when placed in water and forms thixotropic gels when small amounts are added to water. Later investigations showed that this clay was formed by the alteration of volcanic ash in place; thus, the term bentonite was redefined by geologists to limit it to highly colloidal and plastic clay materials composed largely of montmorillonite clay minerals, and produced by the alteration of volcanic ash in place. Many mineralogists and geologists now use the term bentonite without reference to the physical properties of the clay. On the other hand, the term has been used commercially for any plastic, colloidal, and swelling clays without reference to a particular mode of origin. See also: Clay; Gel; Montmorillonite
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