Bentonite

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. 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.

Bentonites have been found in almost all countries and in rocks of a wide variety of ages. They appear to be most abundant in rocks of Cretaceous age and younger. In the United States, bentonites are mined extensively in Wyoming, Arizona, and Mississippi. England, Germany, Yugoslavia, Russia, Algeria, Japan, and Argentina also produce large tonnages of bentonite. Many bentonites are of great commercial value. They are used in decolorizing oils, in bonding molding sands, in the manufacture of catalysts, in the preparation of oil well drilling muds, and in numerous other relatively minor ways. The properties of a particular bentonite determine its economic use.

Bentonite is an absorbent aluminium phyllosilicate generally impure clay consisting mostly of montmorillonite, (Na,Ca)0.33(Al,Mg)2Si4O10(OH)2·(H2O)n. Two types exist: swelling bentonite which is also called sodium bentonite and non-swelling bentonite or calcium bentonite. It forms from weathering of volcanic ash, most often in the presence of water.

Sodium bentonite
Sodium bentonite expands when wet - it can absorb several times its dry weight in water. It is mostly used in drilling mud in the oil and gas well drilling industries.

The property of swelling also makes sodium bentonite useful as a sealant, especially targeted for the sealing of subsurface disposal systems for spent nuclear fuel [1] [2] and for quarantining metal pollutants of groundwater. Similar uses include making slurry walls, waterproofing of below grade walls and forming other impermeable barriers (e.g. to plug old wells or as a liner in the base of landfills to prevent migration of leachate into the soil).

Calcium bentonite
The non-swelling calcium bentonite is sold within the alternative health market for its purported cleansing properties. It is usually combined with water and ingested as part of a detox diet, [3] in a practice known as geophagy. It is claimed that the microscopic structure of the bentonite draws impurities into it from the digestive system, which are then excreted along with the bentonite. It is also claimed that native tribes in South America, Africa and Australia have long used bentonite clay for this purpose.

Pascalite is another commercial name for the calcium bentonite clay.

Bentonite History and natural occurrence

The absorbent clay was given the name bentonite by an American geologist sometime after its discovery in about 1890 - after the Benton Formation (a geological stratum, at one time Fort Benton) in eastern Wyoming's Rock Creek area. Other modern discoveries include Montmorillonite discovered in 1847 in Montmorillon in the Vienne prefecture of France, in Poitou-Charentes, South of the Loire Valley and Pascalite discovered in about 1830 by French-Canadian fur trapper Emile Pascal atop the 8600-foot high Big Horn Mountains in Wyoming, USA.

Most high grade commercial sodium bentonite mined in the United States comes from the area between the Black Hills of South Dakota and the Big Horn Basin of Montana. Sodium bentonite is also mined in the southwestern United States, in Greece and in other regions of the world. Calcium bentonite is mined in the Great Plains, Central Mountains and south eastern regions of the United States. Supposedly the world's largest current source of bentonite is Chongzuo in China's Guangxi province.