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Bentonite - USGSBentonite is an absorbent aluminium phyllosilicate generally impure clay consisting mostly of montmorillonite, (Na,Ca)0.33(Al,Mg)2Si4O10(OH)2·nH2O. 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. Bentonite expands when wet - sodium bentonite can absorb several hundred percent of its dry weight in water. It is commonly used in drilling fluids, used to make slurry walls, and used to form impermeable barriers (ie plug old wells, as a liner in the base of landfills to prevent migration of leachate into the soil).
Much of bentonite's usefulness in the drilling and geotechnical engineering industry comes from its uniqe rheological properties. Relatively small quantities of bentonite suspended in water form a viscous, shear thinning material. Most often, bentonite suspensions are also thixotropic, although rare cases of rheopectic behavior have also been reported. At high enough concentrations (~60 grams of bentonite per liter of suspension), bentonite suspensions begin to take on the characteristics of a gel (material with finite yield strength). Also see montmorillonite.
Bentonite is named after Benton Formation (a geological stratum, at one time Fort Benton Formation) in eastern Wyoming's Rock Creek area. Most high grade commercial sodium bentonite mined in the US 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 US, in Greece, and in other regions of the world. Calcium bentonite is mined in the south eastern US. Bentonite can be used in cement, adhesives, ceramic fillers, cosmetics, and cat litter. Bentonite clay is also used in pyrotechnics to make end plugs and rocket nozzles. Most is used as drilling mud in the oil and gas well drilling industries. It is also sold in health food stores as a drink.