Bentonite: Encyclopedia - Bentonite

Bentonite

Bentonite 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 unique 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).

Bentonite can be used in cement, adhesives, ceramic fillers, cosmetics, and cat litter. Bentonite, in small percentages, is used as an ingredient in commercially designed clay bodies and ceramic glazes. Bentonite clay is also used in pyrotechnics to make end plugs and rocket nozzles. Sodium bentonite is used mostly as drilling mud in the oil and gas well drilling industries. The non-swelling calcium bentonite is sold within the alternative health market for its claimed threapeutic properties. Pascalite is a commercial name for bentonite clay.

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 Great Plains, Central Mountains and south eastern regions of the US.

Adapted from the Wikipedia article "Bentonite", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki