Bentonite. (Courtesy USGS.)

Bentonite is an absorbent clay consisting mostly of the mineral montmorillonite. It is formed by the weathering of volcanic ash, most often in the presence of water. Chemically, it is an impure form of aluminum phyllosilicate. The formula of montmorillonite may be written as (Na,Ca)0.33(Al,Mg)2Si4O10(OH)2·(H2O)n. There are two types of bentonite: sodium bentonite, also called swelling bentonite; and calcium bentonite, also called nonswelling bentonite.


Bentonite is useful for a wide range of applications. For instance, sodium bentonite is used in drilling mud for oil and gas wells, in sealing subsurface disposal systems (such as for spent nuclear fuel), and in preventing metal pollutants from entering groundwater. Calcium bentonite is sold as an agent that cleanses the intestinal tract. In addition, bentonite is used in cement, adhesives, ceramic structures, cosmetics, and cat litter. It may also be used in pyrotechnics to make end plugs, and in therapeutic face packs for treating acne.

History and natural occurrence

Sometime after its discovery around 1890, this absorbent clay was named after the Benton Formation (a geological stratum at what was Fort Benton) in Wyoming. Montmorillonite, discovered in 1847, was likewise named after its place of discovery in Montmorillon in the Vienne prefecture of France, in Poitou-Charentes, South of the Loire Valley.

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, Greece, and other parts of the world. Calcium bentonite is mined in the Great Plains, Central Mountains, and southeastern regions of the United States.

Sodium bentonite

Sodium bentonite expands when wet and can absorb several times its dry mass in water. It is mostly used in drilling mud for oil and gas wells and for geotechnical and environmental investigations.

The property of swelling also makes sodium bentonite useful as a sealant, especially targeted for sealing 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—for example, to plug old wells, or as a liner in the base of landfills to prevent migration of leachate into the soil.

Calcium bentonite

The nonswelling calcium bentonite (commercially known as Pascalite) is sold in 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.[4]

Calcium bentonite may be converted to sodium bentonite and exhibit sodium bentonite's properties by a process known as "ion exchange." Commonly, this means adding 5-10 percent of sodium carbonate to wet bentonite, mixing well, and allowing time for the ion exchange to take place.

Uses for both types


Bentonite can be used in cement, adhesives, ceramic bodies, cosmetics, and cat litter. Fuller's earth, an ancient dry cleaning substance, is finely ground bentonite, typically used for purifying transformer oil. Bentonite, at low 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 it can also be used as a therapeutic face pack for the treatment of acne or oily skin.





See also


  1. The Slurrying Test in the Äspö HRL, SKB, 2007. Retrieved May 24, 2007.
  2. Hydraulic behaviour of bentonite based mixtures in engineered barriers: The Backfill and Plug Test at the Äspö HRL, Universitat Polytecnica de Catalunya, 2007. Retrieved May 24, 2007.
  3. Bentonite, Alternative Medicine, 2007. Retrieved May 24, 2007.
  4. Bentonite clay for internal healing,, 2007. Retrieved May 24, 2007.



External links

All links retrieved June 2, 2016.


