Bentonite

Not to be confused with the Ba,Ca,Mg-mixed carbonate mineral Benstonite.
For the chromium-rich ettringite mineral, see Bentorite.
For the barium titanium cyclosilicate mineral, see benitoite.

Bentonite layers from an ancient deposit of weathered volcanic ash tuff in Wyoming
Gray shale and bentonites (Benton Shale; Colorado Springs, Colorado)

Bentonite (/ˈbɛntənaɪt/ BEN-tə-nyte)[1][2] is an absorbent swelling clay consisting mostly of montmorillonite (a type of smectite) which can either be Na-montmorillonite or Ca-montmorillonite. Na-montmorillonite has a considerably greater swelling capacity than Ca-montmorillonite.

Bentonite usually forms from the weathering of volcanic ash in seawater, or by hydrothermal circulation through the porosity of volcanic ash beds,[3][4] which converts (devitrification) the volcanic glass (obsidian, rhyolite, dacite) present in the ash into clay minerals. In the mineral alteration process, a large fraction (up to 40–50 wt.%) of amorphous silica is dissolved and leached away, leaving the bentonite deposit in place.[citation needed] Bentonite beds are white or pale blue or green (traces of reduced Fe2+
) in fresh exposures, turning to a cream color and then yellow, red, or brown (traces of oxidized Fe3+
) as the exposure is weathered further.[5]

As a swelling clay, bentonite has the ability to absorb large quantities of water, which increases its volume by up to a factor of eight.[5] This makes bentonite beds unsuitable for building and road construction. However, the swelling property is used to advantage in drilling mud and groundwater sealants. The montmorillonite / smectite making up bentonite is an aluminium phyllosilicate mineral, which takes the form of microscopic platy grains. These give the clay a very large total surface area, making bentonite a valuable adsorbent. The plates also adhere to each other when wet. This gives the clay a cohesiveness that makes it useful as a binder and as an additive to improve the plasticity of kaolinite clay used for pottery.[6]

One of the first findings of bentonite was in the Cretaceous Benton Shale near Rock River, Wyoming. The Fort Benton Group, along with others in stratigraphic succession, was named after Fort Benton, Montana, in the mid-19th century by Fielding Bradford Meek and F. V. Hayden of the U.S. Geological Survey.[4] Bentonite has since been found in many other locations, including China and Greece (bentonite deposit of the Milos volcanic island in the Aegean Sea). The total worldwide production of bentonite in 2018 was 20,400,000 metric tons.[7]

  1. ^ "bentonite". Lexico UK English Dictionary. Oxford University Press. Archived from the original on April 27, 2021.
  2. ^ "bentonite". Dictionary.com Unabridged (Online). n.d.
  3. ^ Nesse, William D. (2000). Introduction to mineralogy. New York: Oxford University Press. p. 257. ISBN 9780195106916.
  4. ^ a b Sutherland, Wayne M. (September 2014). "Wyoming Bentonite" (PDF). Wyoming State Geological Survey. Retrieved 12 January 2021.
  5. ^ a b Jackson, Julia A., ed. (1997). "Bentonite". Glossary of geology (Fourth ed.). Alexandria, Virginia: American Geological Institute. ISBN 0922152349.
  6. ^ Nesse 2000, pp. 252–257.
  7. ^ T. Brown et al. 2020. World Mineral Production 2014–18 Archived 2022-04-18 at the Wayback Machine. British Geological Survey, Nottingham, England.