noun Mineralogy.
a clay formed by the decomposition of volcanic ash, having the ability to absorb large quantities of water and to expand to several times its normal volume.

1895–1900; named after Fort Benton, Montana; see -ite1

bentonitic [ben-tn-it-ik] , adjective Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
bentonite (ˈbɛntəˌnaɪt)
a valuable clay, formed by the decomposition of volcanic ash, that swells as it absorbs water: used as a filler in the building, paper, and pharmaceutical industries
[C19: from Fort Benton, Montana, USA, where found, + -ite1]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Encyclopedia Britannica


clay formed by the alteration of minute glass particles derived from volcanic ash. It was named for Fort Benton, Mont., near which it was discovered

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
Bentonite slurry, a gelatinous blend of clay and water, was pumped into the
  growing hole to prevent the sides from caving in.
Bentonite in large concentrations is sticky, used for drilling muds on oil rigs
  and also for forming clay molds for metal casting.
The mud is made from a variety of materials such as the clay mineral bentonite.
Bentonite is an extremely absorbent, granular clay formed from volcanic ash.
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