Bentonite

n. clay made of volcanic ash

Bentonite is an absorbent aluminium phyllosilicate generally impure clay consisting mostly of montmorillonite, (Na,Ca)0.33(Al,Mg)2Si4O10(OH)2·(H2O)n. 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.
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Noun
1. an absorbent aluminum silicate clay formed from volcanic ash
(hypernym) clay

n. bentonite, clay made of volcanic ash

A colloidal clay, expansible when moist, commonly used to provide a tight seal around a well casing.