Bentonite is an absorbent aluminium phyllosilicate generally impure clay consisting mostly of montmorillonite. The absorbent clay was given the name bentonite by an American geologist sometime after its discovery in about 1890 - after the Benton Formation (a geological stratum, at one time Fort Benton) in Montana's Rock Creek area. 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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