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Bentonite is a fine-grained clay formed from volcanic ash by the hydration (formation of hydrates or absorption of water) of, and loss of bases and perhaps silica from, tiny particle of volcanic glass present in the ash. Sodium bentonites, which expand considerably when saturated with water, are used diversely, as in the making of paper, and sealing of dams. Calcium bentonites are used in the making of fuller's earth.

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Source: U.S. Geological Survey
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