Bentonite

Uncommon card · No. 114670

Bentonite ( BEN-tə-nyte) 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, which converts (devitrification) the volcanic glass (obsidian, a volcanic glass with a chemical composition equivalent to rhyolite) 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. 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. As a swelling clay, bentonite has the ability...

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