Bentonite clay is used in desiccant bags. The name comes from Benton Shale in Wyoming. It consists mostly of montmorillonite, a mineral named after Montmorillion in France.
The individual crystals of montmorillonite clay are not tightly bound. Water can therefore enter and cause the clay to swell. Chemically, it is hydrated sodium calcium aluminium magnesium silicate hydroxide (Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2·nH2O. Potassium, iron, and other cations are common substitutes, and the exact ratio of cations varies with source.
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