In contrast, beidellite is smectite with greater than 50 % tetrahedral charge originating from isomorphous substitution of Al for Si in the silica sheet.
3.
Montmorillonite is a subclass of smectite, a 2 : 1 phyllosilicate mineral characterized as having greater than 50 % octahedral charge; its cation exchange capacity is due to isomorphous substitution of Mg for Al in the central alumina plane.
4.
Unlike permanent charges developed by isomorphous substitution, pH-dependent charges are variable and increase with increasing pH . Freed cations can be made available to plants but are also prone to be leached from the soil, possibly making the soil less fertile.
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