A dioctahedral member of the smectite group of minerals. A representative formula is: R0.33(Al1.67Mg0.33)Si4O10(OH)2^.nH2O. Montmorillonite is Al-rich and capable of cation exchange (the exchangeable cation is depicted as R in the formula, n is a rational number, not necessarily an integer). The origin of the layer charge is in the octahedral sheet. In older literature, montmorillonite was used as a group name, which is replaced by the group name, smectite.
See also Part 1 of the Glossary for terms that are obsolete: Wyoming-type, Otay-type, Chambers-type, Tatatila-type, beidellite-type (ideal and non-ideal), and non-ideal montmorillonite.
Cf., smectite