The Relation between “Illite,” Beidellite, and Montmorillonite1

Margaret D. Foster
U. S. Geological Survey, Washington
1 Publication authorized by the Director, U. S. Geological Survey.

Abstract: Formulas calculated from analyses, written in the fractional notation used by Ross and Hendricks, and with a notation of tetrahedral and octahedral charge, reveal relationships between the hydrous micas called “illite,” montmorillonites (using the term in the restricted sense), and beidellites not otherwise apparent. These relationships are shown graphically by means of histograms made up of five columns, representing respectively, total charge, tetrahedral charge, octahedral charge, potassium, and water.

“Illites” have a much higher total charge and a much higher tetrahedral charge than montmorillonites, of which the tetrahedral charge is usually insignificant. Consequently simple removal of fixed K and its replacement by exchangeable cations would not convert an “illite” to a montmorillonite nor would mere fixation of K between the layers of a montmorillonite result in the formation of an “illite” like any that have been described.

From the standpoint of octahedral-tetrahedral charge relationships, beidellites and “illites” should be more similar than montmorillonites and “illites.” However, considerable doubt has been cast by Grim and Rowland on the purity of most of the materials that have been called beidellites; they interpreted the differential thermal curves of the beidellites examined by them as indicating mixtures of kaolinite or halloysite, “illite,” and montmorillonite. Formulas based on calculated mixtures of these minerals in varying proportions were very similar to formulas calculated from analyses of the so called beidellites. Thus the existence of a montmorillonite clay like that which would be formed if the fixed K in “illite” were removed and replaced by exchangeable cations has not been authenticated and the concept that montmorillonite and “illite” are analogous except for the presence or absence of nonexchangeable potassium is misleading and is an oversimplification of the relationship between the two groups of minerals.

Clays and Clay Minerals; 1953 v. 2; no. 1; p. 386-397; DOI: 10.1346/CCMN.1953.0020132
© 1953, The Clay Minerals Society
Clay Minerals Society (www.clays.org)