CHEMICAL composition. X-ray diffraction and other analytical data of a series of samples of clays that are composed essentially of illite are presented. The physical properties of these samples are given and related to variations in the composition and character of the illites. Data indicate that these illtes have less potassium than well-crystallzed micas and 

some of them are not essentially mixed-layer structures. It is concluded, therefore, that illite is not necessarily a mixed-layer material.