Illite: Illite mineral information and data. - MindatMica Group . This clay-like series is essentially a K-deficient muscovite , but frequently contains randomly sequenced montmorillonite / beidellite layers. Illite is.illite clay mineral,Clays and Clay Minerals - UCLOne of the commonest processes of clay formation is the chemical decomposition of . The main group of clay minerals are kaolinite, illite and montmorillonite.