Chemical characteristics of glauconite and ferric illite derived from alteration of K-feldspar in three Proterozoic formations of India are studied. The phases show widely varying octahedral and tetrahedral cation contents, but K-content is nearly constant. Co-existence of glauconite and ferric illite is interpreted as due to influx of fresh water in a marine-dominated sequence. A continuum in composition between illitic and glauconitic minerals at high alkali content is established. No correlation between composition of glauconite and environment of deposition or degree of diagenesis is deduced. Ferric illite in fluvial environment contains less Mg than that in a marine-dominated one. Direct pseudomorphism of K-feldspars by glauconite and ferric illite in their respective chemical environments controlled largely by the availability of Fe and Mg is indicated in this study. Chemical characteristics of glauconite and ferric illite in an environment with high a K (super +) induced by dissolution of K-feldspar are markedly different from those described earlier.