The Clay Minerals in Illinois Limestones and Dolomites

Studies of the clay minerals contained in the fine fraction of the insoluble residues from thirty-five Illinois limestone and dolomite samples representing the major stratigraphic units of the Illinois geologic column reveal the presence of illite in all samples, kaolinite in twenty samples, and beidellite (?) in three samples. Much of the illite is thought to be authigenic and probably derived from beidellite; the kaolinite is considered to be detrital. Kaolinite characterizes all the Pennsylvanian and Silurian samples studied. Some Mississippian and some Devonian samples contain kaolinite, others do not. The Ordovician samples contained only illite. On the basis of data relating to the climatic conditions under which various day minerals are formed, it is believed that the joint presence of illite and kaolinite in a formation indicates a source area some parts of which had a warm climate and other parts a cool climate. The results of the study suggest that clay-mineral data may be valuable for correlation purposes, and also that they are of considerable potential economic value.