Abstract: X-ray diffraction techniques were used to identify the clay mineral assemblages of Pennsylvanian underclay samples taken mainly from beneath the Colchester (No. 2) and Herrin (No. 6) Coals of Illinois and the Middle Kittanning Coal of Ohio. The clay minerals present are kaolinite, illite, chlorite, vermiculite, montmorillonite, and mixed-layer clay minerals. The minerals are not restricted to any particular stratigraphic unit, and their areal distribution is related to paleogeographic position in the original basin of deposition. Kaolinite shows a decrease in abundance from near shore to more basinward areas; illite is not present in near shore regions, becomes common basinward, and then shows a decrease with increasing distance from shore; chlorite is not found in near shore or in extreme basinward areas but is most common slightly basinward of the illite maximum; montmorillonite is fairly rare but is prominent in the most basinward positions; vermiculite is uncommon but usually occurs where kaolinite is the dominant mineral; mixed-layer clay minerals are present in all samples, constituting a small fraction of the clay minerals in the near shore regions but increases to the major constituent in the most basinward areas.
Clay minerals entering the basin of deposition during underclay formation were mainly kaolinite and mixed-layer clay minerals. As the mixed-layer clays were carried basinward into waters of increasing salinity, some of the mixed-layer clays, formed from weathered illite and chlorite, preferentially adsorbed potassium and magnesium and were regraded. Mixed-layer clay minerals that had undergone severe structural damage during weathering were not regraded and were deposited further basinward.
Maps showing lateral variation of clay minerals in an underclay also show the paleogeography. These maps may be used in outlining positive areas and deltas, or specific types of clay deposits.