Abstract: The Middle Paleozoic shales of northern Ohio contain a well-crystallized variety of illite as their main constituent. Powder diagrams relate the illite to the two-layer monoclinic muscovite crystallization. The low asymmetry of the 10 A reflection indicates that interlayering of non-mica or hydrated-mica units is small. The relative intensities of the basal reflections show that the illites approach the ideal muscovite composition but have about one fourth the octahedral aluminum replaced by iron and are deficient about one-half or less the interlayer potassium. The interlayer positions are essentially anhydrous. One-dimensional Fourier syntheses made before and after thermal treatment of the illites indicate that intensity changes of the basal reflections due to thermal treatment are related to electron density changes confined to the octahedral layer. Dehydroxylization begins at moderately low temperatures and continues until an anhydride structure is formed.