Phyllosilicates in samples from Gulf Coast cores and the Martinsburg Formation near Lehigh Gap, Pennsylvania (which together represent a continuous sequence from mudstone to slate) are shown by TEM studies to possess imperfections on a massive scale and to exhibit a progressive increase in perfection through diagenesis and low-grade metamorphism. In shallow portions (1,750 and 2,450 m) of the Gulf Coast core, smectites have irregular and curved layers with high dislocation densities and many small-angle, grainlike boundaries. In both the deeper portion (5,500 m) of the Gulf Coast sediments and in the Martinsburg mudstone samples, gram perfection increases in illites, but a complex mosaic structure with small-angle grain boundaries and edge dislocations still prevails. In the Martinsburg slate, phyllosilicates exhibit layers which are continuous and straight and have few imperfections. Original clays which form at low temperatures have highly imperfect and metastable structures. Changes with increasing diagenesis are akin to annealing processes, with increasing perfection of grains being an approach to a more stable, strain-free structure. Temperature, pore water, time, and other variables affecting these changes act only to promote reaction rates to the defect-free, stable structures.