Authigenic illites occur in the pores of many sandstone formations which are known to contain hydrocarbon reservoirs. In particular the Norphlet Formation (Jurassic, Mississippi), Wilcox Formation (Eocene, Texas) and Fort Union Formation (Paleocene, Wyoming) contain large amounts of authigenic illites. These illites occur as laths with perfectly developed morphologies. Laths have widths of 0.1 to 0.3 microns, thicknesses up to 200 A, and lengths ranging up to 30 microns. Elemental analyses of the laths reveal Si, Al, and K as major constituents. Transmission electron images show that these "hair-like" illites are associated with irregular "blob-like" cores which have a chemistry very similar to the laths. The laths have a 1M stacking sequence and a -axis elongation. They are extremely fragile and easily break into very fine particles (less than 0.1 mu m in length). The presence of hairy illites in sandstone pores greatly increases micropososity and pore tortuosity, and decreases the permeability. The fragile nature of the illites can result in a migration of fincs problem during hydrocarbon production.