Abstract: Mafic chlorite from Benton, Arkansas was comminuted by rotary blending of a suspension, and the −2 µm fraction separated by sedimentation in H2O. Droplets of suspension of the < 2 µm fraction were dried on a layer of Epoxy resin and then additional Epoxy was added and heat-cured at 48°C to form a resin sandwich. Cross-sections of 600–900 Å thickness were cut on a Reichert automated ultramicrotome. The sections were collected on standard electron microscope specimen screens, reinforced by vacuum evaporated C and examined by transmission electron microscopy (TEM). The Phillips EM 200 electron microscope was equipped with a “microgun” source to minimize heating of the specimen and to improve contrast and high resolution (HREM). Images of the (001) chlorite crystallographic planes spaced at 13·9 Å intervals were visible on many of the particle sections. Imaging of the planes depended upon their being nearly parallel to the electron beam (within 0° 10′) and therefore, many particles which had other orientations did not show the 13·9Å image. Micrographs made before appreciable irradiation by the electron beam revealed images of fringes corresponding to the 7·22 Å (002) spacing of chlorite. Loss of the 7·22 Å fringes and reinforcement of those at 13·9 Å resulted from heating of the chlorite in the electron beam. This behavior is analogous to the well-known crystallographic effects of heating chlorite at 550–760°C.