Ultrathin sections of reference 2:1 layer silicates treated with octadecylammonium cations were examined using high-resolution transmission electron microscopy (HRTEM) to establish the layer structure. Hitherto, few HRTEM ultrathin-section data existed on the expansion behavior of smectite-group minerals with different interlayer-charge values. Without such information, the expansion behavior of both low-charge and high-charge smectite minerals cannot be characterized and the structures observed in HRTEM images of clay-mineral mixtures cannot be interpreted reliably. Reference smectite-group minerals (Upton, Wyoming low-charge montmorillonite; Otay, California high-charge montmorillonite; a synthetic fluorohectorite; and a Jeanne d’Arc Basin offshore Newfoundland clay sample) with a range of layer charge values were examined. To prevent possible intrusion of epoxy resin into interlayers during embedding, the clay samples were first embedded in epoxy, sectioned with an ultra microtome, and then treated with octadecylammonium cations before examination using HRTEM. Lattice-fringe images showed that lower-charge (<0.38 eq/O10(OH)2) 2:1 layers had 13–14 Å spacings, whereas higher-charge (>0.38 eq/O10(OH)2) 2:1 layers had 21 and 45 Å spacings. These differently expanded silicate layers can occur within the same crystal and an alternation of these layer types can generate rectorite-like structures. For comparison, clay samples were also treated with octadecylammonium before epoxy embedding and sectioning and then examined with HRTEM. These samples mostly had highly expanded interlayers due to epoxy intrusion in the interlayer space. The reference clay minerals embedded in epoxy resin, sectioned, and treated with octadecylammonium cations were used to characterize smectite-group minerals in a natural clay sample from the Jeanne d’Arc Basin, Eastern Canada. Smectite-group minerals in this sample revealed similar structures in lattice-fringe images to those observed in the pure reference clay samples. Rectorite-like structures observed in lattice-fringe images were in fact smectite crystals with short, alternating sequences of low-charge and high-charge smectite layers rather than illite-smectite (I-S) phases with expanded smectite layers and non-expanded 10 Å illite layers.