Results of XRD, TEM, and chemical analyses are combined to characterize the true chemistry and structural formula of the smectite-group minerals present in selected samples of deep-sea sediments from the north-central Pacific and Angola, Lau, and North Fiji Basins. In most cases, the abundant Fe and K in the <0.2-μm fraction is associated with accessory minerals such as mica, phillipsite, and iron oxides. Bulk analytical data obtained from the <0.2-μm fraction, corrected for the presence of these minerals, reveal a structural formula very close to that of ideal montmorillonite. The extensive chemical variability among members of the clay-mineral group usually is attributed to the occurrence of solid solution. The variability in smectite-group minerals also could be due to (1) the occurrence of mixed-layer structures, (2) the presence of compositionally distinct domains in the structure, or (3) a mixture of two or more distinct phases. A clear distinction between a monomineralic solid-solution phase and a mixture of coexisting end-members based on chemical composition or even conventional XRD and HRTEM techniques is of crucial importance. The data presented in the literature concerning the nature of the smectitegroup phase and related I/S mixed layers do not necessarily reflect the true nature of the individual clay-mineral phases. The application of the term “smectite” to a material characteized by a structural formula should be avoided.