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Aridisols with smectite B horizons are usually classified as Argids although the argillic characters may not always be present. In the Chihuahuan Desert of northern Mexico, some Aridisols have clayey horizons that do not seem to result from clay illuviation. Our purpose was to determine the origin of the clay enrichment and the mineralogical relationships between the parent materials and the B horizons. The study area is the Mapimi Playa, derived from a lutite lagoonal sediment parent material. A soil sequence from Orthids to Argids was identified. Four profiles were analyzed. The particle-size distribution was determined by the sedimentation and centrifugation methods. The clay fractions were studied by x-ray diffraction and peak decomposition, infrared spectroscopy, and chemical analyses. Soil water extracts were analyzed with an absorption spectrometer. From the Orthids to the Argids, clay enrichment in B horizons is controlled by the <0.2-mu m clay size fraction and small amounts of palygorskite. The B horizons contain prominent smectite, which has more Mg than the lutite smectite (parental material). The existence of the AlMgOH infrared vibrations in the soil clay suggests the neogenesis of a smectite different from the lutite smectite. The composition of the soil water extract suggests that Mg-smectite and palygorskite may form in Mapini soil profiles. The Argids of Mapimi Reserve result principally from clay neogenesis under current climatic conditions.