Smectites of sedimentary series from a playa lake system from Calatayud Basin have been studied by XRD and TEM chemical microanalyses. Clay particle microanalyses show complete continuity between dioctahedral aluminous and trioctahedral magnesian smectites. Aluminous smectites have been classified as montmorillonites and ferribeidellites, both with compositional variability. They represent detrital phases resulting from weathering in the source area. Compositionally-intermediate smectites (beidellite-saponite and montmorillonite-stevensite) seem to correspond to weighted mean compositions of di- and trioctahedral phases stacked together in the same particle. They represent intermediate transitional stages from detrital to authigenic smectites in more distal basin facies. Trioctahedral smectites, although called saponites and stevensites, are actually random mixed-layer kerolite-Mg-smectite with a high percentage of expandable layers. They represent authigenic clays formed from solutions with high pH and Mg content as a result of evaporative concentration. The variability in composition of smectites, both within the same sample and amongst different samples, is a possible consequence of the heterogeneity in the local chemical environment, and so, equilibrium may be only reached locally in a large variety of microsystems.