Smectite Mineralogy and Charge Characteristics along an Arid Geomorphic Transect

Smectites are a major mineral group often found in the clay fraction of soils. Our objective was to evaluate the effect of pedologic environments on smectites in the arid Carrizo Plain, California. Six pedons (one Argixeroll, three Haploxeralfs, one Natrargid, and one Salorthid) were described and sampled along a geomorphic and salinity gradient transect. The transect soils have formed in alluvial parent materials derived from Miocene marine sandstones, siltstones, and shales. The fine clay fractions of transect soils were studied using x‐ray diffraction, interlayer swelling with alkylammonium cations, and chemical analyses. Both beidellitic and montmorillonitic smectites occurred in the transect soils, suggesting multiple origins for these minerals. The chemical composition of the beidellites are similar to those of a tetrahedrally charged, dioctahedral mica. Since mica was identified in shale parent materials and in the fine silt and clay fractions of the soils, the beidellites are apparently a weathering product of mica. Minor variations in the chemical composition of mica precursors may account for observed differences in beidellite charge characteristics. The chemical composition of montmorillonites was variable, making a common origin for these minerals unlikely. Montmorillonites from saline pedons contained more Fe and Mg and less Al in the octahedral sheet than montmorillonites from nonsaline pedons. Montmorillonites in the nonsaline pedons are probably inherited from montmorillonites identified in parent materials, whereas montmorillonites in saline soils may be neoformed. Groundwater within the Salorthid was supersaturated with respect to both montmorillonite and beidellite.