Sand, silt and clay fraction mineralogy as well as the weathering processes of two soils with excess of sodium (SPL and SS) were studied in the state of Ceará, Brazil. The sand coarse and silt were studied using polarized microscopy and phase contrast, while the fine silt and clay were analyzed by X-ray diffraction. Additional information on the mineralogical transformations that occurred in the horizons of both pedons was obtained using scanning electron microscopy. The Solodic Planosol showed in the clay fraction almost equivalent amounts of kaolinite and montmorillonite in the 2Btn horizon, the same occurring with the proportions of kaolinite and vermiculite in the A horizon. In the other fractions, variable contents of quartz, plagioclase, hornblende and biotite were found. On the other hand, the Solodized-Solonetz had essentially kaolinit clay fraction all over the profile, with variable contents of quartz, feldspar and biotite in the other fractions. In both profiles, the clay enrichment of the argillic horizon seemed to be a result of the weathering of the primary minerals from the underlying parent material. The assumption of alternate oxi-reduction processes in the A and, or, E horizons due to the low 2Btn horizon permeability, which induces a process of ferrolysis on those horizons, is also an alternative genesis which cannot be disregarded to explain the high textural contrast of these soils. The weathering of albite, as well as of the other plagioclases rich in sodium, associated with the semi-arid climate and impeded drainage, are the factors that cause the high sodium percentage observed on both pedons. Also on both soils, the contrasting quartz contents, the geomorphology of the region and the presence of a stone line separating the A or A + E horizons from the 2Btn one, reveal a duplicity of parent materials.
mineralogical transformations; alkalinity; semi-arid region