Morphological, physical, chemical and mineralogical properties, as well as the weathering processes were studied in a Red Yellow Argisol (Aridic Kandiustalf) (well drained) and a Solodic Planosol (Aridic Haplustalf) (partially drained), developed from gnaisses, in a topossequence of the central semiarid region of Ceará State, Brazil. The study had the purpose of investigating the influence of soil locations in the landscape (internal drainage) and on the diversification of soil properties, as well as to study processes of surface transport and deposition on the homogenous parent material. Coarse sand and silt were studied using polarized microscopy and phase contrast, while fine silt and clay were analyzed by X-ray diffraction. Additional information on mineralogical alterations were obtained using a scanning electron microscope. Sand and coarse silt presented large contents of biotite in the Red Yellow Argisol (PVAe) and biotite plus hornblende in the Solodic Planosol (SXen), with a considerable decrease from lower horizons towards the surface. In both soils, quartz and plagioclases were also found within these fractions. Dominant clay minerals of the PVAe were kaolinite and vermiculite in the clay fraction with occurrence of quartz, and kaolinite in the fine silt with variable contents of biotite, plagioclases and quartz. On the other hand, kaolinite and montmorillonite are dominant in the SXen, but quartz and a regular interstratified mineral (vermiculite + mica) were also observed in some horizons. In the fine silt, together with the above-mentioned primary minerals, kaolinite, illite, vermiculite, montmorillonite and the regular interstratified mineral were observed in variable contents for every horizon. The internal drainage of both soils promoted different conditions for the transformations of biotite, hornblende and plagioclases: the first two initially form 2:1 clay minerals, while plagioclases change directly to kaolinite. Clay contents in the argillic horizon in both profiles were essentially a result of weathered primary minerals from the subjacent rock.
mineralogy; mineralogical transformations; internal drainage; semiarid region; Brazil