Montmorillonitic Soils Developed in Nile River Sediments

Calcareous soils developed in widely varying textures of Nile River alluvium of the UAR and the Republic of Sudan during the Quaternary show many physicochemical and mineralogical properties in common, including neutral to alkaline reaction (pH 7 to 8), and about 2 percent organic matter. X-ray diffraction analysis shows that the clay mineralogy is dominated by montmorillonite, with moderate contents of mica, vermiculite, chlorite, and kaolinite. Regularly and irregularly mixed-layer phyllosilicates are abundant. A conspicuous amount of hydroxy (Al, Fe) coatings occur on the montmorillonite layer surfaces. Quantitative analyses of the clay fraction showed a dominance of montmorillonite (40 to 59 percent), with appreciable kaolinite (7 to 18 percent), mica (6 to 13 percent), vermiculite (1 to 11 percent), chlorite (3 to 27 percent), and amorphous material (7 to 12 percent). Quartz and feldspars (0.7 Na feldspar to 0.3 K feldspar) make up 1 to 9 percent and 1 to 6 percent of the clay fraction, respectively. The sand and silt size fractions are dominated by quartz (6 to 65 percent), feldspars (6 to 29 percent), and mica (1 to 19 percent). Weathering of basic volcanic rocks, underlying sandstone and granite in the headwater areas appears to have dominated the mineralogical composition of the alluvium. Detrital minerals wind-carried from the desert appear to have added mica, vermiculite, and quartz to the soils, more vermiculite south of Khartoum and more mica in the delta.