A mössbauer and Mineralogical Investigation of Soils from Central Sudan

Quaternary soil samples from four localities (El Gezira, El Gunaid, El Mukabrab, and El Gash) in central Sudan were examined by Mössbauer spectroscopy, x-ray diffraction, transmitted light microscopy, and chemical analyses. Quartz, feldspars, and rock fragments are the main components of the detrital portions together with clay minerals, Fe and Ti oxides, and traces of pyroxene and amphibole. The main sources of these materials are the basic igneous and metamorphic rocks of the Abyssinian plateau as well as the eolian sands to the east of the Nile River (particularly for El Mukabrab soil). The less than 2-μm fraction contains smectite and chlorite as the dominant clay minerals with subordinate amounts of kaolinite. Mössbauerdata indicate that about 95% of the iron is in the ferric state which is mainly (up to 85%) located in the structure of smectite and chlorite. Variable amounts (15 to 30%c) of the ferric iron occur in hematite and goethite with roughly equal proportions. A substantial part of these iron oxides have minute grain size (less than a few hundred angstroms), thus showing superparamagnetic behavior in the Mössbauer spectra. Pedogenic-diagenetic effects on these soils are reflected by (1) precipitation of calcite mottles and nodules and gypsum aggregates, (2) relatively homogeneous distribution of hematite and goethite with depth, and (3) alteration of some smectite and chlorite into kaolinite.