Soil Mineral Composition in Contrasting Climatic Regions of the Great Dyke, Zimbabwe

Abstract

Ultramafic-derived soils impact agricultural productivity and environmental quality. The mineralogy of soils developed from ultramafic parent materials of the Great Dyke, Zimbabwe, at Mpinga (∼800 mm mean annual rainfall) and Bannockburn (∼500 mm) were investigated to evaluate possible mineral transformation pathways under different climatic regions and landscape positions. Mineral characterization was conducted by x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). Clay minerals observed at both study sites included talc, chlorite, vermiculite, Fe oxides, and Fe- and Cr-rich smectite. Serpentine occurred at the Mpinga site, but only at specific landscape positions, suggesting the influence of local transport and weathering phenomena as well as parent rock composition. Palygorskite was observed at the Bannockburn site only, in association with calcic horizons. Unique to these soils was the absence of micas, the negligible presence of feldspars and relatively smaller amounts of kaolinite. The majority of the clay minerals were inherited from the parent materials by physical and chemical weathering. Iron- and Cr-smectite and Fe oxides were formed by precipitation from saturated solutions and the hydrolytic oxidation of the Fe-rich parent materials, respectively. The occurrence and composition of the primary and secondary clay minerals of the Great Dyke reflect the composition of parent materials and the localized variations in climatic conditions. Localized differences in mineralogy point toward the importance of understanding the localized mineralogy and chemistry as an aid in developing effective site-specific agricultural and environmental management strategies.