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Characterization by Mössbauer spectroscopy of Fe phases in highly weathered serpentinitic soil from southern Cameroon

¹ Department of Subatomic and Radiation Physics, Ghent University, Proeftuinstraat 86, 9000 Ghent, Belgium, ² Department of Geology and Soil Science, Ghent University, Krijgslaan 281 (S8), 9000 Ghent, Belgium, and ³ Geovic Cameroon PLC, BP 11555, Yaounde, Cameroon

^* E-mail: caroline.vancromphaut{at}ugent.be

(Received 1 April 2007; revised 27 September 2007)

Weathered soil material derived from tectonically emplaced serpentinized ultrabasic intrusive rocks of southern Cameroon has received considerable attention from mining companies due to its extractable-metal (i.e. Ni, Co) potential. As these cations can be incorporated into Fe oxides, it was deemed appropriate to study the mineralogical assemblage of a highly weathered serpentinite soil profile from the area. This study focuses on the different Fe-oxide phases, which were investigated using ⁵⁷Fe Mössbauer spectroscopy, showing goethite and hematite as the dominant Fe oxides throughout the weathering profile. These minerals, in association with gibbsite and kaolinite, indicate an advanced degree of weathering. The clay fraction of the ‘Lower Limonite’ layer, above the saprolite and at a depth of 7 m, is very rich in goethite, whereas hematite and magnetite are almost absent. Above this layer, the hematite content in the fine-earth and clay fractions increases upwards, while the goethite content remains constant. The significant substitution and change in the particle size of the goethite and the poor crystallinity of hematite, as indicated by the hyperfine parameters and XRD, suggest that the upper material evolved under different pedological conditions compared to the deeper layers. The mixed composition of the upper layers (above 7 m), which contain muscovite and a relatively chaotic distribution of trace elements, suggests ancient mica-schist capping and possibly different cycles of erosion and pedimentation.

KEYWORDS: serpentinitic soil, Cameroon, Mössbauer spectroscopy, X-ray diffraction