Comparative Study of Heavy Metal-Host Minerals in Ultramafic-Derived Soils in Zimbabwe and Texas, US.

Ultramafic-derived soils are particularly significant to agriculture and environment because of their unique chemistry and mineralogy. Soil chemistry in these environments is largely influenced by the minerals present in the soil at a particular locality, which control the release and sequestration of trace metals. We have studied the mineralogy and chemistry of soils of ultramafic origin from Texas and Zimbabwe. The objectives of the study were to determine trace metal concentrations in soil and to identify the trace element-host minerals, and assess their relative significance to human health and overall environmental quality. Metal concentrations were determined by neutron activation analysis and dissolution techniques. Mineral characterization was conducted by x-ray diffraction, Fourier transform infrared spectroscopy, transmission electron- and scanning electron microscopy. The results showed wide variations in soil mineral type, compositions and abundances across the sites. Serpentine, talc and chlorite occurred in all soil particle size fractions. Pyroxenes and amphiboles were predominant in the sand and silt fractions. All the soils contained high total concentrations of Cr, Ni, V, Ti and Fe, and their concentrations were related to mineral composition and abundance. The potential impact of element-host minerals to human health and the environment are discussed in terms of their chemical composition, morphology and relative ease of transformation and weatherability.