TRACE-METALS IN NATURAL IRON-OXIDES FROM LATERITES - A STUDY USING SELECTIVE KINETIC EXTRACTION

To determine the extent of metal association in natural iron oxides, a combination of XRD and chemical selective dissolution techniques was applied to four samples from laterites developed on peridotites in East Africa. The reagents used were dithionite-citrate-bicarbonate (DCB), citrate-bicarbonate (CB), hydroxylamine hydrochloride (HH), and oxalic acid-oxalate (Tamm). From the results obtained, it appears that: (1) the difference DCB minus CB is a better estimator of the metal fraction in Fe oxides than the difference DCB minus oxalic acid-oxalate, especially in presence of gibbsite; (2) the use of HH allows to be distinguished the specific contribution of Mn oxide; and (3) geochemical models for goethite must provide for the existence of ternary solid solutions (Fe, Al, Cr). With regard to the geochemical properties of the elements, it can be concluded that: (1) Cr substitutes for Fe in the same proportions in goethite, hematite, and maghemite; (2) in contrast, Al substitutes largely for Fe in goethite, little in hematite, and not at all in maghemite; (3) Mn substitutes partly for Fe, but forms discrete phases when total Mn content is high; (4) Ti substitutes for Fe in hematite, but not in goethite; (5) the major Cu-bearing phase is a spinel; and (6) Ni is closely associated with goethite and not with Mn oxides and spinel.