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 Eas t Africa. The reagents used were dithionite-citrate-bicarbonate (DCB), citrate-bicarbonate (CB), hydroxylamine hydrochloride (HH), and oxali c acid-oxalate (Tamm). From the results obtained, it appears that: (1) the difference DCB minus CB is a better estimator of the metal fracti on in Fe oxides than the difference DCB minus oxalic acid-oxalate, esp ecially in presence of gibbsite; (2) the use of HH allows to be distin guished the specific contribution of Mn oxide; and (3) geochemical mod els for goethite must provide for the existence of ternary solid solut ions (Fe, Al, Cr). With regard to the geochemical properties of the el ements, it can be concluded that: (1) Cr substitutes for Fe in the sam e proportions in goethite, hematite, and maghemite; (2) in contrast, A l substitutes largely for Fe in goethite, little in hematite, and not at all in maghemite; (3) Mn substitutes partly for Fe, but forms discr ete 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 sp inel; and (6) Ni is closely associated with goethite and not with Mn o xides and spinel.