DISSOLUTION RATES OF METALS IN FE OXIDES - IMPLICATIONS FOR SAMPLING FERRUGINOUS MATERIALS WITH SIGNIFICANT RELICT FE OXIDES

A study of the pattern of dissolution of synthetic and natural Fe oxid es in 6 M HCl indicates that the rate of element release from syntheti c Fe oxides is strongly influenced by mineralogy and the level of elem ent incorporation. Synthetic maghemite (gamma-Fe2O3) samples are subje ct to much more rapid dissolution than goethite (FeOOH) and hematite ( alpha-Fe2O3). In samples dominated by hematite and maghemite, Cu, Zn a nd particularly Pb, in comparison to Fe, are preferentially released d uring the early stages of dissolution. Similar patterns are apparent f rom the dissolution of hematite- and maghemite-dominated samples deriv ed from natural gossan. Comparison of XRD scans with data from the dis solution of natural gossan samples transformed by incremental heating to hematite- and maghemite-dominated assemblages suggests that the deg ree of crystallinity may also be a significant factor in the release o f elements incorporated in the Fe oxides. Ferrginous materials made up of varying proportions of goethite, hematite, maghemite, kaolinite an d quartz are important sampling materials in a range of regolith envir onments. These are products of complex chemical and mechanical mobiliz ation over long periods of geological time. If the patterns of Fe oxid e dissolution in 6 M HCl and the release of incorporated metals reflec t stability in such weathering regimes, knowledge of the retention cha racteristics of incorporated metals in different Fe oxide phases, as i ndicated by this study, will be useful in the planning and interpretat ion of geochemical surveys in such regions. (C) 1998 Elsevier Science B.V. All rights reserved.