GEOCHEMICAL DIFFERENTIATION IN LATERITE AND BAUXITE FORMATION

The geochemical differences between laterite and bauxite formation are discussed under the aspect that both weathering products are predomin antly formed by residual enrichment of iron and aluminium. Residual en richment is corroborated by various arguments and clearly proved for l aterite formation on ultramafic rocks. Tropical weathering of the most widespread acidic rocks leads, in spite of their high Al/Fe ratio, ge nerally to formation of laterites showing a high enrichment of iron wh ereas aluminium is not or only slightly enriched. This chemical variat ion indicates that high amounts of Al are removed additionally to Si i n soil solutions when saprolites on acidic rocks are transformed into laterites containing goethite, hematite and kaolinite as main constitu ents. Dissolved free aluminium exist in equilibrium with kaolinite onl y in minimal contents, hence the removal of Si and Al probably takes p lace in form of freshly formed colloidal or subcolloidal, X-ray amorph ous particles with kaolinitic or similiar composition. The frequently observed occurrence of two generations of secondary kaolinite in later ites corroborates this conclusion. There is general agreement that bau xites are formed by stronger leaching compared with laterites. Increas ed leaching lowers the concentration of dissolved silica, thus enablin g gibbsite instead of kaolinite stability. The concentration of dissol ved free aluminium, however, is increased in equilibrium with gibbsite . Free aluminium shows in contrast to iron an effective solubility at pH = 5, giving rise to downward leaching and re-precipitation at highe r pH below a laterite crust which covers the bauxite zone in most of t he bauxite deposits. In spite of the stronger leaching, bauxite format ion implies a lower mass loss (mainly Si) during weathering than the f ormation of laterites (loss of Si + Al) related to their highly increa sed iron content.