Geochemical differentiation in laterite and bauxite formation

Abstract

The geochemical differences between laterite and bauxite formation are discussed under the aspect that both weathering products are predominantly formed by residual enrichment of iron and aluminium. Residual enrichment is corroborated by various arguments and clearly proved for laterite formation on ultramafic rocks. Tropical weathering of the most widespread acidic rocks leads, in spite of their high Al/Fe ratio, generally to formation of laterites showing a high enrichment of iron whereas aluminium is not or only slightly enriched. This chemical variation indicates that high amounts of Al are removed additionally to Si in soil solutions when saprolites on acidic rocks are transformed into laterites containing goethite, hematite and kaolinite as main constituents. Dissolved free aluminium exist in equilibrium with kaolinite only in minimal contents, hence the removal of Si and Al probably takes place in form of freshly formed colloidal or subcolloidal, X-ray amorphous particles with kaolinitic or similiar composition. The frequently observed occurrence of two generations of secondary kaolinite in laterites corroborates this conclusion. There is general agreement that bauxites are formed by stronger leaching compared with laterites. Increased leaching lowers the concentration of dissolved silica, thus enabling gibbsite instead of kaolinite stability. The concentration of dissolved free aluminium, however, is increased in equilibrium with gibbsite. Free aluminium shows in contrast to iron an effective solubility atpH= 5, giving rise to downward leaching and re-precipitation at higher pH below a laterite crust which covers the bauxite zone in most of the bauxite deposits. In spite of the stronger leaching, bauxite formation implies a lower mass loss (mainly Si) during weathering than the formation of laterites (loss ofSi + Al) related to their highly increased iron content.