Brazilian bauxites

Lateritic bauxites are accumulations of aluminium developed, in situ, from quartz-free or poorly quartzitic parent rocks under humid tropical climates. Iron in goethite or hematite also accumulates in bauxitic profiles, associated with or dissociated from aluminium, occurring in gibbsite, boehmite or kaolinite. Four major types of lateritic bauxites ; protobauxites, orthobauxites, metabauxites and cryptobauxites are defined as function of the nature of iron and aluminium minerals, as well as their relative distribution in profiles. Protobauxites are lateritic soils where gibbsite and goethite are formed together under very humid climates. Orthobauxites are gibbsitic and do not exhibit a concretionary structure. Iron as goethite or hematite is concentrated with aluminium. Conakrytes (non-nodular iron-rich accumulation) close to the top of the bauxitic profiles are, thus, frequently associated with orthobauxites. The presence of kaolinite at the bottom of the profiles is not necessary. Metabauxites are boehmitic and show a concretionary or pisolitic structure ; iron is dissociated from aluminium and is frequently concentrated as hematite in a kaolinitic ferricrete, located at the bottom of the bauxitic profile. Kaolinite appears always at the bottom of metabauxites profiles and, less frequently, at the base of orthobauxites. In cryptobauxites, kaolinite is abundant at the top and at the bottom of the profiles, so that the gibbsitic layer is interbedded between two kaolinitic horizons. The lateritic bauxite classification is based on reactions of hydration-dehydration and of silication-desilication, regulated by temperature, water activity and chemical composition of the parent material. (D'après résumé d'auteur