brazil is one of the world's leading producers of bauxite. little is known, however, about the chemical and mineralogical characteristics of brazilian bauxite. the objective of this paper is to characterize bauxite in both natural and thermally and chemically activated forms. the transformations occurring during these two processes are described. the results show that the raw bauxite is basically composed of the mineral gibbsite, followed by semicrystalites of goethite and silicon oxide. its characteristics are those of a solid without mesoporosity, with small total surface area and pore diameter and volume. the chemical and physical modifications that occur in the material during the calcination process are governed mainly by the dehydration of the aluminum hydroxides phases, leading to the formation of the boehmite, hematite and alpha alumina phases. calcination results in an increase in the porosity and surface area of the bauxite as well as the appearance of the characteristics of mesoporous and thermally stable materials. the activation of bauxites in an acid medium showed that the transformations in the solid are directly related to the concentration of the acid solution used and result in an increase in the total surface area and pore diameter and volume.