The mobility of zirconium and identification of secondary Zr-bearing phases in bauxite from Pocos de Caldas, Minas Gerais, Brazil: A mass-balance andx-ray absorption spectroscopic study

The geochemistry and mineralogy of Zr have been studied in a bauxitic profi le derived from a nepheline microsyenite at Pocos de Caldas, Minas Gerais, southeastern Brazil. The Zr in the parent rock is hosted by the sorosilicat e hainite (5-6% ZrO2) and a Ca-Na amphibole (0.1-0.2% ZrO2). Comparison of the variations in Zr, Ti, Nb and Th contents as function of the apparent de nsity of different zones in the top few meters of the saprolite shows Th to be the least mobile element of the profile. Therefore, it was chosen as an invariant constituent in order to evaluate the mobility of other elements, especially the Zr. Mass-balance calculations demonstrate the leaching of Z r all along the profile, with a rate of removal of up to 40% at the weather ing front. In the residual bauxitic materials, Zr is essentially localized in ferruginous products such as goethite-rich plasma Filling the pores, and granules within the gibbsite-dominant matrix. Its mineralogical expression appears to be within polyhedra with an average of 6.6 O atoms, and average Zr-O distances of 2.16 Angstrom, probably in adsorption on the surface of goethite in the pores and in granules of the gibbsite-rich matrix.