An application of the Rietveld refinement method to the mineralogy of a bauxite-bearing regolith in the Lower Amazon

A comparison of Rietveld refinement results for a bauxite-bearing regolith and its clayey cover in the Amazon region was made with stoichiometric calculations from chemical analysis and partly from thermogravimetric results. For this investigation a profile in the bauxite-bearing regolith occurrence in the ALCOA bauxite mine at Juruti, Brazil was studied. The different minerals, their compositions and their low crystallinity in the different horizons were investigated and the contents determined. It is evident that some minerals show several generations and some chemical composition changes that must be included in the Rietveld refinement. Al-rich hematites and goethites are common along the bauxite profile. Amorphous contents were determined with rutile added as an internal standard and shown to have gibbsite- or kaolinite-like composition. The minerals could be quantified in the different horizons and the difficulties were mainly related to variable crystalline aspects of the phases. Rietveld refinement can be a powerful tool in bauxite prospecting, quality control and during mining and beneficiation of ore minerals using the adapted refinement strategies.