Estudo cristaloquímico de minerais de nióbio e tântalo: desordem e mudança de simetria cristalográfica em ixiolita do pegmatito de Volta Grande-MG

Abstract

The context of production and use of niobium and is very strategic for Brazil, because its used by all developed economies and our country concentrates approximately 97 % of all known reserves of this mineral on the planet. This is a chemical element used mainly as adittive for steel, giving them more lightness and strength among many other applications. Niobium is obtained from mineral sources, and the mineralogical study of these sources is very important because in general niobium minerals are also sources of tantalum - another chemical element with economical and technological interest with applications mainly in the construction of electronic components. And also often sources of rare earths, a chemical family consisting of the lanthanide elements plus ytrium with a huge myriad of applications in high-tech devices. The mineralogical characterization of these sources is very important because the abundance of the mineral phases of interest and mineral context of the source material are determinant of the material and energy cost of the separation process, as well the routes that will be used for the processing and added value and use of the end products. Mineral characterization is often grounded in crystallographic studies, where the phases are properly characterized in its crystalline structure and chemical composition. In general these studies are performed through X-ray diffraction together with analysis for chemical composition (classical wet or instrumental methods); currently very frequently are accompanied by analyzes of scanning electron microscopy . The studies of X-ray diffraction in this case are generaly made by powder diffraction , with some studies using micro -diffraction and in rarer cases single crystal diffraction. Despite being applied with much less frequency in studies of mineral matrices in the context of routine, single crystal diffraction is a very important tool because it allows to more effectively make crystal structure determinations , determine the placement of atoms in the lattice and in substitution models. Although it is not possible to use the single crystal diffraction to determine the composition of mineralogical phases of rocky bodies - area of mastery of powder diffraction -is a very important tool for the study of minor phases (usually difficult to form a representative samples of powder and sometimes of greater economic interest) and the study of new minerals. The use of single crystal diffraction however clashes with some solid barriers when applied to the study of minerals. Generally the mineral formation and crystallization processes are not controlled and not completely known, which often does not produce single crystals of suitable quality. Moreover, it is necessary to analyze representative single crystals to generate conclusions about the parent sample, which forces us to deal with various problems which generally we seek to avoid on the use of this technique, such as disorder, twinning and embedding information into the refinement. This paper presents the work, results and conclusions obtained after analysis thought single crystal diffraction , stucture resolution and crystallographic refinement of a single crystal of ixiolite derived from a pegmatite sample obtained in Volta Grande - MG . Potential use of the technique is discussed as well suggestions for pointing conditions for making crystallographic refinements in problem samples.Results of refinement with models of crystal structure, atomic coordinates, sites occupation and thermal displacement parameters for the real disordered ixiolite single crystal studied and compared with the ordered systems in the literature are also presented.