Aplicações do Método de Rietveld na petrografia de rochas kimberlíticas

Abstract

The use of X-rays began in 1895, by Wilhelm Conrad Roentgen. Initially it was used for medical studies, until in 1912 Max Von Laue applied the technique on crystals, discovering X-ray diffraction. Soon a powerful tool for scientific studies was obtained, which guided the discoveries of all structures known to date. Qualitative analysis by X-ray diffraction started from Bragg's studies, where it was determined that when a monochromatic X-ray beam falls on a crystalline material, the phenomenon of diffraction occurs. Thus, to analyze the crystalline phases that are present in an analyzed powder, a diffractogram is made, which is a graph of observed intensity. With the diffractogram the peaks of maximum intensity are calculated. The Rietveld method was created in 1969 by Hugo. M Rietveld, was initially designed to refine the interpretation of diffractogram data generated from neutron diffraction. The method allows the calculation of the standard diffractogram of a given sample through an algorithm with a diffraction pattern suitable for the phase to be studied, applying for this the method of least squares. Thus, comparing the experimental peaks obtained in the unknown sample and the pattern of calculated points, it is possible to identify the unit cell of the crystallographic structure. Some of these values can be found in articles that report similar structures. The classical petrographic study of kimberlites is hampered by the presence of exotic mineralogy, xenoliths, xenocrystals, and little-known minerals of mantle origin. Furthermore, the geochemistry of these volatile rocks favors the late processes of serpentinization, carbonatization and oxidation, transforming the exposed rocks on the surface and often making it impossible to correctly identify the mineral phases present in the sample. For these reasons, the refinement of X-ray diffraction analysis using the Rietveld method is a powerful tool in the petrographic interpretation of exotic and altered mineralogy rocks. Such information is essential for a better understanding of the nature and genesis of this important mineral prospect. This study aims to apply the Rietveld quantification method through X-ray diffraction analysis in the total-powder method to broaden the understanding of the petrography of Lamproíto Aroeira. For this, the software that DIFRAC.TOPPAS will be used in the laboratory of X-ray mineral technology (LAPAG). The work methodology developed in this study focused on the selection of two representative samples of Lamproíto Aroeira, these two samples were laminated in three orthogonal directions, thus obtaining 6 thin-polished blades and 6 tablets of these same fractions, which were ground and generated the powder analyzed by DRX. Thus, the study of petrography together with the application of the Rietveld method in its respective powder gave a greater reliability to petrographic descriptions and thus help the identification of exotic and serpentinized phases.