RIETVELD METHODOLOGY APPLIED IN THE PETROGRAPHY OF KIMBERLITIC ROCKS
Petrography, X Ray Diffraction, Rietveld Refinement, Lamproite, Nordestina Kimberlitic Province
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.