Navegando por Autor "VEGA PORRAS, David Enrique"

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Cristaloquímica de esmectitas dioctaédricas em bentonitas brasileiras
(Universidade Federal do Pará, 2025-07-01) VEGA PORRAS, David Enrique; PAZ, Simone Patrícia Aranha da; http://lattes.cnpq.br/5376678084716817; https://orcid.org/0000-0002-5880-7638
Bentonites are mineral inputs of great industrial interest due to the physicochemical properties of clay minerals from the smectite group. These properties, resulting from the complex crystal chemistry and small particle size of smectites, include various layer charge, swelling capacity, and cation exchange capacity. In Brazil, the main bentonite deposits are located in the municipality of Boa Vista, Paraíba, which accounts for >70% of the country’s production. However, the progressive depletion of these varieties has motivated the search for new deposits. Among them, the Formosa deposit, in the state of Maranhão, stands out. Although both the Boa Vista and, to a lesser extent, the Formosa bentonites have been extensively studied in their <2 μm or coarser fractions, the crystal chemistry of their smectites has not yet been fully elucidated. This is due not only to the typical chemical and structural complexity of smectites, but also to the difficulty of obtaining pure smectite phases and the interference of accessory minerals. In this context, this thesis presents a detailed study of the crystal chemistry of smectite from the Formosa deposit, comparing it with three commercial varieties from Boa Vista: Bofe, Chocolate, and Verde Lodo. The <2 μm, 0.1-2 μm, and <0.1 μm fractions were analyzed, with the latter being the main focus of the study. The analyses included XRD (Rietveld method), FTIR, TEM-EDS, SEM, XRF, AAS, TG, and Mössbauer spectroscopy. These smectites, previously described as montmorillonites, also exhibit significant tetrahedral layer charge. In fact, the four samples show a tetrahedral charge of approximately 0.15 per half unit cell, sufficient to induce swelling after the Hofmann-Klemen treatment. The octahedral symmetry of all four samples was trans-vacant, influenced by both the Fe content and the tetrahedral charge. The structural and chemical similarities between the Formosa and Boa Vista smectites, along with similar smectite content in the <2 μm and <0.1 μm fractions – as quantified by the Rietveld method – demonstrated that the Formosa deposit is a potential alternative to the Boa Vista bentonites.
Acesso aberto (Open Access)
Quantificação mineralógica de bentonitas via DRX usando um método combinado Rietveld-Le Bail-Padrão Interno
(Universidade Federal do Pará, 2020-04-09) VEGA PORRAS, David Enrique; PAZ, Simone Patrícia Aranha da; http://lattes.cnpq.br/5376678084716817; https://orcid.org/0000-0002-5880-7638
In the bentonite industry ore quality control has been traditionally carried out using the swelling parameter, denominating those that swell as naturally sodium bentonites and those that do not as naturally non-sodium bentonites. However, the properties of the bentonites can vary due the quantity and cationic type of montmorillonite. These variations cannot always be predicted, because there is no consolidated and practical method for clays mineralogical quantification. A quantification via PXRD using the conventional Rietveld method via X-ray diffractometry is only reliable when all the crystalline structures of the mineral phases are known. This is not the case for bentonites, since the montmorillonite shows turbostratic disorder that is not considered in the available structural models, making it the mineralogical quantification a major challenge. Thus, in this work, was generated a calibrated hkl phase model for a so-called Formosa Mgmontmorillonite applying the combined Rietveld-Le Bail-Internal Standard method developed by Paz et al. (2018). The montmorillonite was obtained via centrifugation according to the Stokes law by particle size separation of the clay fraction (< 2 μm) from the Formosa bentonite. The sample was characterized by XRD, XRF, FTIR, SEM, MS and PSD, finding a low concentration of other mineral phases (impurities). According to the XRF and MS analysis, it is classified as a beidellitic montmorillonite with octahedral charge of >50 %. Quantitative results were satisfactory using the hkl model for montmorillonite-fluorite binary mixtures with montmorillonite content >50 % (statistical indices χ2 and RBragg < 5). The method was reproducible for three replicates of the montmorillonite-fluorite binary mixture (80-20%). The greater precision and reproducibility of the results, shows that the calibrated hkl phase model can be used satisfactorily for the mineralogical quantification of bentonites (60-80% montmorillonite content). These results showed that the combined method is a convenient and fast procedure, essential requirements for an industrial application, allowing deal with phases that have partial crystallographic information and handling severe diffractometric effects of structural disorder, such as turbostratic disorder, typical of clay minerals.