Navegando por Assunto "Minerais evaporitos"

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Aspectos geoquímicos de gleissolos em pântanos salinos da região Bragantina, Pará
(Universidade Federal do Pará, 2011-06-08) CAMARGO, Paloma Maria Pinto; CORRÊA, José Augusto Martins; http://lattes.cnpq.br/6527800269860568
The knowledge about the nature of depositional processes in the tidal flats, where the salt marsh are included, is very important to understand the tropical coastal ecosystem. Evaporitic minerals, like in Sabkhas, are observed in Bragança salt marsh during the dry season. Halite is deposited on the surface of the soil and gypsum and pyrite precipitate in the subsurface via capillary action out of brines brought up from the water table. The pedogenic processes in the salt marsh where dominate by chemical reactions like halomorphism, hydromorphism, sufate reducing. The aim of this study is characterize the soil of Bragança salt marsh using the textural characteristics, mineralogical and geochemical composition of sediments. The sampling was performed in two transects located in both sides of the PA-458. Each transects have 3 profiles that arise 1 meter depth. The samples were collected by a dutch auger for augering shallow depths (20 cm). The granulometry of sediments was obtained with a laser granulometer and the mineralogical composition was performed by X-ray diffraction techniques. The sand grains were observed using a petrographic microscope with digital image capture. The neoformed minerals and silt+clay grains size were described using a scanning electron microscopy (SEM). In the soil were also studied the pH, electric conductivity, salinity, organic matter content, major, minor and trace elements using classical chemical methods, X-ray fluorescence and atomic absorption spectrometry. Gleysol present in salt marsh show silty-clayey grain size, massive texture, slightly sticky to sticky, very plastic to slightly plastic and friable to compact. Mineralogical it was composed of quartz, kaolinite, illite, smectite, pyrite, jarosite, polyhalite and halite. The pH values ranged from high acidity to acid and the lowest value was found in the depth of 70-90 cm (pH = 3.85), profile P4. Some profiles shown in the Zone 2, where pH is much acid, yellow to orange mottles which corresponds to pyrite oxidation. Based on composition of the adsorption complex of studied soil profiles, the profiles showed high CEC due to the large amount of organic matter and clay minerals. The dominance of Na+ and Mg2+, followed by Ca2+ and K+ assigning to soil eutrophic character. Only the profile P4, B transect, showed dystrophic character. All profiles shown high sodium concentration, characterizing the salt marsh soil as "Solonchak" and therefore may be classified as Salic Sodic Gleysol. The salt marsh soil contains high values of SiO2, Al2O3 and Fe2O3, which reflect its major minerals. Illite, smectite, polyhalite, halite and pyrite are characteristics of salt marsh environment, where the sedimentation occur under the influence of sea water which contributes to Na+, K+, Ca2+, and Mg2+. The presence of jarosite indicates the oxidation of sulfides. The high amount of Na2O reflect the strong evaporation rates under which the salt marsh environment is submitted, and support the neoformation of halite at different depths along a profile. The results and the field observations suggests that the new mineral phases are essentially the result of high evaporation rate, organic matter content, primary mineral phases.