Mineralogia e geoquímica supergênicas do urânio - Província Uranífera de Lagoa Real, Caetité - Bahia

AUTOR(ES)
DATA DE PUBLICAÇÃO

2005

RESUMO

X Ray Diffraction and Infrared Spectroscopy (FTIR) analysis reveal that the following uranyl hidroxisilicates mainly constitute the uranium secondary mineralogy from albitites of the Cachoeira and Laranjeiras uranium anomalies (Lagoa Real BA Brazil): β-Uranophane-Ca[(UO2)(SiO3OH)]2.5(H2O); Uranophane-Ca[(UO2)(SiO3OH)]2.5(H2O); Sklodowskite-Mg[(UO2)(SiO3OH)]2.6(H2O); Haiweeite-Ca[(UO2)Si5O12(OH)2].3(H2O). Using only the X Ray Diffraction, the uranyl phosphates below were also characterized: Autunite - Ca[(UO2)(PO4)]2 . 8-12(H2O); Meta-autunite - Ca[(UO2)(PO4)]2 .6-8(H2O); The natural yellowish color of the above minerals was also found by the same analytical methodology in slightly weathered calcites and albites of the sampled material. Thus, yellowish materials from uraniferous albitites do not represent only uranyl minerals. β-Uranophane, Uranophane, Sklodowskite and Haiweeite resulted from the interaction between Ca+2, Mg+2 and UO2+2 cationic species and hidroxisilicate anionic complexes. These ions originated from the weathering that affected the mineralogy of the albitites, granites and gneisses found in the Lagoa Real uranium district. The Ca+2 ion came from pyroxenes, calcium plagioclase and calcite. The Mg+2 came from amphiboles and biotites. The mobile ion uranyl (hexavalent U) came from the uraninite (UO2) by the U+4 oxidation. The Ca+2 and uranyl íons of the autunite and meta-autunite originated in the same way as above. The PO4-3 ions came from the weathering that affected the apatites and monazites of the granites and gneisses. A geochemical model allowing the formation of the aforementioned uranyl hidroxisilicates could beas follows: 1. U+4 oxidation to U+6 which appears as the mobile uranyl ion [UO2]+2. 2. Uranyl hydrolysis and formation of the uranyl hydroxide complexes (low stability). 3. Dissociation of the uranyl hydroxide complexes and hydrolysis of the carbonate ions from calcite, resulting in groundwater pH increasing which improves the silica dissolution from silicatic minerals and consequent precipitation of the uranyl hidroxisilicates with Ca e Mg. 4. Intensification of the dissociation of uranyl hydroxide complexes, which dislocates the equilibrium of the carbonate ion hydrolysis, allowing the formation of the uranyl carbonate complexes (stable in aqueous solution), and limits the formation of uranyl hidroxisilicates. The Lagoa Real uranium secondary mineralogy indicates the existence of a permanent groundwater table in the weathering profile during a recent geological past.The climatic oscilation of it maintains the uraniferous albitites in permanent contact with water, allowing the formation of [UO2]+2 and its gheochemical immobilization by the [XO4]x-ou {[XO4][OH]}y- found in the rockpores as well. The erosion in a more recent geological past lowered the groundwater sheet and exposed this uranium secondary mineralogy at the surface. The inexistence of uranyl sulfates and carbonates corroborates the lack of a strong evaporation setting during the formation of the Lagoa Real uranium secondary mineralogy.

ASSUNTO(S)

lagoa real uranium minerals caetité minerais de urânio caetité lagoa real depósitos de urânio uranium deposits mineralogia

ACESSO AO ARTIGO

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