Original paper

The role of uranium (V) ion in the chemical composition of meta-autunite from pegmatites of Quintos de Baixo, Brazil

Bermanec, Vladimir; Wegner, Reinhard; Kniewald, Goran; Rakvin, Boris; Palinkaš, Ladislav A.; Rajić, Maša; Tomašić, Nenad; Furić, Krešimir

Abstract

Meta-autunite from the pegmatite mineralization of Quintos de Baixo, Brazil was investigated. The total uranium content and elemental analysis yield the formula Ca(UVIO2)2–x(UVO2)x(H3O+)x(PO4)2·(6–x)H2O for the meta-autunite. The natural sample crystallizes tetragonally in the space group P4/nmm having unit cell parameters a = 6.990 (1) Å, c = 8.445 (1) Å and V = 412.6 (1) Å3 calculated from X-ray diffraction data. Thermal analysis showed a weight loss of 16.03 wt % in four distinct processes up to 1500 °C. Electron paramagnetic resonance (EPR) spectroscopy provides evidence that part of the uranium present in meta-autunite is in the pentavalent state. High temperature diffraction data show lowering of the symmetry, decreasing the unit cell parameters and finally, the breaking of the structure of the phosphate phase and formation of monoclinic uranium oxide and an amorphous phase. At 80 °C meta-autunite I transforms into the meta-autunite II with orthorhombic symmetry, while further heating produces changes in the diffraction pattern due to dehydration, although all the heating products could be by analogy identified as meta-autunite II. The calculated unit cell parameters for this phase decrease from a = 7.253 (4) Å, b = 6.537 (3) Å, c = 7.008 (4) Å at 82 °C to a = 6.38 (2) Å, b = 5.82(1) Å, c = 6.68 (1) Å at 204 °C. Uranium oxide phases originating from the breakdown of the phosphate structure show temperature dependant stability. At 1050 °C hexagonal U2O3 analogous with α-Al2O3 is present with symmetry (R3 c) and unit cell parameters a = 4.806 (1) Å, c = 13.138 (4) Å, V = 262.9 (1) Å3.

Keywords

meta-autunitepentavalent uraniumnon-stoichiometric uranium oxidesx-ray diffractionir spectroscopythermal analysisdifferential pulse polarographyelectron paramagnetic resonanceborboremaquintos de baixo pegmatite