Paper No. 5
Presentation Time: 8:00 AM-6:00 PM
TOPOLOGICAL RELATIONS IN THE SYSTEM U-SI-O-H FORMED DURING THE WEATHERING OF THE ALBITITE-TYPE URANIUM CACHOEIRA DEPOSIT, CAETITÉ, BAHIA, BRAZIL
The alteration pattern at the Cachoeira Mine due to weathering results in the formation of uranophane [Ca (UO2)2(SiO3)2(OH)2], soddyite [UO2)5(SiO4)2(OH)2.5H2O], kasolite Pb [UO2)(SiO3)(OH)2 with very subordinate amounts of coffinite [USiO4)0.5(OH)2], formed under high silica activity. Hydration of primary uraninite or secondary pitchblende, which could be partly destabilized during weathering, under silica- and/or water-rich conditions form the encountered mineral association. Nine key reactions have been assembled in µSiO2-µH2O diagram: uraninite + ½ SiO2 + H2O = coffinite (1); 2 uraninite + 2 SiO2 + 6 H2O = uranophane (2); uraninite + SiO2 + H2O = kasolite (3); 5 uraninite + 2 SiO2 + 6 H2O = soddyite (4) ; uranophane = 2 soddyite + 4 H2O (5); uranophane = 2 coffinite + SiO2 + 4 H2O (6); soddyite + 3 SiO2 = 5 kasolite + H2O (7); 4 coffinite + H2O = soddyite + SiO2 (8); coffinite + ½ SiO2 = kasolite (9). It could be shown that increasing µH2O would make stable uranophane. Under silica-rich conditions kasolite and uranophane form and moderate µSiO2-µH2O soddyite and coffinite are the stable phases. Although it is difficult to detect and recognize other phases such as becquerelite [Ca (UO2)6O4 (OH)6(H2O)8], sklodowskite [Mg[UO2)2(SiO3)2(OH)2.5H2O] and schoepite [UO2)8O2(OH)12.(H2O)12] because they are poorly crystallized or amorphous, eventually they could exist and included in the reaction set, enlarging the number of reactions to twenty one. The uranyl oxyhydroxides form in U-rich aqueous condition through oxidation and hydration of uraninite. In that way, all eight mineral phases may be represented in a single diagram. Increasing µH2O favor stability of becquerelite, which under still higher H2O conditions yield to schoepite. Increasing both H2O and silica, sklodowskite must form. As these minerals still have not been positively found at Cachoeira, we assume that high silica/H2O ratio prevailed.