Saléeite, Mg(UO2)2(PO4)2, from the Wojcieszyce uranium deposit (Lower Silesia, Poland)

Abstract

The secondary uranium mineral, saléeite, was found in dumps of the former Wojcieszyce Uranium Mine. It occurs as tiny tabular crystals, up to 0.3 cm in size. The chemical formula, which was calculated on the basis of a 12 spot analyses is: (Mg_0.73Fe_0.05K_0.04Ca_0.04)_S0.86(UO₂)_2.16(PO₄)_2.00·10H₂O. The cation position is occupied mainly by Mg. The Mg ions are partially substituted by Fe, K and Ca. The tetrahedral position is occupied by the PO₄^3- ion. The interaction between acidic pore solutions (originating from the reaction of pyrite oxidation), uraninite and minerals of the host rocks (chlorite, apatite) leads to the release of Mg²⁺, UO₂²⁺ and PO₄^3- ions. These ions are adsorbed by amorphous Fe oxyhydroxides and poorly ordered ferrihydrite. Crystallization and recrystallization of these phases leads to the release of adsorbed ions and favours the crystallization of saléeite. The coexistence of saléeite and goethite, which forms by recrystallization of ferrihydrite, points to an important role of sorption and desorption of Mg²⁺, PO₄^3- and UO₂²⁺ ions in the process of crystallization of this mineral species. Crystallization of saléeite occurs at a pH of 5.5–6.0. The supergene zone of the Wojcieszyce deposit is the first occurrence of saléeite in Poland.