GROWTH OF URANYL-HYDROXY-HYDRATE AND URANYL-CARBONATE MINERALS ON THE (104) SURFACE OF CALCITE

Abstract

Interaction of concentrated acidic uranyl-bearing solutions at initial values of pH of 2.5 and 4.5 with calcite at 25°C in an open system results in formation of schoepite, becquerelite and wyartite-II. Interaction of similar solutions with calcite at 100°C results in the formation of dehydrated schoepite after one day and becquerelite after two to three days. Concentrated basic uranyl-bearing solutions in contact with calcite at 25°C in an open system produce uranyl-bearing calcite, wyartite-II, becquerelite and schoepite. Uranyl tricarbonate minerals such as liebigite precipitate on calcite only in more highly concentrated basic solutions. The crystal morphologies of almost all of these minerals were recorded on the calcite surface with an atomic force microscope (AFM). Red crystals of becquerelite grown in acidic solutions are elongate parallel to [010], and the (001) face is defined by the [010], [110] and [100] edges. The (001) face of pink crystals of wyartite-II grown in an acidic solution is defined by the [120], [100] and [120] edges and has striations parallel to [100]. Under basic conditions, uranyl-bearing calcite precipitates as growth hillocks in the first few minutes of the experiments. Green crystals of liebigite are attached to the calcite surface via their (001) or (010) faces. The (001) face of pink-to-violet crystals of wyartite-II grown in basic solutions is defined by the [100], [110], [120], [230] and [010] edges. Small red crystals of becquerelite grown in basic solution have a similar morphology to those grown in acidic solution. The formation of wyartite-II is a result of high CO 2 activity on the calcite surface, which provides the necessary low values of Eh. The formation of schoepite on the calcite surface in Ca 2+-(CO 3) 2-solutions occurs owing to favorable crystal-growth kinetics relative to becquerelite and other uranyl-carbonates. On the basis of observations in the open system, formation of minerals on the calcite surface is predicted for acidic and basic solutions in a closed system.