CRYSTAL GROWTH OF SCHOEPITE ON THE (104) SURFACE OF CALCITE

Abstract

Schoepite. [(UO2)(8)O-2(OH)(12)](H2O)(12), precipitates through interaction of acidic and basic uranyl solutions with calcite. The uranyl solutions have (UO2)(2+) concentrations between 1.5 and 50 mmol L-1, and initial pH values of 2.5, 4.5 and 10.0. Examinations with the atomic force microscope (AFM) reveal different morphologies of schoepite crystals grown on the (104) surface of calcite. Crystals grown during in situ experiments with uranyl nitrate solutions (initial pH = 2.5) have a platy morphology, and their prominent (001) faces are defined by [210] edges. Crystals grown during in situ experiments with uranyl acetate solution (initial pH = 4.5) seem more bulky, with poorly defined rectangular (001) faces. Examinations of calcite surfaces after interaction With a slightly acidic solution for three days (final pH between 5.5 and 6.5) show platy asymmetrical crystals with prominent (001) faces that are defined by [100], [010], [120], and [110] edges. Experiments with basic solutions after three weeks (final pH between 7.5 and 8.0) show bulky rectangular crystals of schoepite, in which the (001) face is mainly defined by [100] and [010] edges. The different thicknesses of schoepite crystals in the in situ experiments are explained by the different growth-rates of S and F faces in solutions with different saturation-indices with respect to schoepite. The occurrence of edges on the (001) face of schoepite crystals is explained by structural parameters of the chains of polyhedra parallel to the edges and the pH of the solution.