DISSOLUTION OF URANYL-OXIDE-HYDROXY-HYDRATE MINERALS. IV. FOURMARIERITE AND SYNTHETIC Pb2(H2O)[(UO2)10UO12(OH)6(H2O)2

Abstract

Dissolution processes of Pb uranyl-hydroxy-hydrate phases with framework and sheet structural units of polymerized uranyl polyhedra were studied in order to understand the role of the interstitial Pb cations and the degree of polymerization of the structural unit during those processes. Batch-dissolution experiments on single crystals of fourmarierite, Pb 1– x (H 2 O) 4 [(UO 2 ) 6 O 3–2 x (OH) 4+2 x ] (sheet structural-unit) and synthetic Pb 2 (H 2 O)[(UO 2 ) 10 UO 12 (OH) 6 (H 2 O) 2 ] ( PbUOH , framework structural-unit) were done in HCl solution of pH 2, in distilled water, in 0.1 mol L −1 Na 2 CO 3 solution of pH 10.5, in 1.0 mol L −1 M Cl solutions of pH 2 ( M = Na, K), and in 0.5 mol L −1 M Cl 2 solutions of pH 2 ( M = Ba, Ca, Sr, Mg). Dissolution features on the basal surface of these phases were examined with atomic force microscopy, scanning electron microscopy and optical microscopy. Hillocks on the basal surface of fourmarierite form in distilled water, in HCl, KCl and SrCl 2 solutions of pH 2 and in a Na 2 CO 3 solution. Etch pits form only on the basal surface in solutions of pH 2, indicating that their formation is promoted by a higher activity of protons in solution. The symmetry and elongation of etch pits formed in electrolyte solutions of pH 2 can vary with the type of cation in solution. The formation of hillocks was not observed on the (001) face of PbUOH , which is in contrast to observations on the basal surfaces of curite, becquerelite, billietite and fourmarierite. The outline of etch pits on the basal surface of PbUOH formed in distilled water and in Na 2 CO 3 solution can be described as a regular parallelogram, whereas etch pits formed in aqueous SrCl 2 , MgCl 2 , CaCl 2 and NaCl solutions of pH 2 have the shape of a distorted parallelogram. The cations in solution have a different effect on the lateral dimensions and depth of etch pits formed on the prominent surfaces of fourmarierite and synthetic PbUOH . The relief of dissolution features on the basal surface of fourmarierite increases in electrolyte solutions in the sequence BaCl 2 2 2 2 , and on the prominent (001) surface of synthetic PbUOH , in the sequence KCl 2 2 2 2 . The relief of dissolution features on the basal faces of both phases is inversely correlated with the size of the cation in solution. This correlation is explained by the higher affinity of large alkali and alkaline-earth cations (Ba, K) to oxide and silicate surfaces, which may result in a stronger adsorption of these cations to specific surface-sites. A stronger adsorption of cations on surface sites lowers the number of protonated anion-terminations (activated sites) and may result in a lower rate of dissolution perpendicular to the basal surface and edges.