DISSOLUTION OF URANYL-OXIDE-HYDROXY-HYDRATE MINERALS. III. BILLIETITE

Abstract

Dissolution experiments on freshly cleaved and non-freshly cleaved single crystals of billietite, Ba(H 2 O) 8 [(UO 2 ) 6 O 4 (OH) 6 ], were done in HCl solutions of pH 2, in ultrapure water, in 0.1 mol L −1 Na 2 CO 3 solutions of pH 10.5, in 1.0 mol L −1 M Cl solutions of pH 2 ( M = Na, K, Li), in 0.5 mol L −1 M Cl 2 solutions of pH 2 ( M = Ca, Sr, Mg) and in a 0.5 mol L −1 Pb(NO 3 ) 2 solution of pH 2. Dissolution features on the (001) basal surface of the billietite crystals were examined with atomic-force microscopy, scanning electron microscopy and optical microscopy. Hillocks on the basal surface form in ultrapure water, and a striped pattern of steps occurs on the billietite surface after treatment in a Na 2 CO 3 solution of pH 10.5. 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. Symmetry and elongation of etch pits formed in electrolyte solutions of pH 2 can vary with the type of surface ( i.e. , freshly cleaved versus non-freshly cleaved) and the cation in solution. Etch pits on non-freshly cleaved surfaces commonly display a lower symmetry than the etch pits formed on freshly cleaved surfaces, most likely the result of the degree of alteration of the non-freshly cleaved surfaces. Etch pits elongate parallel to [100] form in aqueous NaCl, KCl and MgCl 2 solutions, and etch pits elongate parallel to [010] form in aqueous CaCl 2 , SrCl 2 and Pb(NO 3 ) 2 solutions, respectively. A Pb 2+ -containing phase precipitates on the surface in an aqueous Pb(NO 3 ) 2 solution of pH 2. Etch pits formed on billietite in Ca-containing solutions are elongate parallel to the rows of Ca atoms in the bulk structure of becquerelite, Ca(H 2 O) 8 [(UO 2 ) 6 O 4 (OH) 6 ], and to the elongation of the becquerelite crystals. The different elongation of etch pits can be explained with an adsorption model involving the cations in solution and specific sites on the basal surface. In the electrolyte solutions, the relief of dissolution features on the basal surface and edges increases in the sequence KCl 3 ) 2 2 2 = MgCl 2 = LiCl, suggesting that the relief of the basal surface inversely correlates with the size of the cation in solution.