- journal_title：The Canadian Mineralogist
- Contributor：Michael Schindler ; Frank C. Hawthorne ; Peter C. Burns ; Patricia A. Maurice
- Publisher：Mineralogical Association of Canada
- Date：2006-
- Format：text/html
- Language：en
- Identifier：10.2113/gscanmin.44.5.1207
- journal_abbrev：Can Mineral
- issn：0008-4476
- volume：44
- issue：5
- firstpage：1207
- section：Articles
The morphology of etch pits on the basal surface of becquerelite, Ca(H2O)8 [(UO2)6O4(OH)6], is predicted using the bond-valence deficiency of anion-terminations on chains of polyhedra parallel to its edges. The predicted morphology is compared with observations in batch-dissolution experiments on single crystals. These experiments were done in HCl solutions of pH 2 and 3.5, in ultrapure water, in 0.1 and 0.5 mol L−1 Na2CO3 solutions of pH 10.5, in 1.0 mol L−1MCl solutions of pH 2 (M = Na, K), in 0.5 mol L−1MCl2 solutions of pH 2 (M = Ba, Sr, Mg) and in a 0.5 mol L−1 Pb(NO3)2 solution of pH 2. Dissolution features on the (001) basal surface of the becquerelite crystals were examined with atomic-force microscopy, scanning electron microscopy and optical microscopy. Hillocks on the basal surface occur in the pH range 3.5–10.5. Etch pits form on the basal surface only in solutions of pH 2, indicating that their formation is promoted by high activity of protons in solution. Etch pits formed in an HCl solution of pH 2 do not display the symmetry elements of the bulk structure (space group Pn21a), possibly a result of nonstoichiometric detachment of the uranyl-bearing and Caϕn polyhedra. Symmetry and elongation of etch pits formed in electrolyte solutions of pH 2 vary with the type of cation in solution. Etch pits elongate parallel to [100] form in aqueous solutions of BaCl2, KCl, NaCl and MgCl2, whereas etch pits elongate parallel to [010] form in aqueous solutions of SrCl2 and Pb(NO3)2. Etch pits on an untreated basal surface formed during synthesis in a Ca-bearing solution are elongate parallel to [010]. Etch pits formed on becquerelite in Ba-, K-, and Ca-containing solutions are elongate parallel to the rows of Ba, K and Ca atoms in the bulk structures of billietite, compreignacite and becquerelite, and to the elongation of their 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. The stability of the basal surface and its edges in the presence of various cations in solution was determined using the dimensions of the etch pits and the relief of dissolution features. The stability of the basal surface in the electrolyte solutions decreases in the sequence MgCl2 > KCl > BaCl2 > Pb(NO3)2 > NaCl > SrCl2, and the stability of the edges decreases in the sequence MgCl2 > KCl > BaCl2 > SrCl2 > NaCl > Pb(NO3)2. Both rankings indicate that solutions with cations of ionic radius similar to that of Ca2+ have the greatest influence on the dissolution processes of single crystals of becquerelite.