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The Canadian Mineralogist; December 2006; v. 44; no. 6; p. 1379-1385; DOI: 10.2113/gscanmin.44.6.1379
© 2006 Mineralogical Association of Canada
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Articles

THE CRYSTAL STRUCTURE OF DEHYDRATED WYARTITE, Ca (CO3) [U5+ (U6+O2)2 O4 (OH)] (H2O)3

Frank C. Hawthorne1,§, Robert J. Finch1,¶ and Rodney C. Ewing2

1 Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
2 Department of Geology, University of Michigan, Ann Arbor, Michigan 12345, U.S.A.

§ E-mail address: frank_hawthorne{at}umanitoba.ca

Dehydrated wyartite from the Shinkolobwe mine, Shaba, Democratic Republic of Congo, ideally Ca (CO3) [U5+ (U6+O2)2 O4 (OH)] (H2O)3, is orthorhombic, a 11.2610(6), b 7.0870(4), c 16.8359(10) Å, V 1343.6(2) Å3, space group Pmcn, Z = 4. The structure was solved by direct methods and refined to an R1 index of 2.9% based on 2045 observed reflections measured with MoK{alpha} X-radiation on a four-circle single-crystal diffractometer equipped with a CCD detector. The structure consists of neutral sheets of the form [U5+ (U6+O2)2 O4 (OH)] that contains edge- and corner-sharing (U6+{phi}7) polyhedra ({phi}: O2–, OH–, H2O), two of which are pentagonal bipyramids with five equatorial O2– and OH– groups bonded to a central uranyl ion, (U6+O2)2+. The third U atom is part of a unique U5+{phi}7 polyhedron, in which two O2– anions are part of an interlayer (CO3)2– group and another is the O atom of an H2O group. The plane of the (CO3) groups lies perpendicular to the structural sheets, and each (CO3) group is coordinated to U5+ in the structural sheet and to Ca in the interlayer. The sheet is topologically similar to that found in ß-U3O8, and the sheets are bonded to each other through interlayer Ca atoms and interlayer (H2O) groups. The principal difference between the structures of wyartite and dehydrated wyartite is that the sheets in the former are linked only through interstitial hydrogen-bonding, whereas the sheets in the latter are linked directly by Ca–O bonds (plus interstitial hydrogen-bonds).

Keywords: dehydrated wyartite, wyartite, crystal structure, carbonate, uranyl, pentavalent uranium, dehydration, spent nuclear fuel.






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