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A new uranyl carbonate sheet in the crystal structure of fontanite, Ca[(UO₂)₃(CO₃)₂O₂](H₂O)₆

Department of Civil Engineering and Geological Sciences, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, Indiana 46556, U.S.A.

Correspondence: ^* E-mail: pburns{at}nd.edu

The structure of fontanite, Ca[(UO₂)₃(CO₃)₂O₂](H₂O)₆, is monoclinic, space group P2₁/n, a = 6.968(3), b = 17.276(7), c = 15.377(6) Å, ß = 90.064(6)°, V = 1851(1) ų, Z = 4. The structure was solved by direct methods and refined on the basis of F² for all unique reflections using least-squares techniques to an agreement index (R1) of 9.9%. The structure contains two symmetrically distinct uranyl pentagonal bipyramids, one uranyl hexagonal bipyramid, and two CO₃ triangles. The uranyl polyhedra form chains by sharing equatorial edges, and CO₃ groups occur on either side of the chains, where they share equatorial edges of the uranyl hexagonal bipyramids. The CO₃ groups share their third ligand with a uranyl pentagonal bipyramid of an adjacent chain, resulting in uranyl carbonate sheets of composition [(UO₂)₃(CO₃)₂O₂]^2–. The single symmetrically unique Ca²⁺ cation is located between the sheets, and is coordinated by two O atoms of uranyl ions of adjacent sheets, and six H₂O groups. The uranyl carbonate sheet in fontanite is novel, but is based upon the phosphuranylite anion topology that is the basis of uranyl phosphate, uranyl selenite, and uranyl sulfate sheets in a variety of minerals.

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