A new uranyl carbonate sheet in the crystal structure of fontanite, Ca[(UO 2 ) 3 (CO 3 ) 2 O 2 ](H 2 O) 6

The structure of fontanite, Ca[(UO 2 ) 3 (CO 3 ) 2 O 2 ](H 2 O) 6 , is monoclinic, space group P2 1 /n, a = 6.968(3), b = 17.276(7), c = 15.377(6) Å, β = 90.064(6)°, V = 1851(1) Å 3 , Z = 4. The structure was solved by direct methods and refined on the basis of F 2 for all unique reflections using leastsquares techniques to an agreement index (R1) of 9.9%. The structure contains two symmetrically distinct uranyl pentagonal bipyramids, one uranyl hexagonal bipyramid, and two CO 3 triangles. The uranyl polyhedra form chains by sharing equatorial edges, and CO 3 groups occur on either side of the chains, where they share equatorial edges of the uranyl hexagonal bipyramids. The CO 3 groups share their third ligand with a uranyl pentagonal bipyramid of an adjacent chain, resulting in uranyl carbonate sheets of composition [(UO 2 ) 3 (CO 3 ) 2 O 2 ] 2- . The single symmetrically unique Ca 2+ cation is located between the sheets, and is coordinated by two O atoms of uranyl ions of adjacent sheets, and six H 2 O 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.