CRYSTAL-STRUCTURE OF SEELITE FROM RABEJAC (FRANCE)

The crystal structure of seelite from Rabejac has been determined by X -ray diffraction and refined to R = 0.031 for 758 observed reflexions. Monoclinic system; space group C2/m with a = 18.194(3), b = 7.071(1), 6.670(1) Angstrom, beta = 99.70(1), V = 845.8(2) Angstrom(3), D-x = 3 .71 (D-m = 3.70), Z = 2. Chemical formula : Mg[(UO2)(AsO3)(0.7)(AsO4)( 0.3)](2).7H(2)O. The structure consists of layers similar to those of the seelite from Talmessi and those of the silicates of the uranophane structural group. Those layers are connected by octahedral Mg(H2O)(6) cations and by water molecules. The uranyl phosphate ranunculite is p robably isostructural with seelite.