Chemical-composition and Crystal-structure of Phosphuranylite, Ca(uo2)[(uo2)3(oh)2(po4)2]2.12h2o]

Abstract

The crystal structure of phosphuranylite has been determined by X-ray diffraction and refined to R = 0.089 for 963 observed reflexions. The unit cell is orthorhombic, space group Bmmb (n-degrees 63), a = 15.835(7), b = 17.324(6), c = 13.724(4)angstrom, V = 3.765(2)angstrom3, Z = 4. Chemical formula : CaO.7UO3.2P2O5.14H2O with an U/P ratio = 7/4 instead of 6/4 or 8/4 as generally presumed. The structure consists of [(UO2)3(OH)2(PO4) 2]n2n- layers (type dumontite), parallel to (100), connected by Ca2+, UO2+ and H2O. The structure is compared with that of dewindtite. It is shown how two minerals whose formulae are rather different (replacement of 3PbO by 1CaO+1UO3+1H2O) can have similar structures, owing to subtle reorganizations