Molecular structure of the uranyl mineral zippeite - An XRD, SEM and Raman spectroscopic study

Raman spectra at 298 and 77 K and infrared spectra of the uranyl sulfate mineral zippeite, K-2[(UO2)(6)(SO4)(3)O(OH)(6)] center dot 4H(2)O, were studied. Observed bands were tentatively attributed to the (UO2)(2+) and (SO4)(2-) stretching and bending vibrations, the OH stretching vibrations of water molecules and hydroxyls, H2O bending vibrations and libration modes, and 8 U-OH bending vibrations. Empirical relations were used for calculation of U-O bond lengths in uranyl R = f(v(3) or v(1) (UO2)(2+)) angstrom. This was found in agreement with U-O bond lengths from the single crystal structure analysis. The number of observed bands supports the conclusion from single crystal structure analysis that at least two symmetrically distinct U6+ (in uranyl) and S6+ (in sulfate), and water molecules and hydroxyls may be present in the zippeite crystal structure. Some O-H...O bond lengths were attributed to the hydrogen-bonding network in zippeite crystal structure.