No abstract

Raman spectroscopy has been used to study the molecular structure of a series of selected uranyl silicate minerals including uranophane, sklodowskite, cuprosklodowskite, boltwoodite and kasolite. Raman spectra clearly show well resolved bands in the 750 to 800 cm -1 region and in the 950 to 1000 cm -1 region assigned to the ν 1 modes of the (UO 2 ) 2+ units and to the (SiO 4 ) 4-tetrahedra. Sets of Raman bands in the 200 to 300 cm -1 region are assigned to ν 2 δ (UO 2 ) 2+ and UO ligand vibrations. Multiple bands indicate the non-equivalence of the UO bonds and the lifting of the degeneracy of ν 2 δ (UO 2 ) 2+ vibrations. The (SiO 4 ) 4-tetrahedral are characterized by bands in the 470 to 550 cm -1 and in the 390 to 420 cm -1 region. These bands are attributed to the ν 4 and ν 2 (SiO 4 ) 4-bending modes. The minerals show characteristic OH stretching bands in the 2900 to 3500 cm -1 and 3600 to 3700 cm -1 region ascribed to water stretching and SiOH stretching vibrations. The high wavenumber position of the δH 2 O bands indicate strong hydrogen bonding of water in these uranyl silicates. Bands in the 1400 to 1550 cm -1 region are attributed to δSiOH modes. The Raman spectroscopy of uranyl silicate minerals enabled separation of the bands attributed to distinct vibrational units. This enabled definitive assignment of the bands. The spectra are analysed in terms of the molecular structure of the minerals.

Raman and infrared spectra of five uranyl oxyhydroxide hydrates, becquerelite, billietite, curite, schoepite and vandendriesscheite, are reported. The observed bands are attributed to the (UO 2 ) 2+ stretching and bending vibrations, U-OH bending vibrations and H 2 O and (OH) − stretching, bending and libration modes. The U-O bond lengths in uranyls and the O-H· · ·O bond lengths are calculated from the wavenumbers assigned to the stretching vibrations. They are close to the values inferred and/or predicted from the X-ray single-crystal structure. The complex hydrogen-bonding network arrangement was proved in the structures of all the minerals studied. This hydrogen bonding contributes to the stability of these uranyl minerals.

Raman and infrared spectra of the uranyl mineral zellerite, Ca[(UO 2 )(CO 3 ) 2 (H 2 O) 2 ]·3H 2 O, were measured and tentatively interpreted. U-O bond in uranyl and O-H· · ·O hydrogen bonds were calculated from the vibrational spectra. The presence of structurally nonequivalent water molecules in the crystal structure of zellerite was inferred. A proposed chemical formula of zellerite is supported. Raman bands at 3514, 3375 and 2945 cm −1 and broad infrared bands at 3513, 3396 and 3326 cm −1 are related to the n OH stretching vibrations of hydrogen-bonded water molecules. Observed wavenumbers of these vibrations prove that in fact hydrogen bonds participate in the crystal structure of zellerite. The presence of two bands at 1618 and 1681 cm −1 proves structurally distinct and nonequivalent water molecules in the crystal structure of zellerite.