Molecular structure of the uranyl silicates—a Raman spectroscopic study

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 750–800 cm−1 region and in the 950–1000 cm−1 region assigned to the ν1 modes of the (UO2)2+ units and to the (SiO4)4− tetrahedra. Sets of Raman bands in the 200–300 cm−1 region are assigned to ν2 δ (UO2)2+ and UO ligand vibrations. Multiple bands indicate the non-equivalence of the UO bonds and the lifting of the degeneracy of ν2 (δ) (UO2)2+ vibrations. The (SiO4)4− tetrahedral are characterized by bands in the 470–550 cm−1 region and in the 390–420 cm−1 region. These bands are attributed to the ν4 and ν2 (SiO4)4− bending modes. The minerals show characteristic OH stretching bands in the 2900–3500 cm−1 and 3600–3700 cm−1 regions ascribed to water stretching and SiOH stretching vibrations. The high wavenumber position of the δH2O bands indicates strong hydrogen bonding of water in these uranyl silicates. Bands in the 1400–1550 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. Copyright © 2005 John Wiley & Sons, Ltd.