A Raman and infrared spectroscopic study of the uranyl silicates—Weeksite, soddyite and haiweeite

Raman spectroscopy has been used to study the molecular structure of a series of selected uranyl silicate minerals, including weeksite K2[(UO2)2(Si5O13)]·H2O, soddyite [(UO2)2SiO4·2H2O] and haiweeite Ca[(UO2)2(Si5O12(OH)2](H2O)3 with UO22+/SiO2 molar ratio 2:1 or 2:5. 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. For example, soddyite is characterized by Raman bands at 828.0, 808.6 and 801.8 cm−1 (UO2)2+ (ν1), 909.6 and 898.0 cm−1 (UO2)2+ (ν3), 268.2, 257.8 and 246.9 cm−1are assigned to the ν2 (δ) (UO2)2+. Coincidences of the ν1 (UO2)2+ and the ν1 (SiO4)4− is expected. Bands at 1082.2, 1071.2, 1036.3, 995.1 and 966.3 cm−1 are attributed to the ν3 (SiO4)4−. 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 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.