A Raman and infrared spectroscopic study of the uranyl silicates - weeksite, soddyite and haiweeite

Abstract

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 to 800 cm-1 region and in the 950 to 1000 cm-1 region assigned to the ν1 modes of the (UO2)2+ units and to the (SiO4)4- tetrahedra. For example soddyite is characterised by Raman bands at 828.0, 808.6, 801.8 cm-1 (UO2)2+ (￮1), 909.6 and 898.0 cm-1 (UO2)2+ (￮3), 268.2 cm-1 and 257.8 and 246.9 cm-1are assigned to the ￮2 (￤) (UO2)2+. Coincidences of the ￮1 (UO2)2+ and the ￮1 (SiO4)4- is expected. Bands at 1082.2, 1071.2, 1036.3, 995.1, 966.3 cm-1 are attributed to the ￮3 (SiO4)4-.Sets of Raman bands in the 200 to 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 to 550 cm-1 and in the 390 to 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 to 3500 cm-1 and 3600 to 3700 cm-1.