A Raman Spectroscopic Study of the Uranyl Sulphate Mineral Johannite

Raman spectroscopy at 298 and 77 K has been used to study the secondary uranyl mineral johannite of formula (Cu(UO2)2(SO4)2(OH)2.8H2O). Four Raman bands are observed at 3593, 3523, 3387 and 3234 cm-1 and four infrared bands at 3589, 3518, 3389 and 3205 cm-1. The first two bands are assigned to OH- units (hydroxyls) and the second two bands to water units. Estimations of the hydrogen bond distances for these four bands are 3.35, 2.92, 2.79 and 2.70 Å. A sharp intense band at 1042 cm-1 is attributed to the (SO4)2- symmetric stretching vibration and the three Raman bands at 1147, 1100 and 1090 cm-1 to the (SO4)2- antisymmetric stretching vibrations. The ν2 bending modes were at 469, 425 and 388 cm-1 at 77 K confirming the reduction in symmetry of the (SO4)2- units. At 77 K two bands at 811 and 786 cm-1 are attributed to the ν1 symmetric stretching modes of the (UO2)2+ units suggesting the non-equivalence of the UO bonds in the (UO2)2+ units. The band at 786 cm-1, however, may be related to water molecules libration modes. In the 77 K Raman spectrum, bands are observed at 306, 282, 231 and 210 cm-1 with other low intensity bands found at 191, 170 and 149 cm-1. The two bands at 282 and 210 cm-1 are attributed to the doubly degenerate ν2 bending vibration of the (UO2)2+ units. Raman spectroscopy can contribute significant knowledge in the study of uranyl minerals because of better band separation with significantly narrower bands, avoiding the complex spectral profiles as observed with infrared spectroscopy.