Vibrational spectroscopy of selected natural uranyl vanadates

Raman spectroscopy has been used to study a selection of uranyl vanadate minerals including carnotite, curienite, francevillite, tyuyamunite and metatyuyamunite. The minerals are characterised by an intense band in the 800-824 cm−1 region, assigned to the ν1 symmetric stretching vibrations of the (UO2)2+ units. A second intense band is observed in the 965-985 cm−1 range and is attributed to the ν1(VO3) symmetric stretching vibrations in the (V2O8) units. This band is split with a second component observed at around 963 cm−1. A band of very low intensity is observed around 948 cm−1 and is assigned to the ν3 antisymmetric stretching vibrations of the (VO3) units. Bands in the range 608-655 cm−1 may be attributed to molecular water librational modes or the stretching modes ν(V2O2) units. Bands in the range 573-583 cm−1 may be connected with the ν(UOequatorial) vibrations or ν(V2O2) units. Bands located in the range 467-539 cm−1 may be also attributed to the ν(UOequatorial) units vibrations. The bending modes of the (VO3) units are observed in the 463-480 cm−1 range-there may be some coincidence with ν(UOequatorial). The bending modes of the (V2O2) in the (V2O8) units are located in a series of bands around 407, 365 and 347 cm−1 (ν2). Two intense bands are observed in the 304-312 cm−1 range and 241-264 cm−1 range and are assigned to the doubly degenerate ν2 modes of the (UO2)2+ units. The study of the vibrational spectroscopy of uranyl vanadates is complicated by the overlap of bands from the (VO3) and (UO2)2+ units. Raman spectroscopy has proven most useful in assigning bands to these two units since Raman bands are sharp and well separated as compared with infrared bands. The uranyl vanadate minerals are often found as crystals on a host matrix and Raman spectroscopy enables their in situ characterisation without sample preparation.