Molecular structure of the uranyl mineral andersonite - a Raman spectroscopic study

The molecular structure of the mineral andersonite, a uranyl tricarbonate, has been studied by Raman microscopy at 298 and 77 K and ATR infrared spectroscopy. Raman bands are observed at 3558, 3510 and 3415 cm(-1) and infrared bands at 3559, 3502, 3388, 3270 and 3120 cm(-1) enabling estimates of the hydrogen bond distances of 2.98, 2.91, 2.80, 2.77, 2.70(7) and 2.70(1) Angstrom to be estimated. The wide range of values shows the water molecules are not equivalent in the structure of andersonite. Two well resolved Raman bands at 1092 and 1080 cm(-1) assigned to the nu(1) symmetric stretching modes of the (CO3)(2-) units prove the existence of two non-equivalent carbonate units. A single peak at 832 cm(-1) resolvable into two bands at 833 and 831 cm(-1) is attributed to the overlap of the symmetric stretching modes of the (UO2)(2+) units and the nu(2) bending modes of the (CO3)(2-) units. The nu(4) in-plane bending region is characterised by three Raman bands at 745, 726 and 698 cm(-1) confirming the loss of symmetry of the carbonate anion and the non-equivalence of the carbonate units. Raman spectroscopy at 298 and 77 K assists in the assignation of bands to the structure of andersonite. (C) 2004 Elsevier B.V. All rights reserved.