The structure of phurcalite-A vibrational spectroscopic study

The uranyl mineral phurcalite of formula Ca2(UO2)3O2(PO4)2·7H2O from three different origins has been studied by Raman spectroscopy at both room temperature and liquid nitrogen temperature in conjunction with infrared spectroscopy. Raman bands are attributed to the (UO2)2+ symmetric stretching vibrations and are complimented by bands assigned to the (UO2)2+ antisymmetric stretching vibrations. U-O bond lengths in uranyls are calculated from the wavenumbers of observed Raman and infrared bands. Raman and infrared bands are attributed to the (PO4)3− symmetric and antisymmetric stretching vibration. Some alternate attributions of the bands to the (UO2)2+ and (PO4)3− stretching vibrations are given. Coincidences of these bands and also of the bands related to the (PO4)3− bending vibrations and libration modes of water molecules are proposed. Multiple bands in the bending region reflect the complexity of the phurcalite structure, as does the complexity of the 200-300 cm−1 region where the (UO2)2+ bending modes are expected. Three bands observed in the region 1590-1680 cm−1 and assigned to HOH bending modes show the existence of at least three different types of water molecules with different hydrogen bonding strengths in the phurcalite structure. In the case of the mineral sample MR5, infrared bands observed at 3591, 3537, 3516, 3416, 3256 and 3000 cm−1 are related to the OH stretching region bands. This gives rise to hydrogen bond distances of 2.920, 2.809, 2.724 and 2.649 Å.