Raman microscopy of selected autunite minerals

Raman spectroscopy at ambient and liquid nitrogen temperature has been used to determine the molecular structure of selected minerals from the autunite and meta-autunite groups. Six hydroxyl stretching bands for autunite are observed of which three are highly polarised. The hydroxyl stretching vibrations are related to the strength of hydrogen bonding of the water units. Bands in the Raman spectrum of autunite at 998, 842 and 820 cm(-1) are highly polarised. Low intensity band at 915 cm(-1) is attributed to the nu(3) antisymmetric stretching vibration of (UO2)(2+) units. The band at 820 cm(-1) is attributed to the nu(1) symmetric stretching mode of the (UO2)(2+) units. The (UO2)(2+) bending modes are found at 295 and 222 cm(-1). The presence of phosphate and arsenate anions and their isomorphic substitution are readily determined by Raman spectroscopy. The collection of Raman spectra at 77 K enables excellent band separation.