Raman microscopy of autunite minerals at liquid nitrogen temperature

Uranyl micas are based upon (UO2PO4)− units in layered structures with hydrated counter cations between the interlayers. Uranyl micas also known as the autunite minerals are of general formula M(UO2)2(XO4)2·8–12H2O where M may be Ba, Ca, Cu, Fe2+, Mg, Mn2+ or 12(HAl) and X is As or P. The structures of these minerals have been studied using Raman microscopy at 298 and 77 K. Six hydroxyl stretching bands are observed of which three are highly polarised. The hydroxyl stretching vibrations are related to the strength of hydrogen bonding of the water OH 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 ν3 antisymmetric stretching vibration of (UO2)2+ units. The band at 820 cm−1 is attributed to the ν1 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.