Hot stage Raman Spectroscopic Study of the Thermal Decomposition of Saleeite

Hot stage Raman spectroscopy has been used to study the changes in structure of saléeite (Mg(UO2)2(PO4)2.10H2O) during dehydration. Raman spectra were obtained using a Renishaw Raman microscope coupled with a Linkham thermal stage. Temperatures for Raman spectroscopic analysis were selected by studying the high resolution thermogravimetric analysis of the mineral saléeite. Five stages of dehydration are observed at 43, 73, 87, 231 and 376 degrees Celsius with the loss of 3, 3, 2, 1 and 1 mole of water respectively. Three antisymmetric PO4 stretching vibrations are observed at 1007, 994 and 980 cm-1. No symmetric stretching vibration is observed until after significant dehydration has occurred at 75 degrees Celsius, when a low intensity band at 927 cm-1 is observed. Two bands at 843 and 827 cm-1 are attributed to the symmetric stretching modes of the two UO2 vibrational modes resulting from the two non-equivalent UO bonds. The low intensity band at 894 cm-1 is assigned to the UO2 antisymmetric stretching mode. The complexity that is observed in the ν3 antisymmetric stretching region of the PO4 units is also observed in the PO4 ν2 bending region. Four bands are observed at 471, 446, 405 and 376 cm-1. Two bands observed at 283 and 234 cm-1 are assigned to the (UO2)2+ bending modes. After dehydration it is suggested that the structure loses its distortion, takes on a tetragonal structure and hence the Raman spectra become less complex upon thermal treatment.