Chemical and microstructural characterisation studies on natural and heat treated brannerite samples

Two naturally occurring brannerite samples from the Crockers Well and Roxby Downs deposits in South Australia were characterised to assess their compositional, mineralogical, and textural properties. Characterisation was undertaken using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Raman Spectroscopy and an Electron Probe Micro-analysis (EPMA). The samples were also heat treated to examine the effect of temperature on recrystallisation, mineral stability and deportment of impurities. In each sample, the brannerite was found to be thorium-rich (∼8–10 wt.% Th) indicating partial solid solution between brannerite and thorutite (ThTi2O6). Other minerals identified in the samples were thorutite (ThTi2O6), uranothorite ([Th, U]SiO4), a thorianite–uraninite solid solution ([Th, U]O2) and gangue minerals including rutile, quartz, aluminosilicates, zircon, titanite, apatite and unidentified REE-containing phosphates. The brannerite in both samples was amorphous having undergone radiation-induced metamictisation. Crystallinity of the brannerite was restored upon heating of the samples to 1200 °C for 24 h in air. For the Crockers Well sample, radiation damage annealing/recrystallisation began at temperatures as low as 800 °C while the Roxby Downs sample did not begin to show evidence for recrystallisation until 900 °C. The main mineralogical changes after heating and recrystallisation involved a decrease in the amount of thorianite-uraninite phase due to decomposition into ThO2 and UO2 as well as the formation of Pb-rich glass films at grain boundaries. Compositional, textural and microstructural aspects of brannerite are likely to play an important role in any process to extract uranium from brannerite-containing ores.