Large deposits of baddeleyite (ZrO2) are found in the Phalaborwa complex in South Africa, and is found mainly in the foskorite ore zone and to a lesser extent in the carbonatite ore body [1]. Very little data has been reported for the systematic analysis of the baddeleyite microstructure [2]. Previous work [3] has reported on the extremely variable values for uranium (50-2000ppm), lead (19-782ppm) and thorium (0.5-73ppm) in Phalaborwa baddeleyite which is possibly due to large probe diameters and hence large analytical interaction volumes generated by analytical techniques such as SIMS (SHRIMP technique) and LA-ICP-MS analysis which were used for determining the isotope ratios in baddeleyite for geochronology applications [4-5]. In this paper we will describe the characterization and formation of small (0.1-1 μm) uranium oxide (UO2) and lead oxide (PbO) primary inclusions in Phalaborwa baddeleyite.
Polished sections of a Phalaborwa baddeleyite xenocryst were used to prepare FIB lamella for TEM analysis. The FIB lamellae were imaged using the TEM and HAADF STEM mode in a JEOL ARM 200F TEM. Compositional analysis and elemental mapping was performed by energy dispersive spectroscopy (EDS).
The results obtained from the TEM and STEM data indicated that there is an abrupt boundary between the uranium and baddeleyite phases with a crystallographic epitaxial relationship. This suggests that there was an exsolution process of the uranium during cooling after the geological emplacement process. This observation is consistent with the phase diagram [6] for the UO2-ZrO2 system which predicts that the two phases are immiscible below 1 000°C. The presence of a PbO inclusion within the UO2 will be shown to be inconsistent with the miscibility of the two systems.