The Bushveld Complex in South Africa hosts the world's largest resources of platinum group elements (PGE), which are mainly mined from three ore bodies, namely the Merensky Reef, the UG-2 chromitite and the Platreef. In these ores, the PGE are bimodally distributed, occurring both as discrete platinum-group minerals (PGM) and hosted by sulfides. The presence of PGE in sulfides has been demonstrated by electron probe microanalysis, laser ablation induced coupled plasma mass spectrometry, secondary ion mass spectrometry and particle-induced X-ray emission. However, evidence is lacking on the mineralogical siting of the PGE, e.g. whether they occur in solid solution, as nano- and/or micro-inclusions. Therefore, in the present study a combination of focused ion beam and transmission electron microscopy was used which allows to obtain crystal-structural relationships between the host mineral
and incorporated trace elements and revealing the physicochemical state of the PGE in sulfides. The present study confirms the existence of micrometer-sized discrete PGM in the ores. Further, the PGE occur in a number of forms, namely (1) as discrete nanoinclusions of PGM, (2) as patchily distributed solid solution, (3) ordered within the pentlandite crystal structure, substituting for Ni and/or Fe (superlattice), and (4) as homogenous solid solution. Nanometer-sized PGM (nPGM) show no orientation relationship with the host sulfide mineral. Consequently, they are discrete phases, which were trapped within pentlandite during sulfide growth. Heterogeneous and patchy distributions of Rh and Ir within the pentlandite lattice suggest that Rh and Ir were already present within the sulfide liquid. The absence of possible reaction partners (e.g. Bi, As, Sn) necessary for the formation of discrete PGM forced Rh and Ir to remain in the crystal lattice of pentlandite and down-temperature exsolution caused patchy distribution patterns of Rh and Ir. High concentrations of Rh and Ir in pentlandite initiate ordering of the randomly distributed PGE in form of nanometer-sized lamellae resulting in the formation of a superlattice. Palladium is homogenously distributed within the pentlandite lattice, even at high Pd concentrations, and in addition also occurs as nPGM.