Crystal and Electronic Structure of (Fe,Ni,PGE)-Pentlandites: Planetary and Economic Implications

English Abstract:

We aim to test the hypothesis that sulphide minerals such as pentlandite (Pn) are capable of preferentially uptaking platinum-group-elements (PGE) and playing a critical role in PGE mineralization. Our goal is to combine synchrotron X-ray absorption (XAFS) as well as Electron-energy loss spectroscopy (EELS) experiments along with electronic structure calculations to probe Pn crystallization processes that bear relevance to not only the formation of magmatic PGE deposits, but also models of planetary accretion and evolution.

Current models suggest that immiscible sulphide liquids form during the evolution of magmatic PGE deposits. As the immiscible liquid cools, sulphide minerals crystallize. One of the most economically important minerals in magmatic PGE deposits is Pn. Experimental observations show that palladium (Pd) preferentially partitions in Pn, whereas platinum (Pt) is incorporated into other phases. The contrasting behaviour between Pt and Pd is enigmatic, considering their apparently similar geochemical affinities. More recently, some authors have suggested the presence of PGE nanoclusters in the Pn lattice and the potential of Pn for hosting PGEs has come into question.

Pn occurs mostly as a ternary Fe-Ni phase, although abnormaly Fe-rich Pn has been reported to occur in meteorites. It is well known that the stabilities of solids depends largely on their configurational entropies and our modeling will take this into consideration using Special Quasirandom Structures (SQS) as implemented in a novel algorithm. This SQS approach will allow us to determine the optimal distribution of the Fe and Ni atoms by matching a specified set of correlations (or cluster functions) between neighboring atoms to the corresponding correlations of the perfectly disordered state. The optimal ordering of the Fe and Ni atoms will be determined over a wide range of Fe/Ni values, and their relative stabilities will be evaluated quantitatively. Subsequently, PGE (i.e., Pd and Pt) will be incorporated in the disordered lattices of Pn calculated from the SQS approach. Once the optimal ordering of the atoms has been determined, the electronic structures of Pn will be investigated and the ionic degrees of freedom will relax by solving the Kohn-Sham equations within the spin-polarized-generalized-gradient approximation.