Mineralogy and origin of oxygen-bearing platinum-iron grains based on an X-ray absorption spectroscopy study
Résumé
Oxygen-bearing platinum group minerals have been reported from many locations, but their mineralogical identity has been debated. We conducted an X-ray micro-beam absorption study of O-bearing Pt-Fe grains recovered from a stream in southern New Caledonia. The Pt LIII-edge absorption spectrum indicates that Pt is mostly in the metallic state, not combined with O or OH–. The bond length between Pt and the neighboring atoms, based on the extended X-ray absorption fine structures, is much longer than the Pt-O bond length, also indicating that Pt is not bonded with O. A quantitative simulation of the atomic structure around Pt suggests that O-bearing Pt-Fe has a similar, but disturbed, structure compared to isoferroplatinum. The absorption spectra of Fe K-edge for the studied samples have a small peak at 7122 eV and a large peak at 7128 eV. Since isoferroplatinum has a larger peak at 7122 eV, the peak at 7128 eV is explained by the presence of an Fe3+-bearing phase. A 10:90 mixture of ferrihydrite (Fe3+-O-OH) and isoferroplatinum yields the observed absorption spectra. The data suggest that the O-bearing Pt-Fe is a physical mixture of relict isoferroplatinum and newly precipitated Fe3+-O-OH. We suggest that O-bearing Pt-Fe formed by the dissolution of Fe0 followed by the dissolution of Pt0 from isoferroplatinum and precipitation of Fe3+ during weathering of host ultramafic rocks.