Mineralogy and origin of oxygen-bearing platinum-iron grains based on an X-ray absorption spectroscopy study

作者：Hattori Keiko H^*; Takahashi Yoshio; Auge Thierry

来源：American Mineralogist, 2010, 95(4): 622-630.

DOI：10.2138/am.2010.3391

• XANES
• EXAFS
• PGM oxides
• PG E
• weathering
• serpentinization

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 L(III)-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 Fe(3+)-bearing phase. A 10:90 mixture of ferrihydrite (Fe(3+)-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 Fe(3+)-O-OH. We suggest that O-bearing Pt-Fe formed by the dissolution of Fe(0) followed by the dissolution of Pt(0) from isoferroplatinum and precipitation of Fe(3+) during weathering of host ultramafic rocks.

• 出版日期2010-4