• journal_title：American Mineralogist
• Contributor：Kéiko H. Hattori ; Yoshio Takahashi ; Thierry Augé
• Publisher：Mineralogical Society of America
• Date：2010-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2010.3391
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：95
• issue：4
• firstpage：622
• section：Articles

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 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.