4.84 grams 32x24x2mm PLATEAU PUTORANO native iron like MESOSIDERITE meteorite

Description

This is not a meteorite but once was thought to be many years ago.
Putorana
is a place-name in Siberia. Meteorites are traditionally named for their location where they were seen to fall, or were found later, often many years later. The lustrous, rounded blebs of silvery-white metal in a fine-grained groundmass are reminiscent of a mesosiderite, the less-flashy cousins of pallasites, the better-known class of stony iron meteorites. This material was analysed soon after its discovery, and deemed to be an elegant "pseudometeorite", a fine facsimile of a class of meteorite, but nonetheless of terrestrial origin.
Even as a sample of
native iron
, Putorana is of considerable mineralogical interest, for native iron is rare on Earth, where virtually all iron is bound up with oxygen in oxide, silicate, carbonate and other, less-common oxygen-rich mineral species, such as sulphates and arsenates. Iron also occurs as common sulphides, such as pyrite, pyrrhotite (equivalents of troilite in meteorites), and in ore minerals such as the copper-iron sulphide chalcopyrite and the nickel-iron sulphide pentlandite. Reduced iron species, such as carbides, silicides, phosphides and alloys with other metals (such as nickel or platinum) are uncommon to downright rare, and some are known only in meteorites, or in extreme reducing environments like lightning strikes.
The
possible meteoritic provenance
was studied by Treiman
et al.
(2001, 2002). and their negative finding was later cited in the valuable handbook by Norton and Chitwood (2008) in a list of "meteorwrongs" (
ibid.
, pp.175-180). The purported meteorite(s) are from the Putorana plateau of the Noril'sk district of Siberia, a region known to economic geologists for the huge nickel-copper-platinum-palladium deposits of the Noril'sk-Talnakh "ore junction", associated with rift basalts and related magmas of Permian age. The region, east and southeast of Noril'sk, includes a nature reserve (Montaigne and Olson, 2000), near which the sample was apparently recovered.
The samples were first identified not as meteorites but as fine-grained basaltic breccia with basalt, fine-grained "anorthosite" and feldspathic dunite clasts, lacking the cosmogenic radionuclide
26
Al. Plagioclase, olivine and rare ilmenite co-crystallized, followed by pigeonite. There is a rusted exterior, no obvious fusion crust, and a fresh interior. The lack of a crust obviously speaks against a meteoritic origin for such fresh material, although the mineralogy is certainly unusual for terrestrial lavas, including cohenite and rare native copper (Treiman
et al.
, 2001). The rock contains Ni-bearing native iron (essentially, but not quite, kamacite, generally found only in meteorites) but despite this unusual metal, it is not a meteorite - in addition to the evidence above, oxygen isotope ratios appear terrestrial, and some of the silicate mineral-chemical details are not appropriate to meteorites. This "Putorana" rock is presumed to be related to Siberian Trap basalt magmatism. The metallic copper can be seen in hand specimen, or within kamacite. There is no taenite, the typical nickel-rich alloy complement of kamacite in meteorites. Iron carbide (Fe
3
C, cohenite) encloses kamacite. Comparison with native iron on Earth (e.g., Disko Island, west Greenland) and with planetary basalt suites and mesosiderites identifies Putorana as terrestrial (Treiman
et al.
, 2002). Electron microprobe analysis of Putorana iron reveals that the iron contains minor amounts of Ni (2.31%), Co (0.54%) and Cu (0.12%), very comparable to published data on iron from Disko Island (see below).
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