[meteorite-list] Oktibbeha County Super Ataxite? ...was The tale of a falling star

From: Mr EMan <mstreman53_at_meteoritecentral.com>
Date: Sun, 14 Jun 2009 14:49:11 -0700 (PDT)
Message-ID: <963993.53226.qm_at_web55207.mail.re4.yahoo.com>

The Oktibbeha Meteorite is a "triple hitter" for unusual meteorites and unfortunately only a few grams are still in existence.

I believe someone asked about other meteorites which were found in entombed, in burial mounds, etc. The Oktibbeha County, Mississippi meteorite found in 1857 in a burial mound (just West of Columbus, Mississippi in what would have likely been Creek Nation Territory).

The mineral(sic)Oktibbehite was "identifed" in this find but no mineral data appears to exist for it. Here is the link for a descriptive mineralogy page:
<http://www.minersoc.org/pages/Archive-MM/Volume_38/38-297-623.pdf>

Even more preplexing is the microprobe analysis from the 1960's which shows a Ni content of 60%. That might be the record!

Elton
 
Here is the text:
SHORT COMMUNICATIONS
MINERALOGICAL MAGAZINE, MARCH I972, VOL. 38, PP. 623-26
The Oktibbeha County iron meteorite
TAYLOR (I857) first described the unique Oktibbeha County iron meteorite, whichwas found in an Indian burial mound in Mississippi. It was originally egg-shapedand weighed about 15o g, but was split along a fissure by a sledge-hammer blow,dividing it into two nearly equal parts, one of which was forged, while the other was subjected to chiseling, sawing, and filing. According to Hey (I966) about 46 g now remains. Analyses by Taylor and by Cohen 0892) show a nickel content of about6o %, the highest recorded in a meteorite. Hey lists another iron of similar nickel content (La Fayette), but none now remains. The next highest is the Santa Catherina iron with about 34 % Ni.
Doubts have been expressed from time to time about the authenticity of the
Oktibbeha County meteorite, because of its extraordinary nickel content, although a natural terrestrial origin is improbable, since native nickel-iron (awaruite) from N. American localities contains too much nickel (74 to 77 % according to Palache et al., T944). Perry (I942, I944) observed phosphide inclusions (identified by sodium
picrate etching) in a specimen of Oktibbeha County from the American Museum of Natural History, New York, which resembled rhabdite (= schreibersite, (Fe, Ni)~P) found in ordinary iron meteorites. The identity of similar inclusions in a specimen from the Academy of Natural Sciences, Philadelphia (no. Ioo67), has now been confirmed as schreibersite by electron microprobe analysis. Fig. I shows scanning pictures of 'rhabdite' in this specimen, taken with the phosphorus Ka X-ray line. The formation of euhedral rhabdite-like schreibersite requires the very slow cooling typical of iron meteorites.
X-ray diffraction shows the metal to consist of large, well-oriented taenite (9,-nickeliron) single crystals, giving further indication of slow cooling. In ordinary iron meteorites rhabdite occurs as orientated needles in kamacite (a-nickel-iron), and is thought to have formed below 50o ~ The Oktibbeha County 'rhabdite' probably formed at a considerably higher temperature, because diffusion is much slower in taenite than in kamacite at a given temperature. There appears to be a definite orientation relationship between the 'rhabdite' and the face-centred cubic taenite in Oktibbeha County, comparable with that existing between rhabdite and body-centred cubic kamacite in ordinary irons. The Philadelphia specimen consists of a slice probably passing approximately
through the centre of the original ovoid mass. At the edge the original schreibersite has been heated to form rounded metal-phosphide eutectoid areas. The microprobe scanning pictures in fig. 2 show how the heating effect decreases with distance from distance of a few ram. In the centre of the slice schreibersite is unaltered.
This effect is almost certainly due to the thermal gradient produced by shortlived
but intense surface heating caused by atmospheric friction, and is therefore
further evidence for the meteoritic nature of the specimen. Similar effects are found
in other iron meteorites.
The British Museum (Natural History) specimen (B.M. 34595) appears to be an
artificially heated piece of material originally similar to the New York and Philadelphia
specimens. The metal is of granular appearance in the microscope, and X-ray diffraction
indicates disorientated granular taenite. Small irregular areas of barely resolvable
metal-phosphide eutectoid distributed throughout the specimen are presumably
relics of the schreibersite in the unaltered material. There is no evidence of a thermal
gradient. Photomicrographs by Perry (i94z, ~944) of a specimen from Harvard
University show a similar microstructure.
Table I gives microprobe analyses of the B.M. (N.H.) and Philadelphia specimens,
together with earlier analyses. Neither the metal nor the schreibersite were found to
vary appreciably in composition in the Philadelphia specimen, outside the peripheral
heated zone. The metal in the B.M. (N.H.) specimen was only slightly variable in
composition.
TABL~ I. Analyses of the Oktibbeha County meteorite
Previous analyses Microprobe analyses Phil. IOO67 B.M. 34595
Taylor Cohen metal schreiber- metal metal-(I857) (I892) site phosphide eutectoid
Ni 59"7 62-0 ~.I 65q 6r.o 65"3
37'7 37"z 39"0 20-0 38-0 22-5
Cu 0"9 o"3 0"6 -- 0-8 --
Co o'4 o'7 o'5 o'4 o'5 o'4
P o.I o.z o.o I4-6 o.o Iz.I
98"8 IOO'4 ioo.2 Io0.1 IOO'3 lO0"3
The schreibersite in the Philadelphia specimen corresponds approximately to(Fe0.vNi2.3) P, which is the most nickel-rich schreibersite recorded. The association of very nickel-rich schreibersite with taenite of similar nickel content is in accord with the phase diagram (Doan and Goldstein, I969), in which schreibersite and taenite in equilibrium contain about the same wt. % Ni.
Conclusion. The Philadelphia specimen of Oktibbeha County is undoubtedly meteoritic.
Theories of the origin of iron meteorites should therefore take into account the existence of meteoritic metal containing 6o % Ni.

Acknowledgements. This work was carried out in the Mineralogy Dept., British Museum
(Natural History). I am indebted to Mrs. J. M. Hall for assistance with the electron microprobe
analyses, and to Dr. R. J. Davis for X-ray diffraction data. I am very grateful to the
Director of the Academy of Natural Sciences, Philadelphia, Dr. H. R. Roberts, for the loan
of the specimen.
Dept. of Geophysics and Geochemistry S.J.B. REED
Australian National University
Canberra, Australia
REFERENCES
COHEN (E.), 1892. Ann. Naturhist. Hofmus. Wien, 7, 146.
DOAN (A. S.) and GOLDSTEIN (J. I.), 1969. In MILLNAN (P. M.), Meteorite Research, 763. Reidel(Dordrecht).
HEY (M. H.), I966. Catalogue of Meteorites, 3rd edn, Brit. Mus. (Nat. Hist.), London.
PALACI-IE (C.), BERMAN (H.), and FRONDEL (C.), I944. Dana's System of Mineralogy, 7th edn, 1, Wiley, New York.
PERRY (S. H.), I942. Photomierographs of Meteoric Irons, 3, Adrian, Michigan.
-- I944. The Metallography of Meteoric Iron, U.S. Nat. Mus. Bull. 184, Washington. TAYLOR (W. J.), 1857. Amer. Journ. Sci. ser. z, 24, 293.
[Manuscript received I I January 197t]
9 Crown copyright reserved.
Received on Sun 14 Jun 2009 05:49:11 PM PDT