The Gebel Kamil ataxite

--- a nickel-rich iron meteorite from Egypt, north Africa


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Figure 1. A 595-gram individual, like a flattened axe head, with sharp protuberances on the bevelled edges and a characteristic dimpling on one side. This face (illustrated) is the inferred upper, exposed surface that was subjected to natural sand blasting on the desert floor. This iron's most typical form is jagged shrapnel (see, e.g., the example in Brandstatter et al., 2013, p.160). The resemblance to shrapnel is evident.

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Figure 2. A larger, 2527-gram piece, 145x105x38 mm. The upper surface is shown once again (the reverse is smooth in comparison, and the hue is slightly more rusty). Hackly pronged margins are seen, and the thick edge at the front in this photograph may mark part of a larger shear plane along which the parent mass disintegrated upon its hypervelocity arrival at Earth's surface.


"Rock of the Month #151, posted for January 2014" ---

Meteorite

The Gebel Kamil iron meteorite is a distinctive body, comprising many, mostly small and shrapnel-like fragments, similar to Sikhote-Alin in Siberia, Henbury in Western Australia and Whitecourt in Alberta, Canada. It has a very nickel-rich composition and, accordingly, polished and etched surfaces show no Widmanstatten pattern, but instead exhibit a mirror-like finish with scattered crystals of a more bronzey-coloured mineral, the iron-nickel phosphide schreibersite. According to the Meteoritical Bulletin (original listing: No.95. Weisberg et al., 2010, pp.1532-1533), Gebel Kamil is an ungrouped iron meteorite, a Ni-rich ataxite assaying a very high 19.8% Ni, plus 0.75% Co, 464 ppm Cu, 49 ppm Ga and 121 ppm Ge. It is also very rich in platinum group elements (PGE) and Au, a total of 13.12 ppm (Ir + Ru + Rh + Pt + Pd + Au, including 4.8 ppm Pd, 3.5 ppm Pt and 1.57 ppm Au: metal grades to make a terrestrial exploration geologist drool!

The find of more than 5,000 individual fragments, in the East Uweinat desert of southwest Egypt, has a TKW (total known weight) of at least 1600-1700 kg, associated with a crater 45 m in diameter. A new set of analyses of rare gas isotopes and cosmogenic nuclides indicates a preatmospheric radius of >85 cm, and probably 115-120 cm, corresponding to a preatmospheric mass of 50-60 tonnes (Ott et al., 2014).

The groundmass of the iron consists of very fine-grained duplex plessite, a microscopically fine intergrowth of nickel-iron alloys. Accessory minerals include schreibersite and troilite. Blades of schreibersite can be seen in the fine metal groundmass of the Gebel Kamil iron, both in polished and etched slices and occasionally preserved on the outer weathered surfaces (see, e.g., Fig. 1; Warin and Kashuba, 2013; Shanos, 2014, p.32). Daubreelite, near-pure FeCr2S4, is found exclusively within troilite (FeS). Native copper occurs as small blebs nucleated at troilite-kamacite and schreibersite grain boundaries. Most evident are the mm-scale elongate schreibersite inclusions with swathing kamacite in plessite (D'Orazio et al., 2011).

With the notable exception of one large, nicely regmaglypted 83-km individual, the thousands of other Gebel Kamil fragments formed by explosion, and most are small, They range in weight from <1 g up to 3 kg and exceptionally to 34 kg, the second-largest mass. The inferred mass of these angular shrapnel fragments >10 g in size in the strewnfield is some 3.4 tonnes (D'Orazio et al., 2011). Thus it seems that the largest mass may have split from the impactor in the atmosphere, while all the other, angular fragments were formed in the subsequent hypervelocity impact. In other words, roughly 2 percent of the estimated meteorite on the ground is a distinct airburst fragment derived from atmospheric spallation, while the remainder is shrapnel. The impacting body evidently arrived from the northwest, spreading ejecta and the greatest distribution of shrapnel-form fragments for several hundred metres to the southeast of the crater (Folco et al., 2011).

Local Geology

According to Klitzsch and Schandelmeier (1990) "most of southwest Egypt is a cuesta type landscape of late Jurassic to Cretaceous clastics, of small to medium high escarpments with extensive sand and gravel sheets situated between them", though the older basement rocks include Proterozoic and younger granulites and migmatites, marbles, amphibolites and older and younger suites of granites. Dates on gneissic outliers in the Western Desert show that the late Archean Congo craton extends at least as far north as the Gebel Kamil and Chephren quarries. A zircon date of 2629 Ma is reported from Gebel Kamil (hornblende plagioclase gneiss), interpreted as the minimum age of emplacement of a tonalite protolith, with subsequent zircon growth at 2063 Ma and sphene growth or resetting at 1947 Ma (Sultan et al., 1992). The desert floor excavated by the geologically-recent impact is composed of lower Cretaceous sandstones of the Gilf Kebir Formation.

The Gebel Kamil Impact Structure

The impact structure was first noticed in June 2008 in a Google Earth image by Italian scientist V. de Michele (Folco et al., 2010). The 45-m-wide crater is geologically young, most probably formed <5,000 years ago (D'Orazio et al., 2011). The crater extends roughly 7 m below the level of the surrounding desert plain, and the rim height averages 3 m above the plain. 20 of 21 of the 182 terrestrial impact structures (known in 2012) that are <1 km in diameter are <1 Ma old, and 15/21 of these are <100,000 years old. Gebel Kamil is a good example of a small, hypervelocity impact structure, as documented by ground-penetrating radar, differential GPS and magnetic surveys (Urbini et al., 2012). Collection and mapping of microscopic impactor debris around the 45-m-wide crater reveals a downrange ejecta blanket which can be traced some 400 m from the crater (Folco et al., 2015). Work continues on the crater (e.g., Fazio et al., 2014; Sighinolfi et al., 2015; and Lorenz et al., 2015), which increasingly seems to have been formed in the late Holocene, no more than 5,000 years ago.

The small crater and the associated shards of iron meteorite were surveyed by Italian scientists in 2009, with over 5,000 fragments of meteorite recovered (Folco et al., 2010, 2011). The meteorite is a possible source of nickel in archaeological artefacts from pharaonic Egypt, the "iron of heaven" of the ancients. Artefacts derived from meteoritic iron in ancient Egypt include iron beads, blades and amulets, notably funerary or grave goods such as daggers. The appearance of a word for meteorites ("iron from the sky") in the Egyptian language suggests that a significant fall must have been seen: possibly Gebel Kamil, though it would be nice to have an accurate date for the fall, probably in the past 5,000 years (Johnson and Tyldsley, 2013; Sighinolfi et al., 2015).

Subsequent to the ground confirmation of the site in February 2009, many specimens from the site were taken and sold into the international meteorite trade, and the material is now widespread (Broad, 2011).

References

Brandstatter,F, Ferriere,L and Koeberl,C (2013) Meteoriten - Meteorites: Zeitzeugen der Entstehung des Sonnensystems / Witnesses of the Origin of the Solar System. Verlag des Naturhistorisches Museum, Vienna, 270pp. (in Engl. and in Ger.).

Broad,WJ (2011) Everybody wants a piece of an asteroid. International Herald Tribune, 10, 06 April.

D'Orazio,M, Folco,L, Zeoli,A and Cordier,C (2011) Gebel Kamil: the iron meteorite that formed the Kamil crater (Egypt). Meteoritics & Planetary Science 46, 1179-1196.

Fazio,A, Folco,L, D'Orazio,M, Frezzotti,ML and Cordier,C (2014) Shock metamorphism and impact melting in small impact craters on Earth: evidence from Kamil crater, Egypt. Meteoritics & Planetary Science 49, 2175-2200.

Folco,L, D'Orazio,M, Fazio,A, Cordier,C, Zeoli,A, Van Ginneken,M and El-Barkooky,A (2015) Microscopic impactor debris in the soil around Kamil crater (Egypt): inventory, distribution, total mass, and implications for the impact scenario. Meteoritics & Planetary Science 50, 382-400.

Folco,L et al. (2010) The Kamil crater in Egypt. Science 329, 804, 13 August.

Folco,L et al. (2011) Kamil crater (Egypt): ground truth for small-scale meteorite impacts on Earth. Geology 39, 179-182.

Johnson,D and Tyldsley,J (2013) Iron from the sky: meteorites in ancient Egypt. Meteorite 19 no.4, 8-13.

Klitzsch,E and Schandelmeier,H (1990) South Western Desert. In `The Geology of Egypt' (Said,R editor), A.A.Balkema, Rotterdam, 734pp., 249-257.

Lorenz,CA, Ivanova,MA, Artemieva,NA, Sadilenko,DA, Chennaoui Aoudjehane,H, Roschina,IA, Korochantsev,AV and Humayun,M (2015) Formation of a small impact structure discovered within the Agoudal meteorite strewn field, Morocco. Meteoritics & Planetary Science 50, 112-134.

Ott,U, Merchel,S, Herrmann,S, Pavetich,S, Rugel,G, Faestermann,T, Fimiani,L, Gomez-Guzman,JM, Hain,K, Korschinek,G, Ludwig,P, d'Orazio,M and Folco,L (2014) Cosmic ray exposure and pre-atmospheric size of the Gebel Kamil iron meteorite. Meteoritics & Planetary Science 49, 1365-1374.

Shanos,GT (2014) Meteoritic schreibersite and its role in the origin of life. Meteorite 20 no.2, 32-34.

Sighinolfi,GP, Sibilia,E, Contini,G and Martini,M (2015) Thermoluminescence dating of the Kamil impact crater. Meteoritics & Planetary Science 50, 204-213.

Sultan,M, Tucker,RD, Gharbawi,RI, Ragab,AI and El Alfy,Z (1992) On the extension of the Congo craton into the Western Desert of Egypt. GSA Abs.w.Progs. 24 no.7, Cincinnati, 138.

Urbini,S et al. (2012) Geological and geophysical investigation of Kamil crater, Egypt. Meteoritics & Planetary Science 47, 1842-1868.

Warin,R and Kashuba,J (2013) Sulfides, phosphides and iron. Meteorite 19 no.3, 21-25.

Weisberg,MK, Smith,C, Herd,CDK, Haack,H, Yamaguchi,A, Chennaoui Aoudjehane,H, Welzenbach,L and Grossman,JN (2010) The Meteoritical Bulletin, No.98, September 2010. Meteoritics & Planetary Science 45, 1530-1550.

Graham Wilson, 02 October 2013, last updated 11 April 2015.

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