The Agoudal IIAB iron meteorite, Morocco, north Africa

--- a find in 2000 and (most material) late 2012 onwards..

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Figure 1. A small, rather jagged sample of the Agoudal iron. This is said to be typical of many Agoudal specimens. Note the rusty surface with subangular edges and shallow impressions, a flattened shard of "shrapnel" form. Mass 126.10 g. A slice from a larger mass is shown in Fig. 2. Both specimens from Blaine Reed.


"Rock of the Month #193, posted for July 2017" ---

The Agoudal IIAB iron meteorite

was found in the High Atlas Mountains of south-central Morocco. According to Meteoritical Bulletin No.102, summarized in the following paragraph, the total known weight (TKW) of Agoudal is circa 100 kg, though as of the time of writing the actual TKW may be more like 500 kg. The first two samples were found in 2000, and sold to tourists. A piece was only identified as an iron meteorite in 2011, and, late the following year, large amounts of additional material were recovered in a metal detector search (see also Arnold and Beauford, 2014). Most pieces were small, recovered on or near surface. The main mass, 60 kg in weight, was found at a depth of about 50 cm. Most specimens are angular "shrapnel" 2-5 cm in size.However, the small (1-15-gram) pieces are rounded bodies that fell as individuals, developing fusion crust and avoiding the terminal impact of the main mass (Blaine Reed, pers.comm., 18 July 2017). Etched slices show coarse kamacite grains, often with curved boundaries. Some areas show Neumann bands, other are shock-hatched, with incipient recrystallization of finer-grained kamacite. Schreibersite is abundant as skeletal crystals, rhabdites and grain boundary precipitates. Some troilite nodules are present. ICP-MS analyses at the University of Alberta returned bulk values of 5.5 wt.% Ni, 0.41 wt.% Co and 58 ppm Ga, 1 ppm Au and <0.04 ppm Ir. It has been sold and traded under the pseudonym "Imilchil".

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Figure 2. A polished, lightly etched slice of Agoudal, maximum 200x103x4 mm, mass 412.65 g, displays Neumann lines, and numerous schreibersite inclusions aligned towards and along two crystallographic planes within the host kamacite *. This iron is a member of class IIAB, coarsest octahedrite. Some specimens show evidence of heat-induced recrystallization, perhaps attesting to a crater-forming impact.

* The Nomenclature Committee of the International Mineralogical Association has ruled that the ancient term kamacite, applying to domains of iron with appreciable nickel content, is officially referred to the one mineral species, native iron (Burke, 2006). This makes sense in an etymological and taxonomic manner, but the meteoritic terms are unlikely to disappear any time soon, so deeply entrenched are they in the scientific literature, and so convenient for describing metallographic textures in iron, stony-iron, and many chondrite and achondrite meteorite classes. Based on my modest unpublished microprobe data on meteorites, kamacite commonly contains circa 5-9 wt.% Ni and 0.5-1.0% Co, with less / much less nickel than the related Ni-Fe alloy compositions of phases taenite (analogous to and often intergrown with kamacite) and tetrataenite (circa 25-35 and 48-52 wt.% Ni, respectively).


Fine shatter cones have been found in the area (Dickens, 2014), suggestive of an impact structure. A detailed study affirms the presence of such a structure (Lorenz et al., 2015). A heavily- eroded relic impact structure contains shatter cones in limestone basement, and both autochthonous and allochthonous breccias. Iron fragments in the latter contain 5.15% Ni and 19 ppb Ir (similar to the Agoudal meteorite). Agoudal seems to be another example of a small impact structure with an iron impactor and typically "shrapnel"-type fragments of the meteorite. Other examples include Henbury in Australia, Whitecourt in Alberta, Canada, and Gebel Kamil in Egypt.

In a separate context, fine mineral specimens have been recovered from the Agoudal locality in the Bou Azzer mineral district. Erythrite and sphaerocobaltite are two of more than 200 mineral species known in the region (context and examples: Favreau et al., 2007; Polytika, 2016).

References

Arnold,A and Beauford,R (2014) Introductory meteorite fieldwork - Part 2: where to hunt and why to hunt there. Meteorite 20 no.2, 16-20, summer.

Burke,EAJ (2006) A mass discreditation of GQN minerals. Can.Mineral. 44, 1557-1560.

Dickens,D (2014) The Agoudal astrobleme. Meteorite 20 no.1, 10-15 (spring).

Favreau,G, Dietrich,JE, Meisser,N, Brugger,J, Haddouch,LA and Maacha,L (2007) Bou Azzer, Morocco. Mineral.Record 38, 345-407.

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.

Polityka,J (organizer) (2016) Mineral Collections in the American Northeast. Mineral.Record 47 no.4, supplement, 460pp.

Graham Wilson, 15-19 July 2017

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