Iron meteorite

Iron meteorites consist overwhelmingly of nickel-iron alloys. The metal taken from these meteorites is known as meteoric iron and was one of the earliest sources of usable iron available to man.

Occurrence

While they are fairly rare compared to the stony meteorites, comprising about 5.7% of witnessed falls, they have historically been heavily over-represented in meteorite collections. This is due to several factors:

• They are easily recognized as unusual even by laymen, as opposed to stony meteorites. Modern-day searches for meteorites in deserts and Antarctica yield a much more representative sample of meteorites overall.
• They are much more resistant to weathering.
• They are much more likely to survive atmospheric entry, and are more resistant to the resulting ablation. Hence they are more likely to be found as large pieces.

In fact, Iron meteorites account for almost 90% of the mass of all known meteorites, about 500 tons. All the largest known meteorites are of this type, including the largest -- the Hoba meteorite.

Origin

Iron meteorites have been linked to M-type asteroids since both types of objects have similar spectral characteristics in the visible and near-infrared wavelength regions. Iron meteorites are thought to be the fragments of the cores of larger ancient asteroids that have been shattered by impacts. The IIE chemical class may be a notable exception, in that they probably originate from the crust of S-type asteroid 6 Hebe.

Chemical and isotope analysis indicates that at least about 50 distinct parent bodies were involved. This implies that there were once at least this many large, differentiated, asteroids in the asteroid belt - many more than today.

Composition

The overwhelming bulk of these meteorites consists of the Fe,Ni-alloys kamacite and taenite. Minor minerals, when occurring, often form rounded nodules of troilite or graphite, surrounded by schreibersite and cohenite. Schreibersite and troilite also occur as plate shaped inclusions, which show up on cut surfaces as cm-long and mm-thick lamellae. The troilite plates are called Reichenbach lamellae.^[1]

The chemical composition is dominated by the elements Fe, Ni and Co, which make up more than 95%. Ni is always present, the concentration lies between 5 and about 25%.^[2] It can be used to distinguish meteoritic irons from technical products, which contain usually lower amounts of Ni.

Use

The iron nickel alloy was used by several cultures for the manufacturing of tools and weapons. For example the Inuit used parts of the Cape York meteorite.^[3][4]

Classification

Two classifications are in use.^[5] The older structural classification is based on the presence or absence of the Widmanstätten pattern, which can be assessed from the appearance of polished cross-sections that have been etched with acid. This is connected with the relative abundance of nickel to iron. The categories are:

• Hexahedrites: low nickel, no Widmanstätten pattern.
• Octahedrites: average to high nickel, Widmanstätten patterns, most common
• Ataxites: very high nickel, no Widmanstätten pattern, rare

The Octahedrites can be further divided up on the basis of the properties of their Widmanstätten patterns into coarse,medium, and fine octahedrites.

A newer chemical classification based on the proportions of the trace elements Ga, Ge and Ir separates the iron meteorites into classes corresponding to distinct asteroid parent bodies:

• IAB class
• IC class
• IIAB class
• IIC class
• IID class
• IIE class
• IIG class
• IIF class
• IIIAB class
• IIICD class
• IIIE class
• IIIF class
• IVA class
• IVB class
• Ungrouped meteorites. This is actually quite a large collection (about 15% of the total) of over 100 meteorites which do not fit into any of the larger classes above, and come from about 50 distinct parent bodies.

References

1. ^ J.G. Burke: Cosmic Debris, Meteorites in History. University of California Press, 1986.
2. ^ J.T. Wasson: Meteorites, Classification and Properties. Springer-Verlag 1974.
3. ^ T. A. Rickard (1941). "The Use of Meteoric Iron". The Journal of the Royal Anthropological Institute of Great Britain and Ireland 71 (1/2): 55–66. doi:10.2307/2844401. http://links.jstor.org/sici?sici=0307-3114%281941%2971%3A1%2F2%3C55%3ATUOMI%3E2.0.CO%3B2-8. 
4. ^ Buchwald, V F (1992). "On the Use of Iron by the Eskimos in Greenland". Materials Characterization 29 (2): 139–176. doi:10.1016/1044-5803(92)90112-U. http://links.jstor.org/sici?sici=0307-3114%281941%2971%3A1%2F2%3C55%3ATUOMI%3E2.0.CO%3B2-8. 
5. ^ Vagn F. Buchwald: Handbook of Iron Meteorites, University of California Press 1975.

External links

• Meteorite articles, including discussions of iron meteorites, in Planetary Science Research Discoveries