Iron meteorite

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Iron meteorites consist overwhelmingly of nickel-iron alloys.

1 Occurrence
2 Origin
3 Composition
4 Classification
5 References
6 External links

[edit] 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 ^[1], including the largest -- the Hoba meteorite.

[edit] Origin

The majority of iron meteorites originate from M-type asteroids, which are 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.

[edit] Composition

The overwhelming bulk of these meteorites consists of the alloys kamacite and taenite. Minor minerals, when occurring, often form rounded nodules of troilite or graphite, surrounded by schreibersite and cohenite.

[edit] Classification

Two classifications are in use.

The older Structural classification is based on the relative abundance of nickel to iron, which can be assessed from the appearance of polished cross-sections that have been etched with acid. ^[2] The categories are:

Hexahedrites (low nickel)
Octahedrites (average to high nickel), most common
Ataxites (very high nickel), rare

The Octahedrites can be further divided up on the basis of the properties of their Widmanstätten patterns.

A newer Chemical classification based on the proportions of trace elements separates the iron meteorites into classes corresponding to distinct asteroid parent bodies ^[3]:

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.