IIAB irons
Hexahedrites and coarsest octahedrites, including Sikhote-Alin.
- Classification
- Iron meteorites › IIAB irons
- Share of witnessed falls
- about 0.6%
- In our catalogue
- 119 meteorites, 6 seen to fall
- Heaviest
- Sikhote-Alin, 23 t
- Earliest recorded fall
- Braunau, 1847
- Most recent fall
- Raghunathpura, 1986
The IIAB irons have the lowest nickel of the major groups, about 5.3–6.4%. IIA irons are hexahedrites, single kamacite crystals often showing Neumann lines; IIB irons are coarsest octahedrites. Sikhote-Alin, the great Russian iron shower of 1947, belongs here. They formed in the core of one asteroid.
Mineralogy and chemistry
IIAB irons have the lowest nickel of the major groups, about 5.3% to 6.4%. IIA irons, with less than about 5.7% nickel, are hexahedrites: each mass is made of very large single crystals of kamacite, which break along cube faces and show fine parallel Neumann lines when etched, caused by shock. IIB irons, with a little more nickel, are coarsest octahedrites with very broad kamacite bands. They are rich in gallium and germanium and contain schreibersite, troilite and sometimes graphite.
Iridium varies hugely, from less than 0.01 to about 60 parts per million, the steepest trend in any group, showing a long history of crystallisation from a core liquid rich in sulphur.
Parent body and formation
IIAB irons formed in the core of a single asteroid in the inner Solar System. Modelling of their trace elements suggests the core started with several per cent sulphur, and that solid metal crystallised as the liquid evolved, with schreibersite-rich IIG irons possibly representing the last liquid trapped within it. Cooling rates are moderately slow, consistent with a core in a body tens of kilometres across.
Abundance and famous examples
IIAB is one of the larger iron groups, with more than a hundred members. The Sikhote-Alin fall on 12 February 1947, in the Russian Far East, was one of the largest witnessed meteorite falls in history: a fireball brighter than the Sun broke up and scattered thousands of fragments, with about 23 tonnes recovered and the total estimated at 70 tonnes or more. Its pieces show both sharp shrapnel and rounded individuals with regmaglypts. Coahuila hexahedrites from Mexico, North Chile, Negrillos, Old Woman (California, about 2.75 tonnes) and Uwet (Nigeria) are other members. Braunau fell in Bohemia in 1847, Avče in present-day Slovenia in 1908, and Boguslavka in the Russian Far East in 1916.
How to recognise one
IIA hexahedrites break and cut in a way that reveals no Widmanstätten pattern, only fine Neumann lines after etching, and may split along flat cleavage planes. IIB irons show very coarse bands. Both are heavy and strongly magnetic. Sikhote-Alin pieces often have a blue-grey crust or rusty surface with deep thumbprints.
Famous examples
- Sikhote-Alin Iron, IIAB, Russia, 1947 · 23 t
- Coahuila Iron, IIAB, Mexico, 1837 · 2.1 t
- Old Woman Iron, IIAB, United States, 1976 · 2.75 t
- North Chile Iron, IIAB, Chile, 1875 · 300 kg
- Negrillos Iron, IIAB, Chile, 1936 · 28.5 kg
- Braunau Iron, IIAB, Poland, 1847 · 39 kg
- Boguslavka Iron, IIAB, Russia, 1916 · 256 kg
- Uwet Iron, IIAB, Nigeria, 1903 · 54 kg
Related groups
- IIG irons Low-nickel irons with large crystals of schreibersite.
- IIC irons A rare group of plessitic octahedrites.
- IC irons A small magmatic group of coarse octahedrites, low in nickel.
- Iron meteorites Almost pure iron-nickel metal, mostly from the cores of asteroids that melted and were later shattered.
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