IIAB irons
A magmatic iron-meteorite group of low-nickel hexahedrites and coarsest octahedrites.
The IIAB group is a magmatic iron group defined by low nickel contents, from roughly 5 to 6.5 per cent, and relatively high gallium and germanium. Its members span a long iridium range, from iridium-rich early crystallisation products to strongly depleted late ones, a classic signature of fractional crystallisation in a sulfur-rich core. Historically, "IIA" referred to hexahedrites and "IIB" to the slightly more nickel-rich coarsest octahedrites, but they form one continuous group.
Structurally, IIA members are single kamacite crystals with no Widmanstätten pattern and abundant Neumann lines from shock. IIB members show very coarse kamacite and often large schreibersite crystals. Sikhote-Alin, which fell in eastern Russia on 12 February 1947, is the most famous IIAB iron: a coarsest octahedrite that broke up to produce thousands of fragments and over a hundred craters. Coahuila and Braunau are classic hexahedrite members.
Diagnosis combines structure with chemistry. A low-nickel iron showing Neumann lines on etching is a strong hint, but bulk analysis of nickel, gallium, germanium and iridium confirms membership and separates IIAB from rarer low-nickel groups.
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