IIIAB irons
The largest group of iron meteorites, mostly medium octahedrites formed in a once-molten asteroid core.
IIIAB is the most populous iron-meteorite group, accounting for a large share of all classified irons. Its members have nickel contents of roughly 7 to 10 per cent and are mostly medium octahedrites, with kamacite bands around 1 millimetre wide. Trace elements follow well-defined fractional-crystallisation trends, so the group is regarded as the crystallised core of a single parent asteroid, sampled from early to late solids.
Because there are so many specimens, IIIAB has become a benchmark for modelling how cores crystallise, including the role of sulfur and phosphorus and the trapping of melt. Main-group pallasites have similar metal chemistry and oxygen isotopes to some IIIAB irons, prompting suggestions of a link, although whether they share a parent body is debated. Famous IIIAB irons include Cape York from Greenland, Henbury from Australia, which made a field of small craters, and Willamette, the large iron found in Oregon in 1902.
On etched slices, IIIAB irons show a regular Widmanstätten pattern, frequent troilite nodules and lamellae, and schreibersite. Classification relies on bulk nickel, gallium, germanium and iridium, often supported by structure.
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