IVA irons
A magmatic iron group of fine octahedrites, low in gallium and germanium, with puzzlingly varied cooling rates.
Second only to IIIAB among magmatic irons, the IVA group is a large and well-studied family. Its members are mostly fine octahedrites with roughly 7–10 per cent nickel and very low gallium and germanium, reflecting a strongly volatile-depleted parent body. Trace elements follow a fractional-crystallisation trend consistent with one core.
What makes IVA famous is its cooling rates. Metallographic measurements give rates that vary greatly from one meteorite to another and correlate with nickel content, something hard to explain in a core insulated by a rocky mantle. One influential interpretation is that a hit-and-run collision stripped away most of the mantle, leaving a largely bare metal body that cooled from the outside in. Some IVA irons, such as Steinbach and São João Nepomuceno, contain pyroxene or silica-rich silicates, adding further clues.
The best-known IVA irons are Gibeon from Namibia, which fell in prehistoric times and is scattered over a large strewn field, and Muonionalusta from northern Sweden, which carries stishovite and has a very long terrestrial history. Both show crisp fine Widmanstätten patterns when etched. Group assignment requires bulk trace-element analysis.
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