IVA irons
Fine octahedrites, including the famous Gibeon.
- Classification
- Iron meteorites › IVA irons
- Share of witnessed falls
- about 0.4%
- In our catalogue
- 75 meteorites, 4 seen to fall
- Heaviest
- Gibeon, 26 t
- Earliest recorded fall
- Charlotte, 1835
- Most recent fall
- Akyumak, 1981
IVA irons are fine octahedrites with about 7.5–9.5% nickel and very low gallium and germanium. Gibeon, from Namibia, is a IVA iron widely used for jewellery and watch dials because of its beautiful etched pattern. Their cooling rates vary widely, suggesting their parent asteroid was broken apart while still hot.
Mineralogy and chemistry
IVA irons contain about 7.5% to 9.5% nickel and are fine octahedrites with kamacite bands about 0.2 to 0.5 mm wide, giving the delicate, closely spaced pattern that makes them popular for jewellery. They have very low gallium and germanium and contain troilite, chromite and, in a few, silicate inclusions. Their trace elements follow a clear magmatic trend.
Steinbach and São João Nepomuceno are anomalous IVAs that contain abundant pyroxene and silica (tridymite), unusual silicates for an iron meteorite.
Parent body and formation
Cooling rates measured in IVA irons vary enormously, from about 100 to several thousand degrees per million years, and they rise with nickel content. That is the opposite of what a core buried under a mantle would show. The best explanation is that the IVA parent was a core roughly 300 km across stripped of its mantle by a 'hit-and-run' collision with a larger body, then left to cool and crystallise as a naked ball of metal, from the outside in. Lead isotopes in troilite from Muonionalusta give an age of about 4,565 million years, showing that the core formed and cooled very early.
Abundance and famous examples
IVA is one of the larger groups, about 8% of classified irons. Gibeon was found over a huge area of southern Namibia, where Nama people made tools from it; tonnes of fragments are known, and many masses are displayed in Windhoek. Its export is now banned. Muonionalusta, found in northern Sweden in 1906, may have reached Earth around a million years ago and survived in glacial deposits, one of the longest stays on Earth known for any meteorite. Steinbach, found in Saxony in 1724, is packed with pyroxene and silica, so it was long treated as a stony-iron. Bishop Canyon, Duchesne and Altonah from the western United States are other members.
How to recognise one
IVA irons are heavy and magnetic, and when etched show a fine, even Widmanstätten pattern that many people know from Gibeon jewellery and watch faces. Weathered pieces have a brown rind. Classification requires analysis, as other fine octahedrites look similar.
Famous examples
- Gibeon Iron, IVA, Namibia, 1836 · 26 t
- Muonionalusta Iron, IVA, Sweden, 1906 · 230 kg
- Steinbach Iron, IVA-an, Germany, 1724 · 98 kg
- São João Nepomuceno Iron, IVA-an, Brazil, 1960 · 15.3 kg
- Bishop Canyon Iron, IVA, United States, 1912 · 8.6 kg
- Duchesne Iron, IVA, United States, 1906 · 22.7 kg
- Altonah Iron, IVA, United States, 1932 · 21.5 kg
Related groups
- IVB irons Nickel-rich ataxites. Hoba, the largest meteorite, is one.
- IIIAB irons The most common iron meteorites: medium octahedrites from one asteroid core.
- IIIF irons A small group with very low gallium and germanium.
- Iron meteorites Almost pure iron-nickel metal, mostly from the cores of asteroids that melted and were later shattered.
Photos shared under Creative Commons licences (Wikimedia Commons).