IVB irons
Nickel-rich ataxites. Hoba, the largest meteorite, is one.
- Classification
- Iron meteorites › IVB irons
- In our catalogue
- 14 meteorites
- Heaviest
- Hoba, 60 t
IVB irons are ataxites with about 16% to 18% nickel and very high iridium. They show no Widmanstätten pattern to the eye. Hoba, the 60-tonne iron in Namibia, is a IVB.
Mineralogy and chemistry
IVB irons contain about 16% to 18% nickel, so they are ataxites: on an etched surface the eye sees no pattern, but a microscope reveals a fine intergrowth of tiny kamacite spindles in plessite. They have the highest iridium and other refractory metals of any iron group, with about 13 to 38 parts per million iridium, and the lowest gallium and germanium, less than a part per million. They contain little troilite or carbon.
This combination, rich in elements that condense at high temperature and poor in volatile ones, shows that the parent body formed from material that was strongly heated or had lost its volatile elements, and that it was oxidised enough for much of its iron to stay in silicates, leaving nickel-rich metal.
Parent body and formation
IVB irons come from the core of a single asteroid in the outer, carbonaceous region of the Solar System. Their trace elements follow a magmatic trend. Cooling rates are fast, thousands of degrees per million years, suggesting a small core or one exposed early. Hafnium-tungsten ages indicate the core formed within a few million years of the first solids.
Abundance and famous examples
IVB is a small group of about fifteen members, none seen to fall. Hoba, found on a farm near Grootfontein in Namibia in 1920, is the largest known meteorite, a slab about 2.7 m long and 1 m thick weighing around 60 tonnes. It has never been moved, and it was made a national monument in 1955. Cape of Good Hope, found in South Africa in 1793, was one of the first IVBs studied. Tlacotepec and Santa Clara from Mexico, Skookum from the Yukon, Warburton Range from Western Australia, Iquique from Chile and Weaver Mountains from Arizona are other members.
How to recognise one
IVB irons are heavy and magnetic like any iron, but etching reveals no pattern to the eye, only a uniform dull or slightly silky surface. Their high nickel makes them unusually resistant to rust. A laboratory can measure their nickel and iridium.
Why they matter
As cores from the outer Solar System with extreme chemistry, IVB irons show how differently planetesimals could form and evolve. Hoba, as the largest meteorite on Earth, draws visitors from around the world and shows why iron meteorites, tough and resistant to weathering, survive in sizes no stony meteorite can match.
Famous examples
- Hoba Iron, IVB, Namibia, 1920 · 60 t
- Cape of Good Hope Iron, IVB, South Africa, 1793 · 136 kg
- Tlacotepec Iron, IVB, Mexico, 1903 · 71 kg
- Santa Clara Iron, IVB, Mexico, 1976 · 63 kg
- Skookum Iron, IVB, Canada, 1905 · 16 kg
- Warburton Range Iron, IVB, Australia, 1963 · 56.93 kg
- Iquique Iron, IVB, Chile, 1871 · 12.5 kg
- Weaver Mountains Iron, IVB, United States, 1898 · 38.8 kg
Related groups
- Ungrouped irons Irons that fit no known group: each may represent its own asteroid.
- IVA irons Fine octahedrites, including the famous Gibeon.
- 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).