IVB irons
A small magmatic iron group of nickel-rich ataxites, very depleted in volatile elements.
IVB is a small magmatic iron group whose members are almost all ataxites, containing roughly 16–18 per cent nickel. They are extremely depleted in moderately volatile elements such as gallium and germanium, and rich in refractory elements such as iridium, suggesting a parent body formed from material that had lost volatiles in a hot region of the early Solar System. Within the group, element trends are consistent with fractional crystallisation of a core.
Because of the high nickel, IVB irons lack a visible Widmanstätten pattern. Under the microscope, they show fine spindles of kamacite set in plessite. Isotopic studies place the IVB parent body on the carbonaceous-chondrite side of a major isotopic divide among meteorites, suggesting it formed in the outer part of the early Solar System.
The best-known IVB iron is Hoba in Namibia, about 60 tonnes and the largest known meteorite. Tlacotepec from Mexico and Cape of Good Hope from South Africa are other members. In hand, IVB irons are heavy, bright under polish and relatively resistant to rust; group assignment requires trace-element chemistry.
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