IIC irons
A rare group of plessitic octahedrites.
- Classification
- Iron meteorites › IIC irons
- In our catalogue
- 8 meteorites
- Heaviest
- Unter-Mässing, 80 kg
IIC irons are plessitic octahedrites with about 9–11.5% nickel. Only a few are known.
Mineralogy and chemistry
IIC irons contain about 9.3% to 11.5% nickel and are plessitic octahedrites: their Widmanstätten pattern is made of very narrow kamacite spindles in a fine, mixed plessite, almost too small to see without magnification. They are rich in gallium and germanium and contain schreibersite and troilite, and their trace elements follow a magmatic trend.
Isotopes of molybdenum and other elements place IIC in the 'carbonaceous' family, so the parent body likely formed in the outer Solar System, beyond the early orbit of Jupiter.
Parent body and formation
The IIC group formed in the core of one asteroid. The fine structure of their metal points to relatively rapid cooling, suggesting a smaller body or a core that lost its insulating mantle early. With so few members, the details of their crystallisation are still uncertain.
Like other irons from the carbonaceous family, IIC irons formed within a few million years of the first solids, showing that bodies in the outer Solar System also melted and formed cores very early.
Abundance and examples
IIC is a small group with fewer than ten known members, all finds, none seen to fall. Ballinoo and Kumerina, both from Western Australia, are among the larger and better studied. Others come from North and South America and Europe.
Because the group is so small, each newly classified IIC iron noticeably improves the picture of its parent core. Members are recognised by their combination of nickel, gallium, germanium and iridium, and by their distinctive plessitic structure, which can be seen in polished and etched sections under a microscope.
How to recognise one
IIC irons are heavy, magnetic masses. When etched, they show a fine, dense pattern unlike the broad bands of common octahedrites; a hand lens is needed to see the spindles. Classification needs analysis of nickel, gallium, germanium and iridium.
Why they matter
As one of the iron groups from the outer Solar System, the IICs help scientists compare cores formed in the two great isotopic reservoirs. Differences in their composition, such as higher contents of some volatile metals, show that the bodies from which they came were made of different raw material from the inner-Solar-System groups like IIIAB.
Each such group also sharpens the timeline: the cores of carbonaceous bodies seem to have formed slightly later than those of inner bodies, perhaps because these bodies grew later or contained ice that delayed melting.
Famous examples
Related groups
- IID irons Fine to medium octahedrites rich in gallium and germanium.
- IIF irons A small group of magmatic irons.
- IVB irons Nickel-rich ataxites. Hoba, the largest meteorite, is one.
- 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).