IIG irons
Low-nickel irons with large crystals of schreibersite.
- Classification
- Iron meteorites › IIG irons
- In our catalogue
- 6 meteorites
- Heaviest
- Tombigbee River, 43 kg
IIG irons are coarse, low-nickel irons containing large amounts of schreibersite, an iron-nickel phosphide. They are a small group recognised in 2011.
Mineralogy and chemistry
IIG irons are among the lowest in nickel of all iron meteorites, about 4.1% to 4.9%, so they are made almost entirely of kamacite and contain no taenite. They are extremely rich in phosphorus, much of it in large crystals of schreibersite that can make up a sizeable fraction of a cut face, and very poor in sulphur, with little or no troilite. Their structure is coarse, sometimes described as a coarse hexahedrite or coarsest octahedrite with schreibersite.
On element plots they lie close to the IIAB irons but offset from them, with different trends in several trace elements.
Parent body and formation
The leading idea, proposed when the group was defined in 2011, is that IIG irons formed in the IIAB core itself. As that core crystallised, the remaining liquid may have separated into two immiscible melts, one rich in sulphur and one rich in phosphorus. The phosphorus-rich liquid could have crystallised as the IIG irons, trapped in pockets within the IIAB metal. Others prefer a separate parent body. Either way, the IIGs record the unusual chemistry of the last liquid in a crystallising core.
Abundance and members
Six members are known. Bellsbank (South Africa), La Primitiva (Chile) and Tombigbee River (Alabama) were recognised as similar in 1967 and called the Bellsbank trio. Twannberg in Switzerland, found from 1984 onwards in a strewn field near Lake Biel, and Guanaco from Chile, found in 2000, brought the number to five, the minimum for a group, and Auburn from Alabama made six. None has been seen to fall. Tombigbee River was found as a cluster of several masses totalling about 43 kg.
How to recognise one
IIG irons are heavy and magnetic. When cut and etched they show broad kamacite and abundant pale, brittle schreibersite, sometimes as large crystals or skeletal networks. They can be confused with other low-nickel irons, so analysis of nickel, phosphorus and trace elements is needed.
Why they matter
If the IIG irons did form inside the IIAB core, they are a rare sample of the very last liquid in a crystallising asteroid core, telling us how sulphur and phosphorus behave as a core freezes. That has implications for planetary cores, including Earth's, which is also thought to contain light elements such as sulphur.
Famous examples
- Tombigbee River Iron, IIG, United States, 1859 · 43 kg
- Bellsbank Iron, IIG, South Africa, 1955 · 38 kg
- La Primitiva Iron, IIG, Chile, 1888 · 27.4 kg
- Twannberg Iron, IIG, Switzerland, 1984 · 20.69 kg
- Guanaco Iron, IIG, Chile, 2000 · 13.1 kg
- Auburn Iron, IIG, United States, 1867 · 3.63 kg
Related groups
- IIAB irons Hexahedrites and coarsest octahedrites, including Sikhote-Alin.
- 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).