IIIAB irons
The most common iron meteorites: medium octahedrites from one asteroid core.
- Classification
- Iron meteorites › IIIAB irons
- Share of witnessed falls
- about 1.1%
- In our catalogue
- 292 meteorites, 11 seen to fall
- Heaviest
- Cape York, 58.2 t
- Earliest recorded fall
- Rowton, 1876
- Most recent fall
- Sterlitamak, 1990
IIIAB is the largest group of irons, about a third of all irons known. They are mostly medium octahedrites with about 7–10.5% nickel, and their trace elements show clearly how a single asteroid core crystallised from the outside in.
Many famous irons are IIIABs, including Cape York, Willamette and Henbury. The main-group pallasites have a closely related chemistry.
Mineralogy and chemistry
IIIAB irons contain about 7% to 10.5% nickel and are mostly medium octahedrites, with kamacite bands about 0.5 to 1.3 mm wide. They contain troilite nodules, schreibersite and chromite. Gallium and germanium are moderate. Plotted against nickel, iridium in IIIAB irons falls by a factor of about a thousand along a smooth curve: the textbook example of a metal core crystallising, with the earliest solids rich in iridium and the latest liquids rich in nickel and gold.
Parent body and formation
IIIAB irons come from a single large core in the inner Solar System. Cooling rates measured in their metal vary from about 50 to over 300 degrees per million years and fall as nickel rises. That pattern means the core crystallised from the outside in and cooled with little rocky mantle to insulate it, perhaps because collisions had already stripped most of its mantle. Main-group pallasite metal is chemically similar to the most evolved IIIAB irons, but the two are not now thought to share a parent body.
Abundance and famous examples
IIIAB is the largest iron group, about a third of all classified irons. Cape York is a group of masses found in north-west Greenland, where the Inughuit people chipped iron from them for tools for centuries; the explorer Robert Peary shipped the largest, Ahnighito, about 31 tonnes, to New York in 1897, and another, Agpalilik, about 20 tonnes, is in Copenhagen. Willamette, found in Oregon in 1902, about 15.5 tonnes, is the largest meteorite found in the United States and is sacred to the Clackamas people, who call it Tomanowos. Henbury in Australia's Northern Territory and Wabar in Saudi Arabia formed small crater fields. Chupaderos and Morito are giant Mexican masses. Treysa, which fell in Germany in 1916, is classed as an anomalous IIIAB.
How to recognise one
IIIABs are heavy, magnetic irons. Etched, they show the classic medium Widmanstätten pattern, often with rounded troilite nodules. Henbury and Wabar fragments are often twisted shrapnel from cratering impacts. Only chemistry confirms the group.
Many famous irons on public display, in museums, parks and university halls, are IIIABs, simply because the group is so common. Displayed masses should never be chipped or filed; sample requests go through the institution that holds them.
Famous examples
- Cape York Iron, IIIAB, 1818 · 58.2 t
- Willamette Iron, IIIAB, United States, 1902 · 15.5 t
- Henbury Iron, IIIAB, Australia, 1931 · 2 t
- Chupaderos Iron, IIIAB, Mexico, 1852 · 24.3 t
- Morito Iron, IIIAB, Mexico, 1600 · 10.1 t
- Wabar Iron, IIIAB, Saudi Arabia, 1863 · 2.55 t
- Grant Iron, IIIAB, United States, 1929 · 525 kg
- Boxhole Iron, IIIAB, Australia, 1937 · 500 kg
- Tamentit Iron, IIIAB, Algeria, 1864 · 510 kg
Related groups
- Main-group pallasites Nine in ten pallasites: magnesium-rich olivine in metal related to the IIIAB irons.
- IIIE irons Coarse octahedrites, separated from IIIAB by their chemistry.
- IVA irons Fine octahedrites, including the famous Gibeon.
- 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).