IIIE irons
Coarse octahedrites, separated from IIIAB by their chemistry.
- Classification
- Iron meteorites › IIIE irons
- In our catalogue
- 15 meteorites
- Heaviest
- Armanty, 28 t
IIIE irons are coarse octahedrites with about 8–9% nickel. They resemble the IIIAB irons but differ in trace elements and carbon content, and come from a separate parent body.
Mineralogy and chemistry
IIIE irons contain about 8.2% to 9% nickel and are coarse octahedrites. They closely resemble IIIAB irons in nickel and in gallium and germanium, but differ in several trace elements and, characteristically, contain haxonite, an iron-nickel carbide, together with graphite. Their kamacite bands are often broad and swathed around inclusions.
These distinctions, first recognised in the 1970s, show that IIIE irons crystallised in a different core from the IIIABs, despite their similar overall chemistry. Their trace elements follow a magmatic trend.
Parent body and formation
The IIIE parent body was a melted asteroid in the inner Solar System whose core contained more carbon than the IIIAB core, so carbides formed as it cooled. Isotopes of molybdenum and tungsten place it in the 'non-carbonaceous' family. Little is known of its size or history, though cooling rates suggest a core under a substantial mantle.
Abundance and famous examples
IIIE is a small group of about fifteen members, none seen to fall. Armanty, found in Xinjiang, China, in 1898, weighs about 28 tonnes and is one of the largest meteorites known. Kokstad from South Africa (about 340 kg), Staunton from Virginia, Rhine Villa from South Australia and Tanokami Mountain from Japan are other members.
How to recognise one
IIIE irons look like other coarse octahedrites, heavy and magnetic, with broad kamacite bands when etched. Carbides and graphite may appear under the microscope. Only trace-element analysis separates them from IIIAB or IAB irons.
Why they matter
IIIE shows how two iron groups with almost the same major chemistry can come from different cores, and why trace elements are essential to classification. The group also helps scientists understand how carbon behaves in crystallising metal, which matters for estimating the light elements in the cores of planets.
Armanty, at around 28 tonnes, is among the ten largest meteorites on Earth, showing that a small group can still include giants. Its sheer size has made it a landmark of Chinese meteoritics; it is now kept in Urumqi.
Famous examples
- Armanty Iron, IIIE, China, 1898 · 28 t
- Kokstad Iron, IIIE, South Africa, 1884 · 341 kg
- Staunton Iron, IIIE, United States, 1869 · 43.5 kg
- Rhine Villa Iron, IIIE, Australia, 1900 · 3.33 kg
- Tanokami Mountain Iron, IIIE, Japan, 1885 · 174 kg
Related groups
- IIIAB irons The most common iron meteorites: medium octahedrites from one asteroid core.
- IAB irons Irons with stony inclusions, from a partly melted asteroid. Canyon Diablo and Campo del Cielo.
- 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).