IIIF irons
A small group with very low gallium and germanium.
- Classification
- Iron meteorites › IIIF irons
- In our catalogue
- 9 meteorites
- Heaviest
- St. Genevieve County, 244.5 kg
IIIF irons are medium to coarse octahedrites with about 7–8% nickel and very low gallium and germanium.
Mineralogy and chemistry
IIIF irons contain about 6.8% to 8.5% nickel and are medium to coarse octahedrites. They have extremely low gallium and germanium, a hundred times or more lower than in common groups such as IIIAB, and relatively high chromium. Their trace elements follow a magmatic trend.
The low gallium and germanium show that the parent body was strongly depleted in moderately volatile elements, perhaps because it formed from material that had been heated and lost these elements before it accreted. Isotopes place IIIF in the 'carbonaceous' family.
Parent body and formation
IIIF irons come from the core of a single melted asteroid that formed in the outer Solar System. Its depletion in volatile elements is unusual for a carbonaceous-family body, and is shared with the IVB irons, which also come from that family. Their cooling rates and the size of their parent body are poorly known because there are so few samples.
Together, groups like IIIF and IVB suggest that some outer-Solar-System bodies lost volatile elements, either before they formed or while they melted, so that the carbonaceous family includes very varied cores.
Abundance and famous examples
IIIF is a small group with fewer than ten members, none seen to fall. Clark County (Kentucky), Moonbi (New South Wales) and Nelson County (Kentucky) are representative.
Most IIIF irons were found in the 19th and early 20th centuries, often by farmers ploughing fields, and were recognised as a separate group only when trace-element analysis became routine in the second half of the 20th century. Their very low gallium and germanium had at first been taken as an oddity of individual irons rather than the mark of a shared parent.
How to recognise one
IIIF irons are ordinary-looking medium to coarse octahedrites: heavy, magnetic and patterned when etched. Their distinguishing feature, very low gallium and germanium, is invisible to the eye and needs laboratory analysis.
Why they matter
Small groups like IIIF reveal how many different cores existed in the early Solar System. Each represents a body that melted completely and separated its metal, then was destroyed so thoroughly that only its core metal survives. The very low volatile content of IIIF also gives clues to how and where moderately volatile elements were lost from planet-forming material, a question that matters for understanding the composition of Earth.
For collectors, IIIF irons are rarely available, and most known material is in museums and research collections.
Famous examples
- Clark County Iron, IIIF, United States, 1937 · 11.3 kg
- Moonbi Iron, IIIF, Australia, 1892 · 13.2 kg
- Nelson County Iron, IIIF, United States, 1856 · 73.03 kg
Related groups
- IVA irons Fine octahedrites, including the famous Gibeon.
- IVB irons Nickel-rich ataxites. Hoba, the largest meteorite, is one.
- IIIAB irons The most common iron meteorites: medium octahedrites from one asteroid core.
- 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).