Iron meteorites
Almost pure iron-nickel metal, mostly from the cores of asteroids that melted and were later shattered.
- Classification
- Iron meteorites
- Share of witnessed falls
- about 4.9%
- In our catalogue
- 1,072 meteorites, 49 seen to fall
- Heaviest
- Hoba, 60 t
- Earliest recorded fall
- Elbogen, 1399
- Most recent fall
- Kavarpura, 2006
Iron meteorites are made almost entirely of two iron-nickel minerals, kamacite (low nickel) and taenite (high nickel). Iron, nickel and cobalt make up more than 95% of the metal, and nickel is always present, usually between 5% and 25%. Minor minerals include troilite (iron sulphide), graphite, schreibersite and cohenite; plates of troilite seen on cut faces are called Reichenbach lamellae.
Most irons are pieces of the metal cores of asteroids that melted early in the Solar System's history, heated by radioactive aluminium-26 and iron-60. Their chemistry shows they come from at least 50 different parent bodies, many more large melted asteroids than survive today. Their spectra resemble the metallic M-type asteroids.
Irons make up only about 5% of falls but dominate the mass of meteorites on Earth: they are tougher, survive entry better, resist weathering and are easy to recognise, so all the largest known meteorites, from Hoba down, are irons. Before smelting was invented, meteoritic iron was the only metallic iron people had.
Cut, polished and etched with acid, most irons show the criss-cross Widmanstätten pattern, which can only form when metal cools by a few degrees per million years. Irons are classified two ways: by this structure, and by their chemistry.
Chemical groups
Structure alone cannot show which irons share a parent body, so irons are also grouped by chemistry. In the 1950s and 1960s, scientists found that the trace elements gallium and germanium separate irons into a few clusters, numbered I to IV. Later work, led above all by John Wasson, added nickel, iridium, gold and other elements and refined these into today's groups: IAB, IC, IIAB, IIC, IID, IIE, IIF, IIG, IIIAB, IIIE, IIIF, IVA and IVB. A group needs at least five unrelated members; smaller clusters are grouplets, and irons matching none are ungrouped.
Within a magmatic group, elements such as iridium fall steeply as nickel rises, the pattern expected when a metal core crystallises gradually and the remaining liquid changes composition. Non-magmatic groups, IAB and IIE, show flatter, scattered patterns and often contain silicate inclusions, suggesting they formed in pools of impact melt rather than in a single core.
Ages, cooling and parent bodies
Hafnium-tungsten dating shows that the cores of many iron parent bodies separated within about 1 to 3 million years of the first solids, before most chondrites had finished forming. Isotopes of molybdenum, tungsten and other elements split irons into two families: groups such as IAB, IIAB, IIIAB, IIIE and IVA from the inner Solar System, and groups such as IIC, IID, IIF, IIIF and IVB from the outer, carbonaceous region. The Widmanstätten pattern records cooling, typically tens to thousands of degrees per million years, which reveals how deeply buried each core was.
Counting groups and distinct ungrouped irons, around 50 or more parent bodies are represented, far more than for all stony meteorites together, perhaps because metal survives destruction of its parent asteroid much better than rock.
Recognising an iron meteorite
Iron meteorites are dense, about 7.5 to 8 grams per cubic centimetre, and very heavy in the hand. They stick strongly to a magnet. Fresh ones have a thin dark crust and smooth, thumbprint-like regmaglypts; old finds are rusty brown. A small corner ground with a file shows bright, silvery metal. Gas holes, bubbles or a glassy, slaggy texture suggest industrial slag rather than a meteorite. Etching a polished face with dilute acid reveals the Widmanstätten pattern, but this is work for trained adults with proper safety equipment, never for children.
Structural classes
- Hexahedrites (H): under about 6% nickel. Made of a single kamacite crystal, with no Widmanstätten pattern, but often fine parallel Neumann lines caused by shock.
- Octahedrites (O): about 6% to 16% nickel, with a Widmanstätten pattern. Named by the width of the kamacite bands: coarsest (Ogg, over 3.3 mm), coarse (Og, 1.3–3.3 mm), medium (Om, 0.5–1.3 mm), fine (Of, 0.2–0.5 mm), finest (Off, under 0.2 mm) and plessitic (Opl).
- Ataxites (D): over about 16% nickel, with no visible pattern. Hoba is an ataxite.
Classes and groups
IAB irons
Irons with stony inclusions, from a partly melted asteroid. Canyon Diablo and Campo del Cielo.
261 in the catalogue
IC irons
A small magmatic group of coarse octahedrites, low in nickel.
12 in the catalogue
IIAB irons
Hexahedrites and coarsest octahedrites, including Sikhote-Alin.
119 in the catalogue
IIC irons
A rare group of plessitic octahedrites.
8 in the catalogue
IID irons
Fine to medium octahedrites rich in gallium and germanium.
21 in the catalogueIIE irons
Irons full of silicate fragments, possibly from asteroid Hebe.
22 in the catalogueIIF irons
A small group of magmatic irons.
6 in the catalogueIIG irons
Low-nickel irons with large crystals of schreibersite.
6 in the catalogue
IIIAB irons
The most common iron meteorites: medium octahedrites from one asteroid core.
292 in the catalogueIIIE irons
Coarse octahedrites, separated from IIIAB by their chemistry.
15 in the catalogueIIIF irons
A small group with very low gallium and germanium.
9 in the catalogue
IVA irons
Fine octahedrites, including the famous Gibeon.
75 in the catalogue
IVB irons
Nickel-rich ataxites. Hoba, the largest meteorite, is one.
14 in the catalogue
Ungrouped irons
Irons that fit no known group: each may represent its own asteroid.
113 in the catalogueFamous examples
- Hoba Iron, IVB, Namibia, 1920 · 60 t
- Willamette Iron, IIIAB, United States, 1902 · 15.5 t
- Cape York Iron, IIIAB, 1818 · 58.2 t
- Campo del Cielo Iron, IAB-MG, Argentina, 1575 · 50 t
- Sikhote-Alin Iron, IIAB, Russia, 1947 · 23 t
- Gibeon Iron, IVA, Namibia, 1836 · 26 t
- Canyon Diablo Iron, IAB-MG, United States, 1891 · 30 t
Related groups
- IAB irons Irons with stony inclusions, from a partly melted asteroid. Canyon Diablo and Campo del Cielo.
- IIAB irons Hexahedrites and coarsest octahedrites, including Sikhote-Alin.
- IIIAB irons The most common iron meteorites: medium octahedrites from one asteroid core.
- IVA irons Fine octahedrites, including the famous Gibeon.
- IVB irons Nickel-rich ataxites. Hoba, the largest meteorite, is one.
- Ungrouped irons Irons that fit no known group: each may represent its own asteroid.
- Pallasites Gem-like olivine crystals set in iron-nickel metal.
- Mesosiderites Jumbled breccias of metal and basalt, made in a violent collision.
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