Iron Meteorite

Meteorite composed mainly of iron (Fe) and nickel (Ni) in the form of two alloys, kamacite and taenite. Due to their metallic makeup and extraordinary weight, iron meteorites are easily distinguished from ordinary rocks. Also, because they rarely break up in the air and suffer much less from the effects of ablation during their passage through the atmosphere, they are usually much larger than stony or stony-iron meteorites. All known iron meteorites together have a mass of more than 500 tons, which is ~89% of the entire mass of all known meteorites. Yet they are comparatively rare, accounting for just 5.7% of witnessed falls.

There are two ways of classifying iron meteorites. The older, structural method is based on characteristic crystalline features that show up when the meteorites are sectioned, etched, and polished. This results in three subdivisions: hexahedrites (4–6 wt. % Ni), octahedrites (the commonest type: 6-12 wt. % Ni), and ataxites (> 12 wt. % Ni).

Structural Class Symbol Kamacite (mm) Ni (%) Related Groups
Hexahedrites H >50 4.5 – 6.5 IIAB, IIG
Coarsest octahedrites Ogg 3.3 – 50 6.5 – 7.2 IIAB, IIG
Coarse octahedrites Og 1.3 – 3.3 6.5 – 8.5 IAB, IC, IIE, IIIAB, IIIE
Medium octahedrites Om 0.5 – 1.3 7.4 – 10 IAB, IID, IIE, IIIAB, IIIF
Fine octahedrites Of 0.2 – 0.5 7.8 – 13 IID, IIICD, IIIF, IVA
Finest octahedrites Off < 0.2 7.8 – 13 IIC, IIICD
Plessitic octahedrites Opl < 0.2, spindles 9.2 – 18 IIC, IIF
Ataxites D >16 IIF, IVB

The newer chemical method is far more precise but depends on sophisticated instruments to determine abundances of trace elements such as Ge, Ga, and Ir. The concentrations of the trace elements are plotted against the overall Ni content on logarithmic scales to resolve well-defined chemical clusters, each representing a distinct chemical group. Iron meteorites of each chemical group formed on a common parent body. Iron meteorites come mostly from the cores of small differentiated asteroids that were disrupted by devastating impacts shortly after their formation. Short descriptions of the major groups follow.

  • IAB group irons include the famous Toluca, Campo del Cielo, Odessa, and Canyon Diablo meteorites. Most IAB irons are coarse to medium octahedrites (although other structural classes do occur) and often contain abundant inclusions of troilite, graphite, and cohenite, and various silicates. Recent research suggests that both winonaites and IAB irons originated on the same parent body – a partially differentiated asteroid that was disrupted just as it began to form an Fe core and silicate-rich crust. The impact mixed silicates into molten Ni-Fe metal forming the silicated IAB irons, and mixed olivine-rich residues of partial melts into unmelted silicates, forming the winonaites.
  • IIAB Group irons are classified as hexahedrites or coarsest octahedrites, making them some of the most Ni-poor iron meteorites known. Perhaps the most famous IIAB iron is Sikhote-Alin, a witnessed fall from Russia in 1947 (commemorative stamp and meteorite shown below). Several thousand pieces with a total weight of over 70 tons have been recovered. Trace element abundances suggest that this group formed in the core of a differentiated C-type asteroid that was disrupted by several impact events.
  • IIIAB Group irons consist of two closely related groups. The IIIA subgroup has mostly coarse octahedrite textures, whereas IIIB irons usually display medium textures. However, the two subgroups form a continuous sequence in structure and elemental compositions consistent with a common origin (the subgroups probably represent different parts of an asteroid’s core). Group IIIAB includes some of the largest irons ever found (Cape York, Chupaderos, Morito, and Willamette). Some IIIAB members contain large nodules of troilite and graphite, but silicate inclusions are rare. Recent research suggests a close relationship between the IIIAB irons and the silicate-rich main group pallasites: both groups probably formed on the same parent body – a differentiated asteroid that was disrupted by a single impact event. IIIAB iron meteorites represent fragments of the core, whereas main-group pallasites are samples of the core-mantle boundary.
  • IIICD Group irons mostly belong to the structural classes of fine and finest octahedrites, or ataxites. Several IIICD meteorites contain abundant silicate inclusions, similar to the inclusions in IAB irons. There are additional similarities in elemental compositions suggesting a close relationship between IIICD and IAB irons. However, group IIICD displays some unique features that clearly distinguish them from the IAB irons, e.g., presence of haxonite. This group includes the anomalous, troilite-rich Mundrabilla, one of the largest iron meteorites ever found.
  • IVA Group irons are mostly fine octahedrites, and have extraordinarily low Ge and Ga abundances. Some IVA irons contain sparsely distributed small nodules of troilite and graphite (black spots in image below), although silicate inclusions are rare to absent in most members. Recent research suggests that the IVA irons formed in the core of a small, differentiated asteroid disrupted by a major impact shortly after its formation. After re-accretion, the asteroid was again disrupted ~450 Ma ago. The famous meteorite Gibeon (below) is a typical member of this group; over 30 tons have been recovered from its large, prehistoric strewn field in Namibia.
  • Ungrouped A large number of ungrouped irons don’t fit into any of the existing 14 chemical groups, and display unique structural and elemental compositions. Some have compositions similar to other ungrouped irons, and have been provisionally placed into several grouplets comprising less than five members each. The remaining ungrouped irons are unique, and probably represent single samples of their parent bodies.
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