iron meteorite

iron meteorite

Sample of an iron meteorite, with one cent coin for size comparison.

Sample of the Campo del Cielo meteorite

The author at Dundee Science Centre holding a sample of the Campo del Cielo meteorite, which fell in Argentina.

An iron meteorite is a meteorite composed mainly of iron (Fe) and nickel (Ni) in the form of two nickel-iron alloys, kamacite and taenite. Iron meteorites are easy for even a layperson to tell apart from ordinary Earth rocks because of their metallic appearance and high density. They also tend to be bigger than either stony meteorites or stony-iron meteorites, because they usually survive passage through the atmosphere intact and suffer much less from the effects of ablation. All known iron meteorites have a combined mass of more than 500 tons – about 86% of the mass of all known meteorites. Yet they are quite rare, accounting for about one in 20 of observed falls.


Two ways have been devised to classify iron meteorites. The older, structural method is based on characteristic crystalline features that show up when the meteorites are sectioned, etched, and polished. Octahedrites, the commonest type of iron, with less than 6% by weight of nickel, reveal a characteristic Widmanstätten pattern that results from lamellae (fine sheets) of kamacite intergrown with nickel-rich phases. The width of the kamacite lamellae allows classification into 5 structural groups: the coarsest, coarse, medium, fine and finest octahedrites. Plessitic octahedrites are transitional between octahedrites and ataxites. Ataxites are nickel-rich (> 20% nickel), and are mainly taenite. Hexahedrites contain less than 6% nickel and are comprised of kamacite only; both hexahedrites and ataxites lack a classic Widmanstätten pattern.


The newer chemical method of classifying iron meteorites involves measuring the amounts of trace elements such as germanium, gallium, or iridium present. The concentrations of these trace elements are then plotted against the overall nickel content on logarithmic scales. This technique has enabled 13 distinct groups, labeled by Roman numbers and letters, such as "IIIAB", to be identified. The members of each chemical group are thought to share a common origin on the same parent body. Iron meteorites come mostly from the cores of small differentiated asteroids that were broken apart by cataclysmic impacts shortly after their formation.


Classification of iron meteorites
structural class symbol kamacite (mm) nickel (%) related chemical 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


Largest iron meteorites
meteorite country found structural class group mass (kg)
Hoba Namibia 1920 ataxite IVB 60,000
Campo del Cielo Argentina 1990 octahedrite IAB 37,000
Cape York (Ahnighito) Greenland 1894 octahedrite IIIAB 31,000
Armanty China 1898 octahedrite IIIE 23,500
Bacubirito Mexico 1863 octahedrite UNG 22,000
Cape York (Agpalilik) Greenland 1963 octahedrite IIIAB 20,000
Mbosi Tanzania 1930 octahedrite UNG 16,000
Campo del Cielo Argentina 1570 octahedrite IAB 15,000
Williamette United States 1902 octahedrite IIIAB 14,900
Chupaderos Mexico 1854 octahedrite IIIAB 14,100
Mundrabilla Australia 1901 octahedrite IIICD 12,000
Morito Mexico 1600 octahedrite IIIAB 11,000