iron meteorite

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

Related category

• METEORS AND METEORITES

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