Iron meteorites
Iron meteorites are mostly composed of iron and nickel. They were formed from the cores of small asteroids (planetesimals) that got hot enough to melt and their material separated into layers. This process is called differentiation.
Iron meteorites are further divided into several categories based on the nickel content of the crystal structure:
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Hexahedrites: they contain little nickel (usually less than 6%) and have no visible Widmanstätten structure
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Characteristics: Hexahedrites are composed mainly of kamacite. Due to the low nickel content, they do not contain taenite, which is the reason for the absence of Widmanstätten structure
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Examples: Coahuila (Mexico), Braunau (Austria)
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Octahedrites: They have a higher nickel content (6-20%) and are the most common type of iron meteorites. When etched with acid, they show the characteristic Widmanstätten structure
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Characteristics: Octahedrites contain a mixture of kamacite and taenite. When etched, they show Widmanstätten patterns, which are unique geometric structures formed during the slow cooling of the planetesimal interior.
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Dividing: Octahedrites are further divided into fine-grained, medium-grained and coarse-grained according to the size of the kamacite and taenite crystals.
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Fine-grained octahedrites called IVA: Smaller crystals, e.g. Muonionalusta (Sweden), Gibeon (Namibia)
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Medium-grained octahedrites called IIAB: Medium-sized crystals, for example Sikhote Alin (Russia).
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Coarse-grained octahedrites called IAB: Larger crystals, e.g. Campo del Cielo (Argentina), Nantan (China)
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Ataxites: They have a very high nickel content (more than 16%) and show no visible structure even after acid etching
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Characteristics: Ataxites contain mainly taenite with very little kamacite. Due to the high nickel content, no Widmanstätten structure is formed
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Examples: Hoba (Namibia) - IVB, Cape York (Greenland) - IIIAB
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