IRONS
Iron meteorites are classified according to both structure and chemistry. Structure is determined by nickel content alone, while chemistry depends not only on nickel content but also on gallium (Ga) and germanium (Ge) contents. The structural classification usually serves as the primary framework. Chemistry is then used for further subdivisions. There are three main structural groups for irons: hexahedrites, octahedrites, and ataxites. Other groups exist, but they comprise unusual and rare specimens and will not be described further here. Iron meteorites represent samples from the very core of their parent bodies. The parent bodies were presumably broken up in violent collisions between asteroids, exposing their cores to further fragmentation by impacts.
- Hexahedrites
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Hexahedrites are the irons with the lowest nickel content (4-6% by weight). They consist of large crystals of kamacite and contain no taenite. Kamacite and taenite are both Fe-Ni alloys, but they differ by the relative amounts of iron and nickel, and (consequently) have a different crystal structure; kamacite forms a body-centered cubic lattice whereas taenite forms a face-centered one. Because of the virtual absence of taenite in hexahedrites, polished surfaces of these meteorites are featureless except for the occasional presence of fine striations known as Neumann lines. Neumann lines are formed by shock deformation of the metallic kamacite crystals during violent impacts.
- Octahedrites
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Octahedrites are characterized by an intermediate nickel content (6-17% by weight) and contain both kamacite and taenite. These two metallic minerals occur in a distinctive arrangement of bands intersecting in two, three or four directions, which results in the characteristic Widmanstatten pattern. This beautiful pattern appears conspicuously when a section of an octahedrite is polished and etched in weak acid (usually nitric acid). The larger bands in a Widmanstatten pattern consist of kamacite. This kamacite is surrounded by fine sheets of taenite, while the interslices between the bands are made of plessite, a combination of kamacite and taenite.
- Ataxites
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Ataxites have the highest nickel content among iron meteorites (more than 16% by weight). They consist almost entirely of taenite, with only microscopic plates of kamacite. The largest meteorite known on Earth, the Hoba meteorite in Namibia, is an ataxite. It weighs approximately 55 metric tons and still lies where it fell.
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