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What are hexahedrites, octahedrites, and ataxites?
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Meteorite
The Hoba meteorite, lying where it was discovered in 1920 near Grootfontein, Namibia. The largest meteorite on Earth and an iron meteorite by classification, it consists of a nickel-iron alloy and is estimated to weigh about 60 tons.
Historically, iron meteorites were classified based on their crystal structure, which is revealed by etching a polished cross-section with acid. These groups are defined by their composition of two nickel-iron minerals: nickel-poor kamacite and nickel-rich taenite.12
- Hexahedrites: Composed almost entirely of kamacite, these meteorites are essentially structureless and lack the Widmanstätten pattern.12
- Octahedrites: The most common group, these contain both kamacite and taenite. They are characterized by an interlocking arrangement of kamacite plates in an octahedral structure, which creates the distinct Widmanstätten pattern when etched.123
- Ataxites: The rarest class, these are composed primarily of taenite and can contain high concentrations of nickel. Because they are nearly monomineralic, they are almost featureless and do not display the Widmanstätten pattern.124
While this structural classification remains useful for description, it has been largely superseded in modern science by a chemical classification based on the abundances of gallium, germanium, and nickel.12