Octahedrite
The most common structural class of iron meteorite, showing a criss-cross Widmanstätten pattern when etched.
Octahedrites are iron meteorites with intermediate nickel contents, roughly 6–17 per cent. As they cooled very slowly inside their parent bodies, plates of nickel-poor kamacite grew within nickel-rich taenite along the four sets of planes of an octahedron. Etching a polished slice reveals this as the Widmanstätten pattern of crossing bands.
The width of the kamacite bands defines subclasses: coarsest (more than 3.3 millimetres), coarse (1.3–3.3 mm), medium (0.5–1.3 mm), fine (0.2–0.5 mm) and finest (under 0.2 mm), plus plessitic octahedrites with tiny spindles. Band width depends mainly on nickel content and cooling rate, so octahedrites tell us about the cores and metal pools they came from. Canyon Diablo, from Meteor Crater in Arizona, is a coarse octahedrite; Gibeon from Namibia and Muonionalusta from Sweden are fine octahedrites prized for their crisp patterns.
Most iron meteorites are octahedrites. Uncut specimens look like heavy, rusty or brown-black metal with thumbprint-like regmaglypts; only a cut and etched face shows the pattern. Etching uses acid and should be done only by experienced adults.
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