Octahedrite

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Octahedrites are the most common class of iron meteorites. They are composed primarily of the nickel-iron alloys: taenite - high nickel content, and kamacite - low nickel content.

Due to a long cooling time in the interior of the parent asteroids, these alloys have crystallized into intermixed millimeter-sized bands (from about 0.2 mm to 5 cm). When polished and acid etched the classic Widmanstätten patterns of intersecting lines of lamellar kamacite, are visible.

In gaps between the kamacite and taenite lamellae, a fine-granied mixture called plessite is often found. An iron nickel phosphide, schreibersite, is present in some nickel-iron meteorites as well as an iron nickel cobalt carbide, cohenite.

[edit] Chemical classification

Concentrations of trace elements are used to separate the iron meteorites into chemical classes, which correspond to separate asteroid parent bodies:

• IAB iron meteorites
• IC iron meteorites
• IIAB iron meteorites (includes also some hexahedrites)
• IIC iron meteorites
• IID iron meteorites
• IIE iron meteorites
• IIG iron meteorites (includes also some hexahedrites)
• IIF iron meteorites (includes also some ataxites)
• IIIAB iron meteorites
• IIICD iron meteorites
• IIIE iron meteorites
• IIIF iron meteorites
• IVA iron meteorites

For a full list see iron meteorites

[edit] Mineral

Octahedrite is also an obsolete synonym for anatase, one of the three known titanium dioxide minerals.

[edit] References

• Webmineral
• Meteorites Australia
• Octahedrite Widmanstätten Pattern
• www.meteorite.fr