Octahedrite

Octahedrite from Toluca, Mexico

File:Widmanstatten-patterns-3.jpg

Widmanstätten pattern in NiFe octahedrite meteorite

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.

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

Mineral

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

References

• Webmineral
• Meteorites Australia