Octahedrites are the most common structural class of iron meteorites. The structures occur because the meteoric iron has a certain nickel concentration that leads to the exsolution of kamacite out of taenite while cooling.
|— Structural class —|
Octahedrite from Toluca
A phase diagram showing the link between structural and chemical classification.
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-grained mixture called plessite is often found. An iron nickel phosphide, schreibersite, is present in most nickel-iron meteorites, as well as an iron-nickel-cobalt carbide, cohenite. Graphite and troilite occur in rounded nodules up to several cm in size.
- Coarsest octahedrites, lamellae width >3.3 mm, 5-9% Ni, symbol Ogg
- Coarse octahedrites, lamellae 1.3-3.3 mm, 6.5-8.5% Ni, symbol Og
- Medium octahedrites, lamellae 0.5-1.3 mm, 7-13% Ni, symbol Om
- Fine octahedrites, lamellae 0.2-0.5 mm, 7.5-13% Ni, symbol Of
- Finest octahedrites, lamellae <0.2 mm, 17-18% Ni, symbol Off
- Plessitic octahedrites, kamacite spindles, a transitional structure between octahedrites and ataxites, 9-18% Ni, symbol Opl