Octahedrite

Octahedrites derive their name from the crystal structure paralleling an octahedron. Opposite faces are parallel so, although an octahedron has 8 faces, there are only 4 sets of kamacite plates. 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. ==Subgroups==

Octahedrites can be grouped by the dimensions of kamacite lamellae in the Widmanstätten pattern, which are related to the nickel content: • 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 ==Mineral==

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