Octahedrite

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.

Octahedrite
— Structural class —
Octahedrite from Toluca
Compositional type	Iron
A phase diagram showing the link between structural and chemical classification.

Structure

See also: Widmanstätten pattern § Lamellæ formation mechanism, and Widmanstätten pattern § Shape and orientation

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).^[1] 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.^[2]

Subgroups

The Zacatecas Meteorite found in 1782 in Zacatecas Mexico, weighing 780kg.

See also: Iron meteorite § Structural classification

Octahedrites can be grouped by the dimensions of kamacite lamellae in the Widmanstätten pattern, which are related to the nickel content:^[3]

• 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,^[4] 9–18% Ni, symbol Opl

Mineral

Octahedrite is an obsolete synonym for anatase, one of the three known titanium dioxide minerals.^{[citation needed]}

See also

• Glossary of meteoritics