Octahedrite

Octahedrite from Toluca

Widmanstätten pattern in NiFe octahedrite meteorite

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

Octahedrites are a class of iron meteorites within the structural classification. They 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.

Structure

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. ^[1]

Classification

Octahedrites can be grouped by the dimensions of kamacite lamellae in the Widmanstätten pattern, which are related to the nickel content:^[2]

• 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^[3], 9-18% Ni, symbol Opl

Mineral

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

References

External links

• Webmineral
• Meteorites Australia

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