Octahedrites are the most common structural class of . The structures occur because the meteoric iron has a certain nickel concentration that leads to the exsolution of kamacite out of taenite while cooling.
Structure
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 , these alloys have crystallized into intermixed millimeter-sized bands (from about 0.2 mm to 5 cm).
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.[Vagn F. Buchwald: Handbook of Iron Meteorites. University of California Press, 1975.]
Subgroups
Octahedrites can be grouped by the dimensions of kamacite lamellae in the Widmanstätten pattern, which are related to the
nickel content:
[James H. Shirley,Rhodes Whitmore Fairbridge, Encyclopedia of planetary sciences, Springer, 1997. ]
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Coarsest octahedrites, lamellae width >3.3 mm, 5–9% Ni, symbol Ogg
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Coarse octahedrites, lamellae 1.3–3.3 mm, 6.5–8.5% Ni, symbol Og
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Medium octahedrites, lamellae 0.5–1.3 mm, 7–13% Ni, symbol Om
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Fine octahedrites, lamellae 0.2–0.5 mm, 7.5–13% Ni, symbol Of
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Finest octahedrites, lamellae <0.2 mm, 17–18% Ni, symbol Off
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Plessitic octahedrites, kamacite spindles, a transitional structure between octahedrites and ,
[Geochimica et Cosmochimica Acta, Volume 45, Ed. 9–12] 9–18% Ni, symbol Opl
Mineral
Octahedrite is an obsolete synonym for
anatase, one of the three known
titanium dioxide
.
See also
External links
