Coarse Octahedrite on:
[Wikipedia]
[Google]
[Amazon]
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.
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,Geochimica et Cosmochimica Acta, Volume 45, Ed. 9-12 9-18% Ni, symbol Opl
Webmineral
{{Meteorites
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 parentasteroid
An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere.
...
s, 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
Schreibersite is generally a rare iron nickel phosphide mineral, , though common in iron-nickel meteorites. It has been found on Disko Island in Greenland and Illinois.
Another name used for the mineral is rhabdite. It forms tetragonal crystals w ...
, 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,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 minerals.See also
* Glossary of meteoriticsReferences
External links
Webmineral
{{Meteorites