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

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.

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). 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]

Subgroups

The Zacatecas Meteorite found in 1782 in Zacatecas Mexico, weighing 780kg.
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:[2]

Mineral

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

See also

References

  1. ^ Vagn F. Buchwald: Handbook of Iron Meteorites. University of California Press, 1975.
  2. ^ James H. Shirley,Rhodes Whitmore Fairbridge, Encyclopedia of planetary sciences, Springer, 1997. ISBN 978-0-412-06951-2
  3. ^ Geochimica et Cosmochimica Acta, Volume 45, Ed. 9-12