Octahedrite


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.

Contents

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

  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 9780412069512
  3. ^ Geochimica et cosmochimica acta, Volume 45, Ed. 9-12

External links