Octahedrite

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Octahedrites 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.

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]

Contents

Structural classification

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

  • Coarsest octahedrites, kamacite band width >3.3 mm, 5-9% Ni, symbol Ogg
  • Coarse octahedrites, 1.3-3.3 mm, 6.5-8.5% Ni, symbol Og
  • Medium octahedrites, 0.5-1.3 mm, 7-13% Ni, symbol Om
  • Fine octahedrites, 0.2-0.5 mm, 7.5-13% Ni, symbol Of
  • Finest octahedrites, <0.2 mm, 17-18% Ni,symbol Off
  • Plessitic octahedrites, kamacite spindles, 9-18% Ni, symbol Opl

Chemical classification

Concentrations of the trace elements gallium, germanium and iridium are used to separate the iron meteorites into chemical classes, which correspond to separate asteroid parent bodies. There is a partial correspondence between structural and chemical groups, in that most members of a chemical group belong to the same structural group or groups.[2]

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