New England Meteoritical Services

Iron Meteorites

Gibeon, South Africa. Fine octahedrite, IVA

Unlike stone and stony-iron meteorites, which contain distinct compositional and mineralogical classifications, iron meteorite classifications tend to be variations on a theme and will be discussed as a group for this presentation.

Iron meteorites are identified by two different classifications: structural and chemical. The chemical classification is more definitive for scientific purposes as it is based on the relationship of trace elements gallium and germanium to the nickel content, but the structural classification is presented in the classification chart as it is readily apparent in properly prepared specimens and of interest to collectors. Those interested can review the structural and chemical classification groupings.

The variation in the structure of iron meteorites is a result of the ratio of the two nickel-iron metal alloys, kamacite and taenite, that have crystallized forming the core of the parent asteroids. Many iron meteorites also contain inclusions of graphite, troilite, silicates and other minerals that give them a distinctive appearance.

Hexahedrites have formed entirely from kamacite crystals and show no Widmanstätten pattern when polished and etched. They sometimes exhibit very fine parallel lines called Neumann lines that are the result of an impact event.

The Octahedrites consist of a striking intergrowth of two nickel-iron minerals, kamacite and taenite. When polished and etched with acid they reveal the structure known as the Widmanstätten pattern (20K GIF), a formation that has only been found in meteorites and cannot be duplicated in the laboratory, because of the millennia long cooling time needed for its creation. The rate at which an asteroid core cools as well as the amount of taenite contained in the metal affects the thickness of the kamacite bands in the Widmanstätten pattern. This has lead to descriptive structural classifications such as coarsest, medium and fine.

Kamacite Crystal Band Widths of Octahedrites

  • Coarsest - greater than 3.3 mm; nickel, 5 to 9%
  • Coarse - 1.3 to 3.3 mm; nickel, 6.5 to 8.5%
  • Medium - 0.5 to 1.3 mm; nickel, 7 to 13%
  • Fine - 0.2 to 0.5 mm; nickel, 7.5 to 13%
  • Finest - less than 0.2 mm; nickel, 17 to 18%
  • Plessitic - less than 0.2 mm kamacite spindles; nickel 9 to 18%

The Sikhote-Alin, coarse octahedrite, is a desirable representative of a coarsest octahedrite as many specimens exhibit classic meteorite characteristics.

Ataxites are at the other extreme of the kamacite/taenite ratio range and are made almost entirely of taenite. Polishing and etching of these meteorites shows no visible structure, but under a microscope an extremely fine Widmanstätten pattern can be discerned.

While ataxites are a relatively rare meteorite (no falls have ever been observed), Nature always finds a way of making scientists do a double-take, for the largest known main mass of any meteorite is the Hoba West ataxite, estimated to weigh in at 60 metric tonnes. The meteorite still lies where it was found in 1920 and has been declared a national monument by the government of Namibia.

New England Meteoritical Services