Gibeon, South Africa. Fine
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
graphite, troilite, silicates and other minerals that give them a
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%
octahedrite, is a desirable representative of a coarsest
octahedrite as many specimens exhibit classic meteorite
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