Different Directions

Different Directions

Coahuila

Iron, Hexahedrite, IIA, Ni-poor
Found 1837, Mexico

Like all Irons, Coahuila had been dislodged by impact. It was part of the core of its parent body (probably an asteroid) and came to Earth in Mexico in 1837. It is a Hexahedrite.

Hexahedrites are the irons with the lowest nickel content. They consist of large crystals of kamacite and little if any taenite. Kamacite and taenite are both Iron-Nickel alloys, but they differ by the relative amounts of iron and nickel, and (consequently) have a different crystal structure.

Because of the absence of taenite in hexahedrites, polished surfaces of these meteorites are featureless except for the occasional presence of Neumann lines. Neumann lines are formed by shock deformation of the metallic kamacite crystals during violent impacts.

Here's a picture of the Coahuila in our collection.

Coahuila

Photo from New England Meteoritical Services.

What's wonderful about Coahuila is its rabbdite inclusions. This slice shows the overall prismatic rhabdite inclusions throughout the specimen. The coloration is basically believed to be a kind of space-weathering over 3 billion years.

Here's a second image, a closeup of the rhabdite inclusions.

Coahuila

Photo from New England Meteoritical Services.

Rhabdites are iron nickel phosphides (Fe,Ni)3P which were found in the form of prismatic crystals. True to its name, prismatic defines its typically long, slender, wedged-shaped mineral form, but rhabdites also take a plate-like shape as well.

Typically, hexahedrites have nickel contents in the range of 5.2 to 5.8 weight percent, and the iron-nickel phosphides, develop in hexahedrites, with the result that not all of the nickel is in solution in the kamacite.

Here's a side trip that better shows some of the features of hexahedrites, but specifically Coahuila.

Side Trip

Color micrograph of Coahuila:

Slice

After tint etching we see kamacite (k), Neumann bands (N), prismatic-shaped and plate-shaped rhabdites (r).

From George F. Vander Voort's “Metallography of Iron Meteorites,” ADVANCED MATERIALS & PROCESSES/FEBRUARY 2001

The Coahuila meteorite like most hexahedrites is a single crystal, that is, the kamacite matrix has no internal grain boundaries.

Here's the features that this micrograph shows:

  • All of the kamacite is colored blue.
  • Parallel light blue lines run diagonally left to right; these are Neumann bands.
  • The white particles are phosphides, both prismatic and plate shaped (in three dimensions).
  • One prismatically-shaped rhabdite in the upper left corner has cracked due to the intersecting Neumann band.
  • The variation in color around the large prismatically-shaped rhabdite in the center of the image was caused by compositional variations around the phosphide.
  • The kamacite background has a rough appearance because of many very small phosphides.



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