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TITLE: Irons


[Photo: Asteroid with Metal Core]

These iron meteorites are fragments of the cores of large asteroids shattered by collisions billions of years ago. The cores formed early in asteroids’ lives, when the interiors melted and dense metals such as iron and nickel sank to the center. Iron sulfides, and carbon and phosphorus that were dissolved in the metal, also sank. The diverse chemistry of meteoritic fragments from these cores tells us how many asteroids are represented—more than 60.



See caption at right.

Geometric Beauty
The beautiful markings revealed by cutting and chemically etching these iron meteorites are actually crystals of metal. They took shape as hot, homogeneous nickel-iron alloy cooled very slowly. Two different minerals containing different proportions of iron and nickel crystallized from the molten metal. The width of the crystals in the geometric patterns called Widmanstätten structures varies from meteorite to meteorite because of differences in cooling rate and chemical composition.

The criss-crossing bands in this close-up of Carbo are the iron-rich alloy kamacite. The triangular areas in between are the nickel-rich alloy taenite. The image is 80mm across.

 

See caption at right.

Reading Between the Lines
Formed in a long-lost world, Gibeon's geometric stucture gives us hints about how iron meteorites originated. In this video, witness:

  • Gibeon's crystals growing from molten metal.
  • the Widmanstätten structure forming as the crystals cool.
  • a museum preparator cutting and etching the Gibeon specimen.

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Smithsonian National Museum of Natural History Department of Mineral Sciences website Credits