Octahedrite (Sikhote-Alin Meteorite) (3.1 cm across at its widest)
Iron meteorites are dominated by iron (Fe) metal. They also include some metallic nickel (Ni). They represent metallic core samples from large, once-intact, differentiated asteroids/dwarf planets.
Octahedrite is the most common type of Fe-Ni meteorite that falls to Earth. Other Fe-Ni meteorite rock types include hexahedrite and ataxite. The rock names octahedrite-hexahedrite-ataxite reflect an iron meteorite classification based on the physical crystalline structure of the iron-nickel minerals present. Meteoriticists have since augmented this structural classification with information on trace element content.
Mineralogy: all octahedrites are dominated by two minerals having very similar chemistries: kamacite (FeNi) and taenite (FeNi). Kamacite is a silvery-colored iron-nickel metal alloy rich in iron, with about 5.5 weight-percent nickel. Taenite is a silvery-colored iron-nickel metal alloy rich in nickel, with about 27-65 weight-percent nickel. Octahedrites have much more kamacite than taenite. They also contain minor amounts of other minerals (e.g., troilite (FeS), silicates, etc.).
Physical structure: the physical crystalline structure of octahedrites is quite distinctive. It is best seen on cut, polished, and nitric acid-etched surfaces (see examples elsewhere in this photo album). Upon cooling from magma, the kamacite & taenite crystallize as interlocking plates with octahedral (double pyramid) geometries. Cut & etched surfaces show a distinctive criss-crossing pattern of silvery-gray blades. This is called Widmanstätten structure.
The octahedrite shown above is from the Sikhote-Alin Meteorite. Sikhote-Alin is a group IIB iron meteorite that impacted at 10:38 AM on 12 February 1947 near Novopokrovka, in the Sikhote-Alin Mountains of Maritime Province, far-eastern Russia. No photographs were taken, but an eyewitness to the Sikhote-Aline fireball made a painting of what he'd seen (
www.arizonaskiesmeteorites.com/AZ_Skies_Links/SikhoteBack...).
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More info. at:
www.lpi.usra.edu/meteor/metbull.php?code=23593
