Octahedrite (Canyon Diablo Meteorite) (4.55 Ga) 1
Octahedrite from the Asteroid Belt. (cut, polished, and nitric acid-etched surface; public display, Field Museum of Natural History, Chicago, Illinois, USA)
Meteorites are rocks from space that have passed through Earth's atmosphere and landed at the surface. Most originate from the Asteroid Belt between Mars and Jupiter, but some come from Mars, the Moon, Earth (!), and even from outside the Solar System. Three broad categories of meteorites have been identified - stony meteorites, iron meteorites, and stony-iron meteorites.
Iron meteorites are dominated by iron (Fe) metal. They also all include some metallic nickel (Ni) as well. 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.
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.).
The physical crystalline structure of octahedrites is distinctive - it is best seen on cut, polished, and nitric acid-etched surfaces (see above). 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.
Shown here is a cut surface of the Canyon Diablo Meteorite. When it impacted during the Late Pleistocene, about 49,000 years ago, the famous "Meteor Crater" (Barringer Crater) in Arizona, USA was formed. (www.flickr.com/photos/jsjgeology/albums/72157665304339691)
Canyon Diablo is composed of ~90% kamacite, ~1-4% taenite, and up to 8.5% troilite-graphite nodules (FeS & C). The original mass has been estimated to be 100 feet across & about 60,000 tons. Canyon Diablo rocks have been isotopically dated to 4.55 billion years.