File:Octahedrite (Kossuth Meteorite) 1.jpg

English: Octahedrite from the Asteroid Belt between Mars and Jupiter. (~9.7 centimeters across at its widest; cut, polished, & acid-etched surface; private collection)

Meteorites are rocks from space. Three basic categories exist: 1) stony meteorites; 2) iron meteorites; and 3) stony-iron meteorites. As the name suggests, iron meteorites are dominated by iron metal (elemental Fe). They also include some metallic nickel (Ni), plus minor minerals. They represent core samples from differentiated asteroids/dwarf planets in the Solar System that have been disrupted by one or more large impact events.

Iron meteorites come in three textural varieties: octahedrites, hexahedrites, and ataxites. Octahedrites are the most common type of Fe-Ni meteorites. The textural classification of iron meteorites has been augmented 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 troilite (FeS - iron monosulfide), silicate minerals, and others.

The physical crystalline structure of octahedrites is distinctive. On cut, polished, and nitric acid-etched surfaces, a criss-crossing pattern of silvery-gray blades is evident. This is called Widmanstätten structure - it formed when kamacite and taenite slowly crystallized from cooling magma. The two minerals formed interlocking plates with octahedral (double pyramid) geometries.

Seen here is the Kossuth Meteorite, an octahedrite found in 1975 in Auglaize County, northwestern Ohio. The rock is a group IVA iron meteorite with ~9% nickel (Ni).