Octahedrite from the Asteroid Belt between Mars and Jupiter. (USNM 1492, National Museum of Natural History (Smithsonian Institution), Washington D.C., USA) (public domain image provided by the USNM)
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 metallic 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.
Mineralogically, 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, formed when kamacite and taenite slowly crystallized from cooling magma. The two minerals form interlocking plates with octahedral (double pyramid) geometries.
The octahedrite seen here is an oriented individual of the Henbury Meteorite, which was found west-southwest of the "town" of Henbury, Northern Territory, central Australia. Many samples were collected in the 1930s from a crater field consisting of 14 small to moderately small impact craters. The impact event occurred during the Holocene, at about 4.2 ka. The target rocks are Precambrian fine-grained siliciclastic sedimentary rocks. The meteorite itself is a medium octahedrite. This specimen has a fusion crust and abundant regmaglypts. Henbury is classified as a group IIIAB iron meteorite.
Loading contexts...
