A meteorite is the name given to the remains of a meteor after they have passed through the Earth’s atmosphere and landed on the ground. Virtually all meteorites contain some amount of iron and nickel. Meteorites are classified according to how much iron they contain. Iron meteorites contain the most, followed by stony iron meteorites and stone meteorites.
Iron meteorites originate from the core of extraterrestrial planets, as opposed to the crust, which gives up stone meteorites. Some theories suggest they may also originate from supernovas, because iron is produced by nuclear fusion. They are composed of more than 90 percent iron, with the remaining percentage being mainly nickel. This composition makes an iron meteorite deceptively heavy for its size and highly magnetic. While they represent approximately 10 percent of the number of meteorites recovered, their iron makes them account for 90 percent of the mass.
An iron meteorite is classified by one of two methods: chemical composition or structure. Classification by chemical composition looks at the amount of trace elements present — everything that is not iron or nickel. These are normally elements such as gallium, iridium, and germanium. Based on the relative abundance of these, the iron meteorite is placed into one of 13 groups. A 14th group exists for iron meteorites that do not fit into any of the 13 standard groups.
Structural classification is done by examining the two iron-nickel alloys — kamacite and taenite — that are present in all iron meteorites. The iron meteorite is etched with a mild acid that leaves a distinct lattice arrangement of kamacite and taenite known as a Widmanstatten pattern. By measuring the width of the kamacite lines, the structural class is determined. There are eight standard structural classes. A ninth exists for iron meteorites that do not fit into a standard class.
Some famous iron meteorites include Canyon Diablo, which fell in northern Arizona in the United States approximately 25,000 years ago and was the size of a building. Most of the meteorite was vaporized when it hit the ground, but it left a hole almost 1 mile (1.6 km) wide and 600 feet (183 meters) deep. The 15-ton (13,600-kg) Willamette iron meteorite was discovered in 1902 in Oregon in the United States and is widely thought to be one of the most beautiful iron meteorites in the world. Sikhote-Alin fell in 1947 in eastern Siberia, leaving a crater field with 99 distinct impact zones. The fragments from Sikhote-Alin melted in an unusual way when passing through the Earth’s atmosphere, making them highly sought after by collectors.