A meteorite is a stony or metallic rock that falls to the Earth from space, likely from the asteroid belt. Many of the smallest meteorites are dust from comets. The larger ones that impact Earth can form huge craters.
Iron meteorites are composed mostly of nickel and iron and are very dense. They come from the core of asteroids and are rare among meteorites seen to fall to Earth. Iron meteorites have two mineral phases: taenite and kamacite. When one of these meteorites is cut, polished, and etched in an acid bath, a criss-cross mineral arrangement, called the Widmanstätten pattern, becomes clear in many iron meteorites. The presence or absence of the Widmanstätten pattern was once used to identify iron meteorites from stony-iron and stony meteorites. The presence of the Widmanstätten pattern means the iron meteorite has an average to high nickel content. Today, there are three structural classes for iron meteorites: hexahedrites (no Widmanstätten pattern, may present Neumann lines); octahedrites (presents Widmanstätten pattern; common); and ataxites (very high nickel content with no pattern, rare).
Stony-iron meteorites are made of nickel/iron and silicate minerals. One type of stony-iron meteorites are pallasites – rocks composed of iron-nickel surrounding olivine, a greenish silicate mineral. Another type is mesosiderites. These meteorites are composed of iron-nickel and basalt with no separation between the metal and rock.
Stony meteorites are the most common and maybe the crust or outer layer of a planet. They may represent the most primitive material in the solar system. Stony meteorites can be divided into two groups:
When a meteorite hits the Earth, sediment and rock are often melted and ejected into the air. As this melted rock falls, it quickly cools and forms into a glassy rock called a tektite.