Meteorites are the remains of space rocks that have fallen to Earths surface (a space rock still in space is termed a meteoroid , and when it is plunging through Earths atmosphere it is termed a meteor) .
The small number of meteorites that survive vaporization in the atmosphere relatively intact give a picture of the chemical makeup of meteorites. Meteorites are classified into three main groups according to their composition : stoney, iron, and stoney – iron meteorites .
The large majority of meteorites (about 95%) are stoney meteorites. They come from debris left over from the formation of the solar system or from collisions of asteroids. They are made up of silicates, the same compounds that compose most of Earths crust.
Silcates are minerals made up of a silicate anion ( SiO4)-4 and a metal such as iron (Fe), magnesium (Mg), or aluminum (Al) . In a silicate ion, the silicon ion is bound to four oxygen atoms in a tetrahedral structure . There is a 109° angle between each silicon – oxygen bond. The anion has a charge of -4 and bonds with a positively charged metal cation to form the silicate .
Iron meteorites are composed of an alloy (a metallic chemical mixture) of iron and nickel (Ni) .They make up less than 5 percent of all meteorites, but are more likely to survive to reach the Earths surface and to be recognized as meteorites when found because they tend to be more massive .
They are formed from the metallic cores of asteroids shattered by collisions. The percentage of nickel can range from 5 to over 20%. The Barringer Meteor Crater in Arizona, almost a mile wide, was formed by the impact of a large iron meteor about 50,000 years ago.
Stoney – iron meteorites make up about 1% of meteorites. They contain equal parts of silicates and iron, and they are formed from the mantel – core boundaries of asteroids. The iron – nickel content is similar to that of iron meteorites, and the silicate is mostly olivine (Mg, Fe) SO4, with varying amounts of iron and magnesium .
The Earth is impacted by about 50 tons of meteorites a day. Much of the topsoil of Earth is silcates from meteorites mixed with the silcates of Earth. When a body much larger than a meteorite , such as an asteroid , impacts Earth, it can leave chemical evidence around the globe .
There is a concentration of iridium (Ir) much higher than average in the Cretaceous – Tertiary boundary of rocks . Iridium is found in higher concentrations in asteroids, and was deposited by the asteroid that wiped out the Dinosaurs .