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Meteorites



Meteorites (Greek "heavenly apparition") are large meteors that have survived the intense heat of Earth's atmospheric friction and have reached the earth's surface.

Meteorites are pieces of asteroids, the Moon, Mars or perhaps even comets. Their arrival may be heralded by a ball of light (shooting stars) followed by sonic booms.

The fall of meteorites on Earth is largely random. Meteorites are found from the tropics to the poles. A meteorite is usually named after a place near where is was seen to fall.

Meteorites can fall at any time of day, but most fall in the afternoon or evening. Such falls are favored because, as the Earth spins, the hemisphere with noon to midnight local time encounters objects in orbit around the Sun that are closing in on the Earth. In this case, capture by the Earth's gravity is more likely. On the contrary, objects that pass in front of the hemisphere with local time between midnight and noon are pulling away from the Earth. In this case gravity is less likely to pull in the object.

Bodies in orbit around the Sun are called meteoroids if there is a chance that they may at some time land on the Earth.

Most meteorites are fragments broken from asteroids. Most asteroids, however, orbit the Sun in almost circular, stable orbits in the asteroid belt, between Mars and Jupiter. Meteorites, then, are strays from the asteroid belt that were diverted by the gravity of Jupiter into elliptical, Earth-crossing orbits.

Meteorites are classified in three general categories. Iron meteorites (or siderites) are composed entirely of metal, chiefly nickel and iron. As a result, they are very different from indigenous rocks.

Stony meteorites (or aerolites) show a diversity of mineral elements including large percentages of silicon and magnesium oxides; the most abundant type of Stony meteorite is the chondrite, so called because the metal embedded in it is in the form of grainlike lumps, or chondrules.

Stony-Iron meteorites (or siderolites), rarer than the other types, are composed of both metal and stone in varying proportions.



Iron Meteorite

15% of Falls. Iron meteorites are crystalline alloys of native iron with 4-40% nickel and a small amount of cobalt and copper. This composition of ore does not occur in the Earth's crust. Most Iron meteorites were originally completely molten and formed in the cores of asteroids
Hexahedrite
Iron meteorite with 6-7% nickel content and cubic crystals which can be cleaved along the faces of the cube (hexahedron). When the surfaces are polished and etched with nitric acid a fine, parallel oriented or intersecting striation, called Neumann lines, is formed.
Octahedrite
Iron meteorite with up to 40% nickel, crystallizing with octahedral form. Surfaces polished and etched with nitric acid reveal a system of lamellae which is called Widmanstatten structure. So far, it has not proved possible to produce such a system of lamellae experimentally. Three components of the fabric can be distinguished: kamacite, taenite and plessite. Kamacite (beam iron) Dark gray nickel-iron alloy with 6-7% nickel in the form of thick plates. Taenite (ribbon-iron) Nickel-iron alloy, containing about 30% nickel, with silvery luster bordering the kamacite. Plessite ("filling" iron) Gray black mixture of extremely small crystals of kamacite and taenite which fill in the spaces in the system of lamellae.
Ataxite
Finely crystalline, nickel-rich iron meteorite without a clearly defined structure. Probably derived from octahedrite because of the effects of heating.


Stony Meteorite

82% of Falls. Stony meteorites are similar to terrestrial rocks. Their mineral content corresponds approximately to peridotites or gabbros. Small amounts of nickel-iron are also present. They are more abundant than iron meteorites.
Chondrite
Stony meteorites with individual, small, up to pea-sized spheres (chondrules) made of silicate minerals (such as bronzite, diopside, olivine, and plagioclase), very occasionally of chromite, graphite, magnetite, spinel or of rock-glass. The groundmass consists of the same minerals or of glass. Color usually light to dark gray, sometimes black.
Achondrite
Stony meteorite without chondrules usually possessing a gleaming black fused skin. Rarer than chondrites.


Stony-Iron Meteorite

3% of Falls. Somewhere between stony and iron meteorites in composition. Stony-Iron meteorites consist of about equal proportions of silicate minerals and nickel iron. They account for less than 10% of all meteorites which have fallen on Earth.
Siderophyre
Variety of meteorite with a high nickel-iron content as well as a high proportion of bronzite in a net-like fabric. Tridymite is also present.
Pallasite
Variety of meteorite with large olivine crystals in an octahedrite, nickel-iron groundmass.
Mesosiderite
Variety of meteorite with irregularly distributed nickel-iron in a silicate groundmass of bronzite, olivine and plagioclase.


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