| About Meteorites |
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| When a meteoroid, a piece of rock floating in space, enters the Earth's atmosphere and burns up, it is referred to as a meteor. If the meteoroid survives its journy through the atmosphere to reach the ground, it is called a meteorite. Meteorites have been studied for scientifically for centuries and have been collected and often worshipped since the dawn of recorded history. The first iron tools were made from iron meteorites. |
| Most meteorites come from asteroids that lie within the asteroid belt, between the orbits of Mars and Jupiter. Asteroids are bodies of rock and/or metal up to about 300 km in diameter. They are believed to be debris left over from the formation of the planets about 4.5 billion years ago. The enormous gravity of nearby Jupiter disturbs the orbits of asteroids and causes them to occasionally collide. When this happens fragments of rock can be hurdled in all directions and into new orbits, sometimes crossing Earth's orbit and falling as meteorites. There are 3 general classes of meteorites although they can be subdivided into many more groups. The 3 basic types are stone, iron, and stony-iron. Stone meteorites are composed mostly of olivine ( (Mg, Fe)2SiO4 ), pyroxene ( (Mg, Fe)SiO3 ) and plagioclase feldspar (NaAlSi3O8 and CaAl2Si2O8), irons are made of kamacite and taenite (both alloys of iron and nickle), and stony-irons are a combination of the first 2 classes. Examples of each from my collection are pictured below (2 of them have been cut and polished to show the internal structure). |
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| Common Characteristics of Meteorites |
| Fusion crust When a meteorite first enters the atmosphere it is traveling at an enormous speed, usually around 15 km/s. At this speed, the air in front of the meteorite becomes compressed and heats up causing the exterior of the meteorite to heat up to the point of melting. Once the meteorite has slowed to terminal velocity the molten skin cools and solidifies forming a crust which can be seen on freshly fallen meteorites. Fusion crusts are generally darker than the rest of the meteorite and are usually fairly smooth. |
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| Regmaglypts As a meteorite passes through the atmosphere tiny pieces will be sheared off by the extreme force of the air hitting it. This process, known as ablation, roughly polishes the exterior and can also result in smooth shallow pits called regmaglypts or "thumb-prints". |
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| Magnetism Most meteorites contain at least some nickle-iron metal or magnetite (Fe3O4) which makes them stick to magnets. This effect is very useful as a field test because most Earth rocks don't show significant attraction to a magnet. The metal can also set off a metal detector. |
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| Chondrules Chondrules are millimeter sized spheres of mineral that are unique to meteorites. They can be found in the most common type of stone meteorite; chondrites. They can sometimes be seen protruding from a broken surface but they are most easily observed by cutting and lightly polishing a flat surface. |
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| Metal Metals on Earth are rarely found in their reduced, or uncombined, state, especially nickle and iron. Free metal is however very common in meteorites because most formed in an environment without oxygen or water. Nickle-iron can be found in many stone meteorites in the form of small flakes or in irons and stony-irons as large chunks or as the entire mass of the meteorite. |
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