Classification of meteorites

Meteorites are solid extraterrestrial materials that have survived the descent through the Earth s atmosphere. They are derived from meteoroids, which are natural objects with diameters of up to 10-100 m that orbit in space, while a meteor is the visible path of a meteoroid that enters the Earth s (or another body s) atmosphere (i.e., a shooting star). The meteoroids (and hence all meteorites) are ultimately derived from a parent body, from which they were separated by impact events that launched larger debris into new orbits. Most meteorite parent bodies appear to be in the asteroid belt, as can be inferred from orbits that were calculated for meteorites that were observed to fall [3]. Some meteorites have even been linked to specific asteroids, and there is good evidence that others are derived from Mars and the Moon (Table 10.1). As such, meteorites provide us with direct chemical and isotopic information on a large range of planetary bodies. Many meteorites furthermore 

There are numerous ways to classify meteorites [14,15]. Meteorites jinds, which cannot be directly linked to a specific observed fall, are much more common than falls. The distinction between stones (silicate meteorites), iron meteorites, and stony-irons is straightforward but ignores the significant genetic differences amongst stony meteorites. More recent nomenclature systems make a primary distinction between chondrites and non-chondrites, whereby the latter group includes all types of igneously differentiated meteorites (Table 10.1). 

Categorles/classes Croups (and important examples) Comments 

Chondrites Carbonaceous CIl (Ivuna, Orgueil) Volatiles-rich composition, best 

Ordinary chondrites H, L, LL Petrological types 3-6 most common 

The volatile materials would have vaporised from the surface of the planetesimals once the temperature reached 160 K below this temperature water sticks to silicate surfaces and condenses, ultimately freezing into ice. The new gaseous material is swept away from the planetesimals by the solar wind of particles, leaving bare planetesimals too small to acquire and maintain an atmosphere. The temperature gradient and location within the solar nebula are then important to the ultimate nature and composition of the planets themselves and interplanetary debris. 

Broadly speaking the classification of meteorites follows the geological mineral classification and with 275 mineral species reported so far this quickly becomes complex some classes of meteorite have only one member. The mineral structure does convey essential information about the temperature at which the meteorite formed as well as the reduction-oxidation (redox) environment was the environment in which it formed rich in oxygen Meteorites have been classified into three broad classes  

Irons composition principally of pure metallic nickel-iron. A large piece of unoxidised iron is very rare on Earth because it quickly oxidises to the iron ore. 

Stony principally silicates or rocky meteorites. It is harder to determine the extraterrestrial origin of these meteorites and it usually requires careful laboratory analysis. 

Type Fall (%) Find (%) Fall weight (kg) Find weight (kg) 

Reserve University in Cleveland who, in collaboration with John W. Schntt, has continued to collect large numbers of meteorite specimens on the East Antarctic ice sheet adjacent to the Transantarctic Mountains. 

The meteorite specimens that have been collected annually by the ANSMET scientists are curated at the Johnson Space Center in Houston, Texas, where they are assigned official designations such as ALHA 81005. The letters ALH refer to the Allan HiUs, the letter A indicates that the specimen was collected by ANSMET, 81 refers to the 1981/82 field season, and 005 teUs ns that it was the fifth Antarctic meteorite from that field season examined in Houston. The letter A was dropped after the 1981/82 fieldseason. The bare-ice (or blue-ice) areas adjacent to the Transantarctic Mountains are identified by the letter codes listed in Appendix 18.12.1. The origin of the bine-ice areas was discussed at a workshop in 1988 chaired by Cassidy and WhiUans (1990) who referred to them as stranding surfaces (Faure 1990). 

All of the meteorite specimens collected in Antarctica are listed in the Antarctic Meteorite Newsletters issued twice each year by the Meteorite Working Group at the Johnson Space Center in Houston, Texas. In addition, selected specimens are classified based on petrographic descriptions of thinsections. This information is also published in The Meteoritical Bulletins that are combined with the annual supplements of the journal Meteoritics and Planetary Science. The Meteoritical Bulletins list and describe meteorites recovered from all regions of the Earth including Antarctica. 

The meteorites recovered by Japanese scientists from 1966 to 1994 were listed by Yanai and Kojima (1995) who also provided chemical analyses of a large number of specimens that originated from the Yamato Mountains, Asuka Station, Sor Rondane Mountains, Allan Hills, Bates Nunatak, Derrick Peak, and Mt. Baldr. Three of the specimens listed by Yanai and Kojima (1995) are lunar meteorites Asuka-881757, Yamato-793274, and Yamato-793169. The compilation includes color photographs of 12 specimens of different kinds of stony meteorites including A-881757 from the Moon. 

Antarctic meteorites were also described in the publications of Marvin and Mason (1980,1982,1984), Marvin and MacPherson (1989), Score et al. (1981, 1982), Score and Lindstrom (1990), and in the meteorite catalog of Grady (2000). In addition, Antarctic meteorites have been the subjects of numerous reports 

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See also in sourсe #XX -- [ , , ]

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