Classification and Glosary
Meteorites are classified into three main categories:
Stones, stony-irons and irons, depending on their dominant composition. Stones are similar to common terrestrial rocks in that their mineral composition is dominated by silicates, by far the most prevalent rock-forming minerals on our planet. Irons are mostly metallic in composition; they consist of alloys of iron (Fe) and nickel (Ni), in varying proportions. Stony-irons are combinations of both; they contain silicate and metallic phases in approximately equal amounts.
Stones are subdivided into two classes: chondrites and achondrites. Chondrites get their name from the fact that they all (with some exceptions) contain chondrules, tiny mineral spherules made mostly of silicates. Although some may be as large as a few millimeters in diameter, most chondrules are less than 1 mm across. In chondrites, chondrules are bound within a consolidated and fine-grained background matrix. Chondrites are the most primitive meteorites known. That is, they are the most ancient ones in terms of when their constituents came together to form a rock, and the most unprocessed ones in terms of how little their materials have been altered since this rock formed. Achondrites, on the other hand, lack chondrules and represent more processed materials.
Earth's surface rocks would be achondrites were they meteorites; they lack chondrules and are the result of extensive geological processing (melting, for instance).
Chondrites, achondrites, stony-irons and irons are subdivided into groups and subgroups. These will be presented in more detail below.
Falls are meteorites whose arrival on Earth was witnessed and recorded. Their time of fall is thus relatively precisely known. These meteorites were usually recovered shortly after their arrival, although often enough in the case of showers, additional fragments from a given fall may be recovered a long time after the fall occurred. When all falls exclusively are considered, a reasonably good estimate of the general population of meteorites reaching the Earth may be made. The vast majority of falls are stones (92.8%), most of which turn out to be chondrites (85.7% of all falls). Irons are rare (5.7% of all falls); stony-irons rarer still (1.5%). In other words, most meteorites falling on Earth are by far chondrites.
Finds are meteorites that were not seen to fall but were subsequently discovered on the ground, often long after they landed. Their arrival on Earth (time, circumstances) is thus not well documented. The vast majority of meteorites in museum and private collections around the world are finds, not falls. Because stones tend to look like ordinary terrestrial rocks, especially if they were subjected to weathering, they are easily overlooked. Stone finds are therefore rare in spite of the commonness of stones among falls. Meteorite collections are instead dominated by irons, which not only have a distinctive appearance and are therefore easier to spot, but they resist longer than stones to weathering and are particularly amenable to being found by metal detectors. Stony-irons would also be common among finds if it weren't for their lesser resistance to weathering compared to irons and, more importantly, for their extreme rarity among falls in the first place.
Meteorites, whether falls or finds, are usually given the name of the locality (post office, if any) nearest the site where they were recovered. In cases where many meteorites representing several falls are found within a relative small area (individual blue ice fields in Antarctica for instance), the meteorites are designated by an abbreviated locality name (the same name for all meteorites from that area) followed by a number giving the year of recovery and a serial number. ALH81005, for instance, is meteorite number 5 among those recovered in the Allan Hills area of Antarctica during the 1981-1982 field season (Note: number 5 does not necessarily mean that this meteorite was the fifth one recovered).