chondrite(redirected from Chondrites)
Also found in: Dictionary, Thesaurus.
Classification of Meteorites
Not until the early 19th cent. did scientists fully accept the fact that meteorites came to the earth from outer space. Since then many studies have been made of their composition and crystalline structure; the use of microchemical analysis, X rays, and the mass spectrograph has facilitated such work. The age of meteorites can be determined by measuring their radium and helium contents. Some meteorites might be fragments of comets; others, small asteroids whose orbital paths crossed that of the earth. Geochemical analysis has shown that more than 170 known meteorites are of lunar origin and more than 100 are of Martian origin. One of the Martian meteorites—known as ALH84001—is believed by some scientists to show evidence of there once having been primitive bacterial life on Mars, but most experts disagree with this conclusion. The lunar and Martian meteorites are thought to have been broken away from the moon and Mars by the impact of large asteroids.
Three general categories are used to classify meteorites. The siderites, or irons, are composed entirely of metal (chiefly nickel and iron). The aerolites, or stony meteorites, show a diversity of mineral elements including large percentages of silicon and magnesium oxides; the most abundant type of aerolite is the chondrite, so called because the metal embedded in it is in the form of grainlike lumps, or chondrules. The siderolites, which are rarer than the other types, are of both metal and stone in varying proportions.
As a meteor speeds through the atmosphere, its outer surface becomes liquefied; the friction of the atmosphere finally reduces its velocity (if the meteor is not large), and the surface cools and solidifies into a dark, smooth crust. Lines of flow in the hardened surface can indicate its motions in flight. Cone-shaped meteorites show that one end was directed forward. Others, which are unevenly shaped, probably spun while falling. The smallest meteorites, dust-sized particles known as micrometeorities, may pass through the atmosphere without heating up due to friction because of their very small mass.
Formation of Craters
See K. Mark, Metorite Craters (1995); O. R. Norton and D. S. Norton, Rocks from Space: Meteorites and Meteorite Hunters (2d ed. 1998).
chondrite(kon -drÿt) A type of stony meteorite that contains chondrules. They are the most abundant class of meteorite in the Solar System (about 86%). Chondrites are largely composed of iron- and magnesium-bearing silicate minerals with a wide range of compositions. Their chemical composition is similar to that of the Sun (but depleted in volatile gases like hydrogen and helium). Several types are recognized: carbonaceous chondrites have the highest proportions of volatile elements and are the most oxidized; enstatite chondrites contain the most refractory elements (withstanding high temperatures) and are reduced; ‘ordinary’ chondrites, the most common type, are intermediate in volatile element abundance and oxidation state. Ordinary chondrites plus some carbonaceous chondrites are thought to be primitive samples of early Solar-System material. See also asteroids. Compare achondrite.
the most common type of stony meteorite, accounting for 90 percent of all such meteorites. Chondrites are characterized by the presence of chondrules located in the primary, fine-grained mass of the meteorite (see). They contain a significant amount of nickel-bearing iron in the form of small grains that have the same composition and microstructure as the grains in octahedrites.