The rarer of the two main types of stony
meteorite, accounting for about 9% of all meteorite falls. Achondrites
are made of rock that has crystallized from a molten state. They contain
mostly one or more of the minerals plagioclase,
pyroxene, and olivine,
and generally, but not always, lack the small rounded inclusions known as
chondrules that are typical of chondrites.
Most achondrites are chemically similar to basalts
and are thought to be the product of melting on large asteroids,
moons, and planets. Soon after these worlds formed, they were heated from
within and partially melted. Although this process is still active on Earth,
it ended about 4.4 billion years ago on asteroids, 2.9 billion years ago
on the Moon, and perhaps one billion years
ago on Mars. Heating of the primordial mixture
of stony minerals, metals, and sulfides
(of which chondrites are made) produced liquids, the densest of which sank
to become planetary or asteroidal cores. Lighter stony minerals rose and
solidified to become basaltic rocks, fragments of which were subsequently
broken off by impacts and hurled into space. More than 200 of these evolved
achondrites have been found, covering a wide range of compositions and origins.
|An achondrite that fell in Sioux County, Nebraska.
This eucrite has been broken open, revealing the distinction between
its black, shiny, and smooth fusion crust and its light-colored interior.
The specimen is about 7 cm from left to right. Photo by J. Kurtzmen
The so-called HED group includes the howardites,
eucrites and diogenites,
which appear to share the same parent body, believed to be the asteroid
Vesta. Other evolved achondrites that seem
to have come from partially differentiated asteroids other than Vesta have
been mostly assigned to two distinct groups known as the angrites and the
Although the majority of achondrites are of asteroidal origin, some are
known to have come from the highland regions of the Moon's farside (see
lunar meteorites) and from Mars
(see Mars meteorites). NWA011, a
meteorite found in the Sahara in 1999, is suspected of having originated
As well as the evolved achondrites, there is an entire group of primitive
achondrites whose members all seem to have derived from small chondritic
parent bodies that only partially melted and differentiated through accretion
processes or from impact events, and then rapidly cooled. Primitive achondrites
vary widely in composition and fall into the following main subgroups: acapulocoites,
winonaites, and ureilites.