An achondrite is a stony

meteorite A meteorite is a rock (geology), rock that originated in outer space and has fallen to the surface of a planet or Natural satellite, moon. When the original object enters the atmosphere, various factors such as friction, pressure, and chemical ...

that does not contain

chondrule A chondrule (from Ancient Greek χόνδρος ''chondros'', grain) is a round grain found in a chondrite. Chondrules form as molten or partially molten droplets in space before being Accretion (astrophysics), accreted to their parent asteroids ...

s. It consists of material similar to terrestrial

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

s or

plutonic Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form ''intrusions'', such as batholiths, dikes, sills, laccoliths, and volcanic necks.Intrusive RocksIntrusive rocks accessdate: Marc ...

rocks and has been differentiated and reprocessed to a lesser or greater degree due to melting and recrystallization on or within meteorite parent bodies. As a result, achondrites have distinct textures and mineralogies indicative of igneous processes. Achondrites account for about 8% of meteorites overall, and the majority (about

2/3

) of them belong to the HED clan, possibly originating from the crust of asteroid Vesta. Other types include

Martian Mars, the fourth planet from the Sun, has appeared as a setting in works of fiction since at least the mid-1600s. Trends in the planet's portrayal have largely been influenced by advances in planetary science. It became the most popular celes ...

Lunar Lunar most commonly means "of or relating to the Moon". Lunar may also refer to: Arts and entertainment * ''Lunar'' (series), a series of video games * "Lunar" (song), by David Guetta * "Lunar", a song by Priestess from the 2009 album ''Prior t ...

, and several types thought to originate from as-yet unidentified

asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...

s. These groups have been determined on the basis of e.g. the Fe/ Mn chemical ratio and the ¹⁷O/¹⁸O

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...

ratios, thought to be characteristic "fingerprints" for each parent body.

Classification

Achondrites are classified into the following groups:O. Richard Norton. The Cambridge encyclopedia of meteorites. UK, Cambridge University Press, 2002. . *

Primitive achondrite Primitive achondrites are a subdivision of meteorites. They are classified on the same rank (historically called "Class") and lying between chondrites and achondrites. They are called primitive because they are achondrites that have retained much ...

s *

Asteroidal achondrite An achondrite is a stony meteorite that does not contain chondrules. It consists of material similar to terrestrial basalts or plutonic rocks and has been differentiated and reprocessed to a lesser or greater degree due to melting and recrystalli ...

s * Lunar meteorites *

Martian meteorite A Martian meteorite is a rock that formed on Mars, was ejected from the planet by an impact event, and traversed interplanetary space before landing on Earth as a meteorite. , 277 meteorites had been classified as Martian, less than half a perce ...

Primitive achondrites

Primitive achondrites, also called PAC group, are so-called because their chemical composition is '' primitive'' in the sense that it is similar to the composition of

chondrites A chondrite is a stony (non-metallic) meteorite that has not been modified by either melting or planetary differentiation, differentiation of the parent body. They are formed when various types of dust and small grains in the early Solar Syste ...

, but their texture is igneous, indicative of melting processes. To this group belong: *

Acapulcoite Acapulcoites are a group of the primitive achondrite class of stony meteorites. Naming and history The acapulcoites are named after the only specimen of the group that was a witnessed fall. The Acapulco meteorite fell on 11 August 1976 at 11:00 ...

s (after the meteorite Acapulco, Mexico) *

Lodranite Lodranites are a small group of primitive achondrite meteorites that consists of meteoric iron and silicate minerals. Olivine and pyroxene make up most of the silicate minerals. Like all primitive achondrites lodranites share similarities with ch ...

s (after the meteorite Lodran) *

Winonaite Winonaites are a group of primitive achondrite meteorites. Like all primitive achondrites, winonaites share similarities with chondrites and achondrites. They show signs of metamorphism, partial melting, brecciation and relic chondrules. Their c ...

s (after the meteorite Winona) *

Ureilite Ureilite is a rare type of stony meteorite that has a unique mineralogical composition very different from that of other stony meteorites. This dark grey or brownish meteorite type is named after the village Novy Urey (Cyrillic: ''Новый Ур ...

s (after the meteorite Novy Ureii, Russia) *

Brachinite Brachinites are a group of meteorites that are classified either as primitive achondrites or as asteroidal achondrites. Like all primitive achondrites, they have similarities with chondrites and achondrites. Brachinites contain 74 to 98% (volume ...

s (after the meteorite Brachina)

Asteroidal achondrites

Asteroidal achondrites, also called evolved achondrites, are so-called because they have been ''differentiated'' on a

parent body A parent body in meteoritics is the celestial body from which originates a meteorite or a class of meteorites. Identification It is easiest to correlate a meteorite with a parent body when the parent body still exists. This is the case for Lun ...

. This means that their mineralogical and chemical composition was changed by melting and crystallization processes. They are divided into several groups: *

HED meteorite HED meteorites are a clan (subgroup) of achondrite meteorites. HED stands for " howardite– eucrite– diogenite". These achondrites came from a differentiated parent body and experienced extensive igneous processing not much different from the ...

s (Vesta). They may have originated on the asteroid

4 Vesta Vesta (minor-planet designation: 4 Vesta) is one of the largest objects in the asteroid belt, with a mean diameter of . It was discovered by the German astronomer Heinrich Wilhelm Matthias Olbers on 29 March 1807 and is named after Vesta (mytho ...

, because their reflection spectra are very similar. They are named after the initial letters of the three subgroups: **

Howardite Howardites are achondritic stony meteorites that originate from the surface of the asteroid 4 Vesta, and as such are part of the HED meteorite clan. There are about 200 distinct members known. Characteristics They are a regolith breccia consist ...

s **

Eucrite Eucrites are achondritic stony meteorites, many of which originate from the surface of the asteroid 4 Vesta and are part of the HED meteorite clan. They are the most common achondrite group with over 100 meteorites found. Eucrites consist of b ...

s **

Diogenite Diogenites are a group of the HED meteorite clan, a type of achondritic stony meteorites. Origin and composition Diogenites are currently believed to originate from deep within the crust of the asteroid 4 Vesta, and as such are part of the HED ...

s * Angrites *

Aubrite Aubrites are a group of meteorites named for Aubres, a small achondrite meteorite that fell in 1836 in Aubres near Nyons, France. They are primarily composed of the orthopyroxene enstatite and are often called enstatite achondrites. Their igneou ...

Lunar meteorites

Lunar meteorites are meteorites that originated from the

Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

Martian meteorite

s are meteorites that originated from

Mars Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...

. They are divided into three main groups, with two exceptions (see last two entries): *

Shergottite A Martian meteorite is a rock that formed on Mars, was ejected from the planet by an impact event, and traversed interplanetary space before landing on Earth as a meteorite. , 277 meteorites had been classified as Martian, less than half a perce ...

s * Nakhlites * Chassignites * OPX martian meteorites ( ALH 84001) * Regolith/Soil samples ( NWA 7034 and pairings)

References

External links

Achondrite Images
from Meteorites Australia {{Authority control Planetary science Meteorite types