An achondrite is a stony

meteorite A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or moon. When the original object ...

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 accreted to their parent asteroids. Because chondrites repr ...

s. It consists of material similar to terrestrial

basalt Basalt (; ) is an 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 surface of a rocky planet or moon. More than 90 ...

s or plutonic 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 two-thirds) of them are

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 ma ...

s, possibly originating from the crust of 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, t ...

. Other types include Martian,

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 of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere. ...

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

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers ( mass num ...

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 recrystall ...

s *

Lunar meteorite A lunar meteorite is a meteorite that is known to have originated on the Moon. A meteorite hitting the Moon is normally classified as a transient lunar phenomenon. Discovery In January 1982, John Schutt, leading an expedition in Antarctica for ...

s * Martian meteorites

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 differentiation of the parent body. They are formed when various types of dust and small grains in the early Solar System accreted to form primi ...

, 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, with a witnessed fall. The Acapulca meteorite fell on 11 August 1976 at 11:00 near ...

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 w ...

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) * Ureilites (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 In meteoritics, a parent body is the celestial body from which originates a meteorite or a class of meteorites. Identification The easiest way to correlate a meteorite with a parent body is when the parent body still exists. This is the case fo ...

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

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

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

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 ...

s ** Diogenites *

Angrite Angrites are a rare group of achondrites consisting mostly of Al-Ti bearing diopside, hedenbergite, olivine, anorthite and troilite with minor traces of phosphate and metals. The group is named for the Angra dos Reis meteorite. They are the oldes ...

s *

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

Lunar meteorites

Lunar meteorites A lunar meteorite is a meteorite that is known to have originated on the Moon. A meteorite hitting the Moon is normally classified as a transient lunar phenomenon. Discovery In January 1982, John Schutt, leading an expedition in Antarctica for ...

are meteorites that originated from the

Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

Martian meteorite

Martian meteorites are meteorites that originated from

Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...

. They are divided into three main groups, with two exceptions (see last two entries): * Shergottites * Nakhlites * Chassignites * OPX martian meteorites (

ALH 84001 Allan Hills 84001 (ALH84001) is a fragment of a Martian meteorite that was found in the Allan Hills in Antarctica on December 27, 1984, by a team of American meteorite hunters from the ANSMET project. Like other members of the shergottite–nakh ...

) * Regolith/Soil samples ( NWA 7034 and pairings)

References

External links

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