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.

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 aubrites.

Although the majority of achondrites are of asteroidal origin, some are known to have come from the highland regions of the Moon’s farside (lunar meteorites) and from Mars (Martian meteorites). NWA011, a meteorite found in the Sahara in 1999, is suspected of having originated on Mercury.

Howardites

A type of achondrite meteorite and a member of the HED group believed to have originated on the asteroid Vesta. Howardites are named after the English chemist Edward Howard (1774-1816), one of the pioneers of meteoritics. Consisting mostly of eucritic and diogentic clasts and fragments, howardites are polymict breccias. However, they also contain dark clasts of carbonaceous chondritic matter, other xenolithic inclusions, and impact melt clasts, indicating a regolith origin. The suspicion is that howardites represent the surface of Vesta, a regolith breccia, consisting of eucritic and diogenitic debris, that was excavated by the large impact that created the enormous crater near Vesta's south pole. These fragments have been mixed with parts of the chondritic impactor, and this mixture has been subsequently pulverized and metamorphosed by smaller impacts and the solar wind to form a regolith. Similar regoliths cover the surface of the Moon and as with the howardites, these regolith breccias display high values for noble gases that have been implanted into the rock by the solar wind. Earth never formed any analogous rocks because its atmosphere and magnetic field protect its surface from continuous meteorite bombardment and the destructive radiation of the solar wind.

Eucrites

The most common class of achondrite meteorite and a member of the HED group for which the parent body is believed to be the asteroid Vesta. Eucrites are basalts—volcanic rocks of magmatic origin-composed primarily of the calcium-poor pyroxene pigeonite and the calcium-rich plagioclase anorthite. Based on mineralogical and chemical differences, they have been placed into three distinct subgroups. The non-cumulate eucrites represent the upper crust of Vesta that solidified on a magma ocean after the core and the mantle had already been formed. The rare cumulate eucrites are the products of the gravitational settling of crystallized minerals, mainly pyroxene and plagioclase, within magma chambers trapped below Vesta's early crust. Finally, the polymict eucrites are breccias that contain more than 90% eucritic material and less than 10% diogenitic clasts.

Diogenites

A class of achondrite composed mostly of magnesium-rich orthopyroxene, with minor amounts of olivine and plagioclase. The pyroxene is usually coarse-grained, suggesting an origin for the diogenites in magma chambers within the deeper regions of Vesta's crust. They are intrusive igneous rocks similar to plutonic rocks found on Earth, which cooled slowly and allowed their pyroxene to form sizeable crystals. This is especially true for the Tatahouine meteorite, a unique diogenite that fell in Tunisia in 1931 and is renowned for its green, centimeter-sized pyroxene crystals. Diogenites belong to the HED group, the same family of meteorites as howardites and eucrites. They are named after the Greek philosopher Diogenes of Apollonia, of the fifth century B.C., who was the first to suggest that meteorites come from space (a realization that was subsequently forgotten for the next 2,000 years

Acapulocoites

A primitive (little altered since its formation) achondrite belonging to a small group named after the only witnessed fall, the Acapulco meteorite that fell in Mexico in 1976. Acapulcoites are made mostly of fine-grained olivine, orthopyroxene, plagioclase, nickel-iron metal, and the iron sulfide, troilite, and are transitional between primordial chondritic matter and more differentiated rocks. Their mineral composition is between that of E and H chondrites, but they have an oxygen isotope pattern that sets them apart from all other known chondrite groups. Importantly, some acapulcoites contain a few relict chondrules, and one specimen, from Tissemoumine, Morocco, named NWA 725, shows an abundance of distinct chondrules. These distinct chondrules confirm that acapulcoites are very primitive and that they form a missing link between chondrites and achondrites. They are thought to have come from the same parent body as the closely related lodranites, which show evidence of slightly more melting.

Lodranites

A rare type of primitive achondrite named after the Lodran meteorite that fell in Pakistan in 1868. Initially, the lodranites were grouped with the stony-iron meteorites because they contain components of both stony material, consisting of olivine, orthopyroxene, and minor plagioclase, and nickel-iron metal in nearly equal proportions. However, since the discovery of the closely related acapulcoite group, the lodranites have been classified as primitive achondrites. Because both groups have similar mineralogical and oxygen isotopic compositions, it is thought that they come from the same parent body, most likely an S-class asteroid that has not yet been identified. Lodranites have coarser-grained olivines and pyroxenes and experienced higher temperatures than acapulcoites, suggesting that they originated within the deeper layers of the acapulcoite/lodranite parent body where they were subjected to a more intense and prolonged thermal processing.

Brachinites

A small subgroup of primitive (little altered since their formation) achondrites, named for the Brachina meteorite that was found in Australia in 1974. Originally, the olivine-rich Brachina was thought to be a second chassignite - a Mars meteorite that contains primarily olivine. However, further research showed Brachina to have a distinct trace-element pattern and a unique oxygen isotopic composition. With only a handful of known representatives, brachinites are composed mainly of small, equigranular olivine grains, among which are scattered small amounts of clinopyroxene, orthopyroxene, and plagioclase. Recent studies of the olivine compositions of different asteroids suggest that (289) Nenetta may be the parent body of this group.

Winonaites

A class of primitive achondrite, named for a most unusual find. The Winona meteorite was found in a stone cist in the ruins of the prehistoric Elden pueblo, Arizona, in 1928. The circumstances of the find suggest that the builders of the pueblo kept and venerated the meteorite as a sacred object after they had actually seen it fall. Winonaites are composed largely of fine-grained pyroxenes, with some magnesium-rich olivine, the iron-sulfide troilite, and nickel-iron metal. Recent research suggests that both the winonaites and the IAB group iron meteorites originated on the same parent body - a partially differentiated asteroid that was disrupted just as it began to form an iron core and a silicate-rich crust. This disrupting impact mixed silicates into molten nickel-iron forming the silicated IAB irons, and mixed olivine-rich residues of partial melts into unmelted silicates, forming the winonaites. A few winonaites have anomalous characteristics, however, that suggest they may have a different origin.

Ureilites

A member of a subgroup of primitive achondrites named for Novo Urei, a village in the Mordova Republic, Russia, where several meteorites fell in late 1886. It has been reported that one stone was soon recovered by local peasants, but not to preserve it for science; on the contrary, the stone was immediately broken apart and eaten! The report doesn't reveal why this happened—perhaps the freshly fallen meteorite smelled good, or perhaps because it was shaped like a loaf of bread, which some ureilite are. However, not all of the stones were eaten, and Novo Urei became the type specimen of one of the best-represented achondrite groups.

The are two subgroups. Main group ureilites are composed mainly of coarse-grained olivine and minor pyroxene, mostly in the form of calcium-poor pigeonite, set in a dark carbonaceous matrix of graphite and diamond, nickel-iron metal, and troilite. Polymict ureilites consist of a mixture of different lithologies. Besides clasts from main group ureilites, they contain magmatic inclusions, dark carbonaceous clasts, chondritic fragments of different origins, and various other inclusions. This suggests a surface or regolith origin for the polymict ureilites, an assumption supported by the values for noble gases that have been implanted into the regolith by the solar wind. However, both the origin and the formation history of the ureilites remain enigmatic. Their mineral and oxygen isotopic compositions suggest that they formed as residues from partial melting, and therefore represent primitive achondrites that probably formed on several parent bodies. On the other hand, rare-element patterns and other chemical characteristics indicate that ureilites are highly fractionated igneous rocks that formed in different regions of the same parent body; probably a moderately differentiated C-class asteroid that was disrupted by an impact event and then rapidly cooled. An impact history would also explain the occurrence of high-pressure minerals such as diamond and londsdaleite that are formed by intense shock metamorphism. Even this theory isn't without its problems though. Recently, a ureilite from the Libyan Sahara, named DaG 868, was found to contain diamonds, but paradoxically, appears to be nearly unshocked.