Achondrites are a grab-bag category of meteorites which are:
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- Rich in silicates
- Lack chondrules
- Have undergone relatively advanced geological processing on their parent bodies.
They are generally more coarse-grained than the chondrites and are generally closer to terrestrial igneous rocks in terms of chemistry, mineralogy, and structure than the chondrites. The near-absence of metal phases (Fe, Ni) in achondrites suggests that they are differentiated rocks; that is, that they were produced by melting, decantation and then cooling of a more primitive material (possibly chondritic originally). In this picture, achondrites would be rocks that have crystallized as part of near-surface magmas on their parent worlds, whereas iron meteorites would represent material that sank to their interior to form a core. Stony-irons would then be samples from the boundary between the achondritic upper layers and the core. Achondrites are classified into several groups, each believed to represent either a distinct parent body or, in some cases, a particular location on a common parent body. Some groups may be subdivided into subgroups.
- HED group
- HED stands for Howardite-Eucrite-Diogenite meteorites. These are three subgroups of achondrites which, although different in mineral composition, share a number of other characteristics (such as isotopic chemistry) which suggest they are probably related. The HED meteorites are believed to come from the same parent body, specifically from the large asteroid Vesta. Because of Vesta's very unusual composition among asteroids and the relatively good match between its composition and that of the HED meteorites, the relationship is thought to be relatively well established.
Eucrites are calcium-rich basaltic achondrites. They are fine-grained volcanic rocks, samples of lava flows from the surface of another world! Eucrites are nevertheless very different from terrestrial basalts in that:
- they contain sodium-rich plagioclase
- they have pigeonite as their dominant pyroxene mineral
- they contain no water at all (no hydrous minerals), and
- they have a reduced oxidation state.
- Diogenites are calcium-poor basaltic achondrites, consisting almost entirely of the Mg-rich pyroxene, hypersthene. They contain only minor amounts of plagioclase and olivine. The mineralogy and oxidation state of diogenites are close enough to those of eucrites to suggest a common parent body. Because diogenites have a coarse-grained texture with large interlocking crystals, however, they must have cooled more slowly than the eucrites. Diogenites probably crystallized from a magma at some depth:. They are plutonic rocks (as opposed to volcanic ones).
- Aubrites are calcium-poor achondrites consisting mostly of enstatite as their pyroxene. They are sometimes referred to as enstatite achondrites and might somehow be related to the enstatite chondrites. Aubrites are believed to come from E-type asteroids, although it has also been suggested that they are what meteorites from Mercury might look like. Less than twenty distinct Aubrites are known.
- Ureilites are calcium-poor achondrites, consisting mainly of olivine, pigeonite (their dominant form of pyroxene) and carbon (2.2%). The carbon is in the form of either graphite, diamond, or lonsdaleite (a rare pure carbon mineral like diamond, but with a different crystal structure). The ureilites are the only achondrites containing significant amounts of free metal (Fe, Ni) (5%). They also contain the highest proportions of heavy rare gases such as argon, krypton and xenon, among all meteorites. The origin of ureilites remains an enigmatic. They appear to result from a complex igneous history, involving perhaps carbonaceous chondrite-like material as a starting point.
- Lunar meteorites
- Twelve meteorites from the Moon have been found to this day. All except one, Calcalong Creek (Australia), were recovered in Antarctica. All lunar meteorites are impact breccias, rocks formed from the rewelding during energetic impact events of loose fragments once part of the lunar soil. Some lunar meteorites are dominantly basaltic in composition and thus come from the lunar mare (the dark, concealed flood lavas that occupy large impact basins on the Moon, especially on its near side); others are composed dominantly of the mineral anorthosite, a sodium-rich plagioclase, suggesting these meteorites come from the lunar highlands (the brighter and more heavily cratered terrains on the Moon). Lunar meteorites are of great scientific importance because they come from areas of the Moon that were likely not sampled by the Apollo or Luna missions. On statistical grounds, it is estimated that at least one of the lunar meteorites found so far must have originated on the far side of the Moon. Lunar meteorites are believed to have been blasted off the Moon in the form of high-speed ejecta during impact events. Lunar meteorites may be identified by a fusion crust with slightly greenish hues and by a grayish interior with angular clasts (inclusions) of often brighter materials.
- Martian meteorites
- Twelve meteorites are apparently samples from Mars. They are commonly referred to as the SNC meteorites (after Shergotty, Nakhla, and Chassigny, the representatives of the first three subgroups known). These martian meteorite subgroups are distinguished on the basis of mineralogy, but they all share isotopic signatures, petrologic characteristics and for some, relatively young crystallization ages (less than 1.4 billion years), which together point to a martian origin. About half of the known martian meteorites were found in Antarctica. The other are falls and finds from elsewhere.
The shergottite subgroup is named after Shergotty, an achondrite which fell in India in 1865. Shergottites are pigeonite and augite-dominated basalts, samples of lavas that once flowed on their parent world. Aside from Shergotty, shergottites include Zagami (Nigeria 1962) and the antarctic meteorites ALH77005 and EET79001.
The nakhlite subgroup is named after Nakhla, an achondrite which landed in Egypt in 1911. Nakhlites are augite-rich achondrites. Aside from Nakhla, nakhlites include Lafayette (Indiana find) and Governador Valadares (Brazil find).
The chassignite subgroup is named after Chassigny, an achondrite which fell in France in 1815. Chassignites are olivine-rich achondrites. Aside from Chassigny, chassignites include Brachina (Australia find).
Recently, the antarctic meteorite ALH84001 was identified as having originated on Mars. The meteorite, however, does not fit into any of the SNC subgroups and must be classified into a subgroup of its own. ALH84001 crystallized (solidified as a rock) more than 4 billion years ago. It is believed it came from the heavily cratered highlands of Mars, which would make it the only sample from these terrains found so far.
- Anomalous achondrites
There are a number of other achondrites that do not fit into any of the preceding groups or subgroups. Some are the only specimens known of their kind. More detailed descriptions of these rare meteorites is beyond the scope of this review.