Achondrites
Achondrites represent a little less than 7% of objects in world collections. These rocks come from differentiated bodies of the solar system, composed of a core, a mantle and a crust, such as the Moon, Mars or certain large asteroids such as Vesta. There is also another family, called primitive achondrite, whose meteorites make the transition between chondrites and achondrites. Achondrites come from parent bodies formed before the chondrites, during early stages of the solar system.
Ibitira @Stephant/ASU
Groups
Lunar Meteorites
Lunar meteorites mostly come from lunar regions that were unexplored during the Apollo, Luna and Chang'E missions. Thus, they provide a more extensive sampling of the lunar surface, covering both its visible side and its far side, and allow us to deepen our knowledge of the chemical composition of the Moon and its evolution through geological time.
Lunar meteorites are composed mainly of four minerals: felsdpath, pyroxene, olivine and less than 5% ilmenite (iron and titanium oxide). There are also other minerals present in lower abundances, some of which are very rare and were first identified on the Moon (i.e., tranquillityite). As such, lunar meteorites are very often identifiable thanks to the appearance of white clasts in a black matrix. These meteorites have been classified simply into three categories, depending on their iron (FeO) and aluminum (Al2O3) content:
- the basaltic meteorites (17–23% FeO, 8–12% Al2O3) probably coming from mare and which are thus called mare basalts. They were formed by volcanism. Thus, they are richer in pyroxene, olivine and ilmenite. These meteorites are then subdivided according to their chemical composition, for example their titanium concentration.
- the feldspathic meteorites (low FeO < 7%, Al2O3 > 25%) rich in feldspars, coming from the lunar crust, i.e. lunar highlands. These meteorites are only breccias.
- the intermediate ones, composed of basaltic rocks and feldspathic rocks, probably coming from a limit zone between maria and highlands, or which can contain high concentrations of incompatible elements, in particular potassium, (K), rare elements (REE ) and phosphorus (K), coming from area commonly referred to as KREEP.
The first lunar meteorite identified is Allan Hills 81005 (ALHA 81005), collected during the collection program in Antarctica ANSMET in January 1982, thanks to the comparison with the samples returned during the American Apollo missions (382 kg) and Soviet Luna (301g). As of today (April 2022), 531 lunar meteorites are listed in the Meteoritical Bulletin.
Apollo 12040 (c) Stephant/OU
NWA 10898 (c) Stephant/OU
Lunar meteorite (c) Stephant/OU
Martian Meteorites
Also called by the acronym SNC, in reference to the groups of shergottites, nakhlites and chassignites. The genetic link between these rocks and Mars was established partly thanks to the similarity of the composition of the Martian atmosphere measured by the Viking probe (1976) and the gases trapped in certain shergottites. Martian meteorites are particularly "young", with no crystallisation ages older than 1.3 Ga.
NWA 7034 (c)NASA
NWA 7034 aka Black Beauty (c) Stephant/ASU
Howardites - Eucrites - Diogenites (HED)
These meteorites, commonly referred to as HEDs, likely originate from the asteroid 4-Vesta. This genetic link is partly demonstrated by the similarity of the spectral signatures of HEDs and Vesta, in particular their reflectance. Eucrites are basaltic rocks, which presumably originate from the crust of the parent asteroid. They are mainly composed of pyroxenes and felsdpaths. The diogenites are cumulates formed of larger crystals than in eucrites, illustrating their slower crystallisation. The presence of olivine suggests that these rocks possibly sample Vesta's mantle, since they formed at a greater depth than the eucrites. Finally, howardites are breccia rocks, ie rocks formed during impact and fragmentation which accumulate various debris, which can be a combination of eucrite debris and/or diogenite ones.
Ibitira (c) Stephant/ASU
Pasamonte (c) Stephant/ASU
NWA 6927 (c) Jon Taylor
Tirhert (c) Stephant/ASU
Aubrites - Ureilites - Angrites
Aubrites consist mainly of pyroxene devoid of iron, enstatite. They are therefore highly reduced meteorites. As such, they are close to enstatite chondrites, even if these two groups come from distinct parent bodies. 81 aubrites are currently officially listed.
Ureilites are ultramafic rocks containing olivine and pyroxene, as well as an interstitial matrix that can represent up to 10 % of the volume of the rock. Carbon can be found in this matrix, in greater proportions than any other achondrite as well as the majority of chondrites.
Angrites are basaltic meteorites, formed either by very slow cooling which gave plutonic rocks or by quenching giving volcanic angrites. They are mainly formed of pyroxene, olivine and plagioclase. These meteorites are also among the oldest rock in our solar system, with formation ages estimated between 4 and 11 Ma after the formation of the CAIs, age zero of our solar system.
Aubrite (c)Jon Taylor
Ureilite (c)James St John
Angrite (c)James St John
Primitive Achondrites
These meteorites are the witnesses of the differentiation process of asteroids, and are dated to a few million years after the formation of the CAIs. They have a composition close to chondritic composition, while having an igneous rock texture, having undergone strong thermal metamorphism, which however only allowed a partial differentiation from the parent body. Some of the meteorites in these groups contain relict chondrules, proving that the parent body was chondritic before the differentiation began.
Acapulcoites-Lodranites
IThe acapulcoites and lodranites come from the same parent body, lodranites having undergone greater heating (100-1250°C) than the acapulcoites (950-1050°C). Thus, lodranites have a coarser texture than acapulcoites. The acapulcoites and lodranites take their name from the only two falls of these groups, ie, Acapulco and Lodran.
Winonaites
Winonaites are very close in composition to acapulcoites and are as such often confused for one another. However, winonaites have an oxygen isotopic signature which attests that they come from a parent body distinct from acapulcoites-lodranites. Silicate inclusions present in iron-type IAB meteorites have been genetically associated with winonaites, suggesting that they originated from the same parent body. Winonaites take their name from Winona, found in Arizona.
Tissemouminites
Tissemouminites are primitive achondrites that were initially classified as acapulcoites or winoanites, highlighting the very ambiguous composition of these samples. Indeed, their intrinsic characteristics are either diagnostic of winonaites or of acapulcoites. A new combination of chemical criteria allow to distinguish this new group of primitive achondrites from both acapulcoites and winonaites. For now, eight meteorites are forming this new group, and each of these meteorites present a very pristine state, with for instance the presence of relict chondrules, remnants of the chondritic precursor.
Brachinites
Brachinites are primitive achondrites which are distinguished by the olivine-rich composition, accounting for 80 to 95% of the volume of the samples. The silicate minerals of these brachinites are also richer in iron compared to acapulcoites, lodranites and winonaites. Only 56 brachinites are officially classified (as of April 2022).
Carte minéralogique de trois achondrites primitives: Acapulco (acapulcoite); Lodran (lodranite); NWA 090 (tissemouminite).
Chondrites
Stony irons
Irons
