Brachinites
Synonyms: asteroidal olivine achondrites
General: The meteorites of this group are named for their type sample, Brachina, a primitive achondrite that was recovered from the Australian desert in 1974. Initially, the olivine-rich meteorite was classified as a second chassignite, a unique type of Martian meteorite, primarily composed of olivine. However, further research revealed a distinct trace-element pattern as well as a unique oxygen isotopic composition for Brachina, and other similar finds, leading to the establishment of the brachinite group.
Description: The brachinites in our collections are all more or less fine-grained ultra-mafic rocks of a rather homogenous composition, resembling terrestrial dunites, and peridotites. Their color is light to dark brown, and fresh individuals exhibit a dull-black fusion crust. No relict chondrules have been observed in any group member, so far.
Mineralogy: Brachinites are composed primarily of small, equigranular olivine grains, but scattered among them we also find small amounts of clinopyroxene, orthopyroxene, and minor plagioclase. Metal is rare or completely absent although the brachinites contain up to 20% total iron, mostly in the form of iron-rich olivine.
Formation history: The formation history of the brachinites is still kind of a mystery. On the one hand, these achondrites exhibit a more or less chondritic composition, and REE pattern, but on the other hand their textures are clearly cumulate, and achondritic, suggesting igneous processing, melting, and recrystallization. Future research will hopefully reveal the mysteries of this most intriguing class of primitive (or not so primitive?) achondrites.
Origin: Asteroidal. Recent studies of the olivine compositions of different asteroids suggest that 289 Nenetta might be the brachinite parent body, but there are a few more candidates among the main belt asteroids with similar reflectance spectra that still need observation.