Brachinites are very rare, ultramafic, olivine-rich, achondrite meteorites. They represent pieces of asteroidal dunite. Very few recovered meteorites have been classified as brachinites. The rock shown below is a sample from the NWA 3151 Meteorite, the largest-ever identified brachinite. It was found in northwestern Africa in 2005.
This rock has an ~equigranular crystalline texture and is overwhelmingly dominated by olivine crystals (about 95% of the rock volume). Published chemical analysis has shown that the olivine is forsteritic (as is all brachinite olivine). Minor reported components include clinopyroxene, altered Fe-Ni metal, troilite (FeS), chromite, Na-plagioclase feldspar, and orthopyroxene.
Brachinites have been inferred to be residual mantle rocks or possibly recrystallized igneous cumulate rocks. They appear to have come from a differentiated planetary body in the asteroid belt that has since broken apart. Research has indicated that the Nenetta Asteroid may be the parent body of brachinites. This brachinite shown below does not yet have a published age, but other brachinites date to 4.56 billion years.
Brachinite - front & back of partial slice (1.3 cm across) of the NWA 3151 Meteorite.
Brachinite, tilted in the light to better show crystalline texture & olivine crystal shapes.