What is a brachinite?
A brachinite is a rare olivine-rich achondrite, thought to be the residue left after an early asteroid partly melted and lost its basaltic melt.
Brachinites are a small group of achondrites named after the Brachina meteorite, found in South Australia in 1974.
What they are made of. Brachinites are made mostly of olivine, often 75 to 95 per cent, with smaller amounts of pyroxene, plagioclase, chromite, sulfides and sometimes metal. Their olivine is fairly iron-rich compared with many other achondrites. Their textures are coarse-grained and equigranular, with crystals meeting at roughly 120° angles, showing they were heated for a long time.
How they formed. Most scientists think brachinites are residues of partial melting. Their parent body was heated enough for some rock to melt, and the melt, rich in feldspar and pyroxene components, drained away, leaving an olivine-rich rock behind. Some brachinites may instead be cumulates, formed by olivine crystals accumulating from a melt.
Primitive achondrites. Brachinites are often grouped with primitive achondrites, such as acapulcoites and lodranites, because they retain some chondrite-like features despite melting.
Oxygen isotopes. Their oxygen isotopes are similar to those of HED meteorites from Vesta, but distinct enough to suggest a different parent body.
Ages. Some brachinites formed within a few million years of the Solar System's start, showing early melting.
Brachinite-like meteorites. Some ungrouped meteorites resemble brachinites but differ in chemistry or isotopes, and are called brachinite-like achondrites. They suggest that several parent bodies went through similar processes.
Brachinites help scientists understand how asteroids began to differentiate.