Brachinite
Brachinites are olivine-rich primitive achondrites from an oxidised parent body.
Olivine dominates the brachinites, a small group named after Brachina, a stone of about 200 grams found in South Australia in 1974. They are dominated by olivine, typically around 80 per cent or more, with lesser amounts of pyroxene, plagioclase, chromite, troilite and sometimes a little metal. The olivine is relatively iron-rich, around Fa30 or more, so brachinites are among the more oxidised achondrites. Textures are medium to coarse-grained and equigranular, rather like a terrestrial dunite or wehrlite.
Brachina was initially compared with the Martian meteorite Chassigny, which is also olivine-rich, but trace-element and oxygen-isotope work in the early 1980s showed it was different, leading to a separate group. How brachinites formed is still debated. Many are interpreted as residues left after partial melting removed basaltic melt; others may be olivine cumulates that settled from magma. Some meteorites that are classified as brachinite-like or ungrouped have similar textures but distinct oxygen isotopes.
Brachinites are rare and nearly all are finds from hot deserts and Antarctica. Recognition depends on olivine-dominant mineralogy, mineral compositions, and oxygen isotopes, which separate them from chassignites and other olivine-rich rocks.
Related terms
Level: Expert Advanced