Brachinites
Olivine-rich rocks from an asteroid whose melt drained away.
- Classification
- Stony meteorites › Achondrites › Primitive achondrites › Brachinites
- Share of witnessed falls
- none yet seen to fall
- In our catalogue
- 33 meteorites
- Heaviest
- Northwest Africa 5969, 4 kg
- Type specimen
- Brachina, found in South Australia in 1974
Named after Brachina in South Australia, brachinites are made mostly of iron-rich olivine with some pyroxene. They are thought to be residues left after partial melting removed basaltic melt from their parent asteroid, though some may be cumulates that crystallised from a melt. None has been seen to fall.
Mineralogy and chemistry
Brachinites are made mostly of olivine, typically 80% to over 90% of the rock, and that olivine is rich in iron, about 30–35 mol% fayalite. The rest is calcium-rich pyroxene, minor orthopyroxene and plagioclase, chromite, iron sulphides and phosphates, with very little metal. Grains are medium to coarse, around 0.1 to 1 mm, with an even, recrystallised texture. Their oxygen isotopes lie slightly below the terrestrial line, with a Δ17O of roughly −0.2 to −0.3 per mil.
In composition they resemble olivine-rich residues from an oxidised chondrite, perhaps something like an R chondrite, though their oxygen isotopes rule out a direct link to any known chondrite group.
Parent body and formation
Most researchers interpret brachinites as residues left after partial melting removed basaltic liquid from their parent body, while some may be cumulates formed as olivine crystals settled from a melt. Chromium isotope dating suggests their minerals formed within a few million years of the first solids. A related discovery is the pair of Antarctic meteorites Graves Nunataks 06128 and 06129: andesite-like rocks rich in sodium plagioclase with brachinite-like oxygen isotopes. They may be the kind of melt that was extracted from a brachinite parent body, and they showed that small asteroids could produce rocks with silica-rich compositions similar to some of Earth's continental crust.
Brachinites are important because they are olivine-dominated rocks from a melted asteroid, the very kind of material expected from asteroid mantles, yet such rocks are much rarer than models predict. Their oxidised minerals also show that some melted asteroids formed from material far richer in oxygen than Vesta.
Abundance, type specimen and famous examples
Brachina, the type specimen, was found in South Australia in 1974; only about 200 g is known. No brachinite has been seen to fall. A few dozen are known, mainly finds from Antarctica, Australia and north-west Africa, together with several 'brachinite-like' ungrouped achondrites, such as Divnoe from Russia, that may sample related bodies.
How to recognise one
A brachinite is a granular, greenish-brown to brown rock dominated by olivine, with little metal, so it is only weakly magnetic. It can look like a terrestrial dunite or a weathered stone. Fusion crust, if present, is the best field clue; confirmation needs analysis of olivine composition and oxygen isotopes.
Famous examples
- Brachina Brachinite, Australia, 1974 · 203 g
- Eagles Nest Brachinite, 1960 · 154 g
- Hughes 026 Brachinite, Australia, 1995 · 360 g
- Allan Hills 84025 Brachinite, Antarctica, 1984 · 4.6 g
- Northwest Africa 3151 Brachinite, 2005 · 1.5 kg
Related groups
- Ureilites Carbon-rich rocks with diamonds, from an asteroid shattered by a giant impact. One was tracked in space before it hit Earth.
- Chassignites Olivine-rich Martian rocks; the first known fell in France in 1815.
- R chondrites (rumurutiites) Highly oxidised chondrites with almost no metal.
- Primitive achondrites Rocks that began to melt but were never fully processed: halfway between chondrites and igneous rocks.
Photos shared under Creative Commons licences (Wikimedia Commons).