What is an aubrite?
An aubrite is a pale achondrite made almost entirely of iron-free enstatite pyroxene, formed in extremely oxygen-poor conditions.
Aubrites, also called enstatite achondrites, are named after Aubres, a meteorite that fell in France in 1836. They are pale, often almost white, because they are made mostly of enstatite, a magnesium pyroxene with virtually no iron. Fragments of enstatite can be centimetres across.
Aubrites formed in extremely reducing conditions, with very little available oxygen. In such conditions, elements that normally bind to oxygen instead form unusual sulfides. Aubrites contain minerals such as oldhamite, a calcium sulfide, and alabandite, a manganese sulfide, which are almost never found on Earth. They also contain small amounts of metal with dissolved silicon.
Their oxygen isotopes match those of enstatite chondrites and of Earth and the Moon, so they seem to come from a body that formed in a similar part of the Solar System. They are thought to be pieces of the melted mantle and crust of an enstatite-rich parent asteroid. Most aubrites are breccias, smashed and reassembled by impacts.
The Norton County meteorite, which fell in Kansas and Nebraska in 1948 and weighed about a tonne in total, is the largest aubrite fall. Because their spectra are bright and nearly featureless, aubrites have been linked to E-type asteroids, such as 44 Nysa and 2867 Šteins, which was visited by the Rosetta spacecraft in 2008. Some researchers have also wondered whether certain aubrite-like meteorites could be samples of Mercury, though this is unproven.