Aubrites
Pale, almost iron-free rocks made of enstatite, from an extremely oxygen-poor world.
- Classification
- Stony meteorites › Achondrites › Asteroidal achondrites › Aubrites
- Share of witnessed falls
- about 0.9%
- In our catalogue
- 75 meteorites, 9 seen to fall
- Heaviest
- Al Haggounia 001, 3 t
- Earliest recorded fall
- Aubres, 1836
- Most recent fall
- Mayo Belwa, 1974
- Type specimen
- Aubres, fell in France in 1836
Aubrites, named after Aubres in France (fell 1836), are pale, often white rocks made mostly of nearly iron-free enstatite. Like the enstatite chondrites, they formed with very little oxygen and contain rare sulphide minerals. Their fusion crust can be light brown or even glassy rather than black.
The 1948 Norton County fall in Kansas and Nebraska, about 1 tonne, is the largest aubrite. Aubrites are probably the melted relatives of enstatite chondrites, perhaps from the asteroid 3103 Eger or the E-type asteroids of the Hungaria region.
Mineralogy and chemistry
Aubrites are made of about 75% to 95% enstatite containing almost no iron, with minor forsterite (iron-free olivine), sodium-rich plagioclase, calcium-rich pyroxene, iron-nickel metal and a suite of rare sulphides that form only with almost no oxygen: oldhamite, alabandite, daubréelite and others. Crystals can be several centimetres long. Most aubrites are breccias of crushed enstatite.
Their oxygen isotopes lie on the terrestrial line, matching enstatite chondrites. Their chemistry suggests they formed by extensive melting of enstatite-chondrite-like material, though no aubrite parent has been firmly linked to the EH or EL parent bodies.
Parent body and formation
Aubrites are thought to be crust and mantle rocks of a differentiated body that formed from very reduced material in the inner Solar System. A few contain chondritic clasts, and Shallowater, found in Texas, is an unbrecciated aubrite that seems to have cooled quickly after a collision, suggesting the parent was broken and reassembled. Spectra of the bright E-type asteroids in the Hungaria region, at the inner edge of the asteroid belt, resemble aubrites, and the small near-Earth asteroid 3103 Eger has been suggested as a source. Mercury has also been compared, because its surface is iron-poor and sulphur-rich, but aubrites do not match it in detail.
Aubrites show how material like Earth's building blocks behaves when it melts with very little oxygen: calcium, manganese and chromium go into sulphides instead of silicates. Their cosmic-ray exposure ages are among the longest of stony meteorites, often tens of millions of years.
Abundance, type specimen and famous examples
Aubrites are about 1% of falls. Aubres, the type specimen, fell in the Drôme, France, in 1836. Norton County fell on the border of Kansas and Nebraska on 18 February 1948 after a brilliant fireball; its main mass of over 1 tonne is the largest achondrite seen to fall. Bishopville (South Carolina, 1843), Cumberland Falls (Kentucky, 1919), which contains dark chondritic fragments, Peña Blanca Spring (Texas, 1946), which landed in a spring-fed pool, and Mayo Belwa (Nigeria, 1974) are others.
How to recognise one
Aubrites are pale, white to light grey rocks of large, glassy enstatite crystals, often crumbly. Their fusion crust can be light brown, beige or even nearly transparent glass rather than black, because there is so little iron. They are only weakly magnetic. Moisture attacks their sulphides, so fresh specimens are kept dry.
Famous examples
- Norton County Aubrite, United States, 1948 · 1.1 t
- Aubres Aubrite, France, 1836 · 800 g
- Bishopville Aubrite, United States, 1843 · 6 kg
- Cumberland Falls Aubrite, United States, 1919 · 17 kg
- Peña Blanca Spring Aubrite, United States, 1946 · 70 kg
- Mayo Belwa Aubrite, Nigeria, 1974 · 4.85 kg
- Shallowater Aubrite, United States, 1936 · 4.65 kg
Related groups
- Enstatite chondrites Chondrites formed with almost no oxygen to spare, full of rare minerals unknown on Earth's surface.
- EL chondrites Low-iron enstatite chondrites.
- EH chondrites High-iron enstatite chondrites.
- Asteroidal achondrites Fully melted, differentiated rocks from the crusts and mantles of asteroids, Vesta above all.
Photos shared under Creative Commons licences (Wikimedia Commons).