Ureilites
Carbon-rich rocks with diamonds, from an asteroid shattered by a giant impact. One was tracked in space before it hit Earth.
- Classification
- Stony meteorites › Achondrites › Primitive achondrites › Ureilites
- Share of witnessed falls
- about 0.6%
- In our catalogue
- 327 meteorites, 6 seen to fall
- Heaviest
- Northwest Africa 5201, 11.73 kg
- Earliest recorded fall
- Dyalpur, 1872
- Most recent fall
- Almahata Sitta, 2008
- Type specimen
- Novo-Urei, fell in Russia in 1886
Ureilites, named after Novo-Urei in Russia (fell 1886), are made of olivine and pyroxene with veins of carbon, graphite and tiny diamonds. They are the remains of the mantle of an asteroid that melted extensively, then was smashed apart by a giant collision.
The diamonds may have formed by shock in that collision, though a 2018 study of Almahata Sitta suggested some formed under high pressure inside a large protoplanet. Almahata Sitta is famous because its asteroid, 2008 TC3, was spotted in space about 20 hours before it exploded over Sudan on 7 October 2008: the first time pieces of a tracked asteroid were recovered.
Mineralogy and chemistry
Ureilites are coarse-grained rocks of olivine and pigeonite, a calcium-poor pyroxene, with crystals up to a few millimetres across. Between the grains run dark veins of carbon, mostly graphite with tiny diamonds and lonsdaleite, plus metal and sulphide. Carbon averages about 3% by weight and reaches about 6% in some stones. The rims of olivine crystals next to the carbon are reduced: iron has been removed as metal, leaving bands of almost iron-free olivine dotted with tiny metal grains, as carbon reacted with the silicate at high temperature and low pressure.
Olivine cores range widely in iron content, from about 3 to 25 mol% fayalite, and this correlates with their oxygen isotopes, which spread along a line of slope about 1 rather than clustering on a single value.
Parent body and formation
Ureilites are mantle rocks: residues left after the parent body melted extensively and lost its basaltic melt and much of its metal. Their variable oxygen isotopes show it formed from a heterogeneous mix of material that was never homogenised, unusual for a body that melted so much. While still hot, perhaps about 5 million years after the first solids, it was shattered by a giant collision. Pressure dropped suddenly, carbon reacted with olivine, and the rocks cooled by several degrees per hour, freezing in the reduced rims. Fragments reassembled into smaller daughter bodies, whose surfaces produced the polymict ureilites, breccias full of mixed clasts.
Diamonds were reported in Novo-Urei as early as 1888. Most are thought to have formed by shock, but in 2018 researchers argued that inclusions in Almahata Sitta diamonds required the high, steady pressure inside a body between the size of Mercury and Mars, suggesting the ureilite parent was itself a fragment of a lost protoplanet.
Abundance and famous examples
Ureilites are the largest group of primitive achondrites, with several hundred known and about 0.6% of witnessed falls. Novo-Urei, the type specimen, fell in Mordovia, Russia, on 4 September 1886. Haverö fell in Finland in 1971, and Dyalpur (1872) and Lahrauli (1955) in India. Almahata Sitta fell in the Nubian Desert of Sudan on 7 October 2008 from asteroid 2008 TC3, about 4 m across; students and staff from the University of Khartoum recovered around 600 fragments, mostly ureilites but also several kinds of chondrite.
How to recognise one
A ureilite is dark grey to black, with greenish olivine and pyroxene crystals visible on a cut face and a gritty, carbon-darkened look. It is weakly to moderately magnetic. Its diamonds make it very hard to saw: cutting wheels wear quickly and a polished face may show tiny raised hard spots. It needs laboratory confirmation.
Famous examples
- Novo-Urei Ureilite, Russia, 1886 · 1.9 kg
- Almahata Sitta Ureilite-an, Sudan, 2008 · 3.95 kg
- Haverö Ureilite, Finland, 1971 · 1.54 kg
- Dyalpur Ureilite, India, 1872 · 300 g
- Lahrauli Ureilite, India, 1955 · 900 g
- Goalpara Ureilite, India, 1868 · 2.7 kg
- Kenna Ureilite, United States, 1972 · 10.9 kg
Related groups
- Primitive achondrites Rocks that began to melt but were never fully processed: halfway between chondrites and igneous rocks.
- Lodranites Coarse-grained leftovers after melt drained away.
- Brachinites Olivine-rich rocks from an asteroid whose melt drained away.
- Enstatite chondrites Chondrites formed with almost no oxygen to spare, full of rare minerals unknown on Earth's surface.
Photos shared under Creative Commons licences (Wikimedia Commons).