Category · Stony meteorites › Achondrites

Ungrouped achondrites

Melted rocks that match no known group: each may be the only sample of its own world.

Classification: Stony meteorites › Achondrites › Ungrouped achondrites
In our catalogue: 66 meteorites, 1 seen to fall
Heaviest: Divnoe, 12.7 kg
Earliest recorded fall: Silistra, 1917

Some achondrites fit none of the named groups. Their oxygen isotopes, mineral chemistry or ages set them apart, and many are the only known pieces of their parent bodies. They are recorded simply as 'achondrite, ungrouped'.

Why so many are ungrouped

A meteorite group needs at least five unrelated members. Many achondrites are the only one of their kind, or have only a few paired fragments, so they are listed as ungrouped. They are among the most scientifically valuable meteorites, because each may be the only sample of a melted world that has otherwise vanished. Oxygen, chromium and titanium isotopes show that ungrouped achondrites come from both the inner and outer Solar System, from bodies that were never sampled by any larger group.

Remarkable examples

Northwest Africa 011, found in 1999, looks like a eucrite but has oxygen isotopes like carbonaceous chondrites, showing that basaltic volcanism also happened on bodies that formed farther from the Sun. Northwest Africa 7325, found in 2012, is a pale, green-tinged rock rich in calcium plagioclase and almost free of iron; it attracted attention as a possible piece of Mercury, though most researchers now think it came from a reduced asteroid. Graves Nunataks 06128 and 06129 from Antarctica are andesite-like rocks, rich in sodium plagioclase, and their oxygen isotopes link them to the brachinites.

Erg Chech 002, found in the Algerian Sahara in 2020, is an andesite about 4,565 million years old, the oldest volcanic rock yet dated. It shows that silica-rich crust formed on some planetesimals almost as soon as they melted. Ibitira, which fell in Brazil in 1957, looks like a eucrite but has distinct oxygen isotopes and is now thought to come from a different parent body.

Ungrouped primitive achondrites

Some ungrouped stones are only partly melted. Tafassasset, a large find from Niger, is a recrystallised rock related to CR chondrites by its isotopes, a rare example of partial melting on a carbonaceous parent body. Divnoe, from Russia, resembles the brachinites but differs in detail. Such rocks show that melting occurred on bodies of many different compositions, not only on the parent bodies of the main achondrite groups.

How they are recognised

Ungrouped achondrites are recognised only in the laboratory, usually when a stone classified as an achondrite fails to match any group in its oxygen isotopes, mineral chemistry or iron-to-manganese ratio. Their appearance varies widely, from pale feldspar-rich rocks to dark basalts.

As more meteorites are classified, ungrouped stones sometimes gain partners and become new grouplets, which is how several of today's groups began. Each new one adds to the count of melted bodies sampled by meteorites, now in the dozens, and shows that melting was common among the first planetesimals of both the inner and outer Solar System.

Famous examples

Northwest Africa 7325 Achondrite-ung, 2012 · 345 g
Northwest Africa 011 Achondrite-ung, Morocco, 1999 · 40 g

Related groups

Brachinites Olivine-rich rocks from an asteroid whose melt drained away.
Angrites The oldest volcanic rocks known, from a vanished early planetesimal.
Eucrites Volcanic basalt from the crust of Vesta.
Achondrites Igneous stones from bodies that melted: asteroids, the Moon and Mars.

See all 66 Ungrouped achondrites

Photos shared under Creative Commons licences (Wikimedia Commons).