Achondrites

Igneous stones from bodies that melted: asteroids, the Moon and Mars.

Classification

Stony meteorites › Achondrites

Share of witnessed falls

about 7.6%

In our catalogue

2,129 meteorites, 86 seen to fall

Heaviest

Al Haggounia 001, 3 t

Earliest recorded fall

Mässing, 1803

Most recent fall

Tissint, 2011

An achondrite is a stony meteorite without chondrules. It formed from rock that melted and recrystallised on its parent body, so it looks and behaves like Earth's volcanic or deep-seated igneous rocks, such as basalt.

Achondrites make up about 8% of meteorites seen to fall, and around two-thirds of them are HED meteorites, probably from the crust and upper mantle of the asteroid 4 Vesta. Others come from the Moon, from Mars, and from several asteroids that have not been identified.

Each parent body leaves a chemical fingerprint. Scientists use oxygen isotopes (the ratios of oxygen-17 and oxygen-18) and the iron-to-manganese ratio of the minerals to sort achondrites into groups that share a home.

How melting made them

The first planetesimals formed within a few million years of the Solar System's birth, while radioactive aluminium-26 was still abundant. Its decay heated many of them from inside. At about 950 °C, metal and iron sulphide began to melt together; at somewhat higher temperatures, silicate rock started to melt as well. Small degrees of melting produced the primitive achondrites. Larger bodies melted extensively: dense metal sank to form cores, olivine and pyroxene collected in mantles, and basaltic melt rose to make crusts. Achondrites sample these layers.

Some achondrites are astonishingly old. Angrites and some ungrouped basalts crystallised within about 2 to 5 million years of the first solids, so melting and volcanism began on small bodies even before the planets had finished growing.

Telling parent bodies apart

Achondrites have lost the chondrules and textures that define chondrite groups, so scientists rely on chemistry that melting does not change. Oxygen isotopes are the most powerful tool: each melted body was stirred thoroughly and ended up with a single, characteristic offset from the terrestrial line. Rocks from Vesta, the Moon and Mars, for example, all have different values. The ratio of iron to manganese in pyroxene and olivine is a second fingerprint, because it differs between bodies and is easy to measure. Isotopes of chromium and titanium confirm whether a body belonged to the inner or outer Solar System.

The groups

Primitive achondrites include acapulcoites, lodranites, winonaites, brachinites and the carbon-rich ureilites. Differentiated asteroidal achondrites are the howardites, eucrites and diogenites from Vesta, the angrites and the aubrites. Planetary achondrites come from the Moon and Mars. Many more are ungrouped, each perhaps the only sample of its parent body. Iron meteorites, pallasites and mesosiderites are the metal-rich partners of this story, sampling the cores and deep interiors of melted asteroids.

How to recognise an achondrite

Achondrites look like Earth's igneous rocks and are easily overlooked. Clues include a fresh fusion crust, which on eucrites and some others is notably glossy; little or no visible metal; and weak or no attraction to a magnet. Many are breccias of pale and dark fragments. Their density, about 2.9 to 3.4 grams per cubic centimetre, is similar to basalt, so only laboratory work can confirm them. Never break or heat a suspected specimen; take it to a museum or university, and keep a note of where it was found.

Groups

Primitive achondrites

Rocks that began to melt but were never fully processed: halfway between chondrites and igneous rocks.

494 in the catalogue

Asteroidal achondrites

Fully melted, differentiated rocks from the crusts and mantles of asteroids, Vesta above all.

1,283 in the catalogue

Ungrouped achondrites

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

66 in the catalogue

Planetary achondrites

Rocks knocked off the Moon and Mars by impacts.

286 in the catalogue

Famous examples

• Millbillillie Eucrite-mmict, Australia, 1960 · 330 kg
• Kapoeta Howardite, South Sudan, 1942 · 11.36 kg
• Johnstown Diogenite, United States, 1924 · 40.3 kg
• Nakhla Martian (nakhlite), Egypt, 1911 · 10 kg
• Tissint Martian (shergottite), Morocco, 2011 · 7 kg

Related groups

• Primitive achondrites Rocks that began to melt but were never fully processed: halfway between chondrites and igneous rocks.
• Asteroidal achondrites Fully melted, differentiated rocks from the crusts and mantles of asteroids, Vesta above all.
• Planetary achondrites Rocks knocked off the Moon and Mars by impacts.
• Ungrouped achondrites Melted rocks that match no known group: each may be the only sample of its own world.
• Iron meteorites Almost pure iron-nickel metal, mostly from the cores of asteroids that melted and were later shattered.

Photos shared under Creative Commons licences (Wikimedia Commons).