Carbonaceous chondrites

Rare, often dark chondrites rich in carbon, water-bearing minerals and organic compounds, including the building blocks of life.

Classification

Stony meteorites › Chondrites › Carbonaceous chondrites

Share of witnessed falls

about 4.4%

In our catalogue

1,616 meteorites, 44 seen to fall

Heaviest

Allende, 2 t

Earliest recorded fall

Alais, 1806

Most recent fall

Sutter's Mill, 2012

Carbonaceous (C) chondrites are about 4% of falls. Many are dark and crumbly and contain up to a few per cent carbon, water locked in clay minerals, and a rich mix of organic molecules, including amino acids and nucleobases. Some are among the most primitive materials known.

Each group is named after a type meteorite, with the second letter taken from its name: CI (Ivuna), CM (Mighei), CO (Ornans), CV (Vigarano), CK (Karoonda), CR (Renazzo) and CB (Bencubbin). CH stands for 'high metal'. Related groups are linked in clans: CM–CO, CV–CK, and CR–CH–CB.

Samples brought back from the asteroids Ryugu (Hayabusa2, 2020) and Bennu (OSIRIS-REx, 2023) closely resemble the most water-rich carbonaceous chondrites, confirming that dark C-type asteroids are their source.

What defines them

Despite the name, not all carbonaceous chondrites are rich in carbon. What unites them is chemistry and isotopes: they are enriched in refractory elements such as calcium and aluminium relative to magnesium, compared with ordinary and enstatite chondrites, and nearly all plot below Earth's line on the oxygen three-isotope diagram. They also belong to the 'carbonaceous' isotope family seen in chromium, titanium and molybdenum, which suggests they formed in the outer Solar System, beyond Jupiter's early orbit.

Within the class, groups differ greatly. CI and CM chondrites are dominated by water-formed clays; CO and CV chondrites are dry and full of chondrules and inclusions; CK chondrites were heated; and the CR, CH and CB groups contain abundant metal.

Water and organic matter

Many carbonaceous chondrites were soaked by liquid water inside their parent asteroids a few million years after they formed, as ice melted from the heat of aluminium-26. The water reacted with the rock to make clays, carbonates, sulphates and magnetite. Hydrogen isotopes in this water broadly match Earth's oceans, and carbonaceous chondrite-like material is thought to have delivered much of Earth's water and volatile elements.

Their organic matter ranges from a large insoluble, coal-like material to soluble molecules such as amino acids, carboxylic acids, sugars and nucleobases. Most of it formed without life, in cold clouds or within the asteroids themselves.

Parent bodies

Dark, low-reflectance C-type asteroids, and related B-, D- and P-types, are their likely sources. Samples from Ryugu, returned by Hayabusa2 in 2020, and from Bennu, returned by OSIRIS-REx in 2023, are rich in clays and carbonates and closely resemble CI chondrites. Bennu's sample even contains sodium-rich salts that would quickly be lost in a meteorite on Earth.

Famous examples and recognition

Allende (CV3, Mexico, 1969) and Murchison (CM2, Australia, 1969) fell within months of each other, just as laboratories were preparing for the Apollo samples, and became two of the best-studied rocks on Earth. Orgueil (CI1, France, 1864), Tagish Lake (ungrouped C2, Canada, 2000) and Winchcombe (CM2, England, 2021) are also famous.

Most are dark grey to black, lighter than ordinary chondrites for their size, and only weakly magnetic or not at all. Their fusion crust is often dull and black. Many are fragile and crumble easily, and some smell faintly of organic compounds when freshly broken. Fresh pieces should be kept dry and handled with clean gloves to avoid contamination.

Groups

CI chondrites

The chemically most primitive rocks known: their make-up matches the Sun's.

9 in the catalogue

CM chondrites

Water-rich carbonaceous chondrites full of organic molecules, Murchison and Winchcombe among them.

461 in the catalogue

CO chondrites

Small-chondrule carbonaceous chondrites, close relatives of the CMs.

391 in the catalogue

CV chondrites

Large chondrules and big white inclusions, the oldest solids in the Solar System. Allende is the classic.

263 in the catalogue

CK chondrites

Heated, highly oxidised carbonaceous chondrites, cousins of the CVs.

229 in the catalogue

CR chondrites

Metal-bearing carbonaceous chondrites with some of the most pristine organic matter known.

151 in the catalogue

CH chondrites

'High metal' carbonaceous chondrites with tiny chondrules.

44 in the catalogue

CB chondrites (bencubbinites)

More than half metal, with chondrules made in a planet-sized collision.

18 in the catalogue

Famous examples

• Allende CV3, Mexico, 1969 · 2 t
• Murchison CM2, Australia, 1969 · 100 kg
• Orgueil CI1, France, 1864 · 14 kg
• Tagish Lake C2-ung, Canada, 2000 · 10 kg
• Winchcombe Carbonaceous Chondrite, United Kingdom, 2021 · 602 g
• Sutter's Mill C, United States, 2012 · 993 g

Related groups

• CI chondrites The chemically most primitive rocks known: their make-up matches the Sun's.
• CM chondrites Water-rich carbonaceous chondrites full of organic molecules, Murchison and Winchcombe among them.
• CO chondrites Small-chondrule carbonaceous chondrites, close relatives of the CMs.
• CV chondrites Large chondrules and big white inclusions, the oldest solids in the Solar System. Allende is the classic.
• CK chondrites Heated, highly oxidised carbonaceous chondrites, cousins of the CVs.
• CR chondrites Metal-bearing carbonaceous chondrites with some of the most pristine organic matter known.
• CH chondrites 'High metal' carbonaceous chondrites with tiny chondrules.
• CB chondrites (bencubbinites) More than half metal, with chondrules made in a planet-sized collision.

Photos shared under Creative Commons licences (Wikimedia Commons).