Howardites
Group · Stony meteorites › Chondrites › Carbonaceous chondrites

CR chondrites

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

CR chondrite
Northwest Africa 801, CR2
Classification
Stony meteorites › Chondrites › Carbonaceous chondrites › CR chondrites
Share of witnessed falls
about 0.2%
In our catalogue
151 meteorites, 3 seen to fall
Heaviest
Tafassasset, 114 kg
Earliest recorded fall
Renazzo, 1824
Most recent fall
Kaidun, 1980
Type specimen
Renazzo, fell in Italy in 1824

CR chondrites, named after Renazzo in Italy, contain large chondrules rimmed with iron-nickel metal, plus water-altered matrix. Many are petrologic type 2.

Some CR chondrites are among the least altered meteorites known and preserve organic matter and presolar grains in near-pristine condition. The large asteroid 2 Pallas has been suggested as a possible parent body. CRs are the namesake of a clan that also includes the CH and CB groups.

Mineralogy and chemistry

CR chondrites contain large chondrules, about 0.7 mm on average, many of them iron-poor and surrounded by rims and beads of iron-nickel metal. Metal makes up about 5% to 8% of the rock and has nickel and cobalt in roughly the ratio found in the Sun. CAIs are scarce and small. The matrix, about 30% to 50% of the rock, has been altered by water into clays, magnetite and carbonates to varying degrees.

Their oxygen isotopes plot below the terrestrial line, on a line of their own that also includes the CH and CB chondrites. CRs are especially rich in heavy isotopes of hydrogen and nitrogen, carried by their organic matter, which may have formed in very cold regions of the early Solar System or in the cloud before it.

Organic matter and pristine samples

Some CRs, especially Antarctic finds such as Graves Nunataks 95229 and Queen Alexandra Range 99177, were barely altered by water or heat. They preserve presolar grains, amorphous silicates and organic molecules in near-original form. Graves Nunataks 95229 contains some of the highest concentrations of amino acids measured in any meteorite.

Parent body and formation

CR chondrules formed late, about 3 to 4 million years after the first solids, later than in most other chondrites. The parent body accreted soon after, with ice, and was warmed only gently. The large asteroid 2 Pallas, with a dark surface and signs of water-bearing minerals, has been suggested as one possible source, though this is uncertain. CRs share a clan with the metal-rich CH and CB chondrites.

Abundance and famous examples

CRs are rare among falls, about 0.2%. Renazzo, which fell near Ferrara, Italy, on 15 January 1824, is the type specimen. Al Rais (Saudi Arabia, 1957) is another fall. Many more are known from Antarctica and North Africa, including the large desert find Northwest Africa 801. Kaidun, which fell in Yemen in 1980, is a remarkable breccia containing fragments of many different meteorite types and is classed with the CRs.

Clues to the solar nebula

Because so many CRs escaped heating, their chondrules keep the chemical zoning and metal compositions they had when they formed. Metal grains with near-solar nickel-to-cobalt ratios suggest they condensed or reacted directly with nebular gas. The late age of CR chondrules also shows that the disc around the young Sun was still making chondrules several million years after the first solids, while other bodies were already melting.

How to recognise one

A CR shows dark matrix with large chondrules, each often outlined by bright metal, so a cut face sparkles in rings. The stone is magnetic because of this metal. Weathering quickly turns the metal rims brown in desert finds.

Famous examples

Related groups

See all 151 CR chondrites

Photos shared under Creative Commons licences (Wikimedia Commons).