Chondrites

Primitive stones that never melted, made of the first solids of the Solar System. The most common meteorites of all.

Classification

Stony meteorites › Chondrites

Share of witnessed falls

about 85.9%

In our catalogue

42,565 meteorites, 919 seen to fall

Heaviest

Jilin, 4 t

Earliest recorded fall

Nogata, 860

Most recent fall

Chelyabinsk, 2013

Chondrites are stony meteorites from asteroids that never grew hot enough to melt. They formed when dust and small grains in the early Solar System clumped together more than 4.5 billion years ago, and lead isotopes date them to about 4,567 million years. Apart from the most volatile elements, their chemistry closely matches that of the Sun.

Their name comes from chondrules: millimetre-sized round grains, mostly of olivine and pyroxene, that were once molten droplets floating in space. Chondrules usually make up 20% to 80% of a chondrite. Chondrites also contain calcium-aluminium-rich inclusions (CAIs), the oldest solids known, plus grains of metal and sulphide, all held in fine-grained dust (the matrix). Some of that dust contains presolar grains, older than the Sun itself.

Chondrites are the most common meteorites: about 86% of falls. More than 60,000 are known, and the catalogue figure below is live. The largest single stone ever recovered, 1,770 kg, fell in the Jilin shower of 1976. Falls range from single stones to enormous showers, such as Holbrook, Arizona, in 1912, when an estimated 14,000 stones fell.

After they formed, many chondrites were changed on their parent asteroids: heated (thermal metamorphism, from decaying aluminium-26), altered by water (aqueous alteration, making clays), and shocked by collisions. These changes are recorded in the petrologic type and shock stage.

Chondrules: frozen droplets

Chondrules are the signature of these rocks. Most are 0.1 to 2 mm across and made of olivine, pyroxene and glass. Their textures, such as barred olivine (parallel plates like a grille) or radiating pyroxene (fans of fine needles), show they cooled from melt within minutes to hours. Lead and aluminium-magnesium dating puts most chondrule formation between about 1 and 4 million years after the first solids.

How the droplets were melted is still debated. Ideas include shock waves passing through the dusty disc around the young Sun, lightning-like discharges, and splashes from collisions between molten or partly molten planetesimals. Different groups may have formed their chondrules in different ways.

The other ingredients

Calcium-aluminium-rich inclusions (CAIs) are white or pinkish grains of minerals such as melilite, spinel and perovskite that condense first from hot gas. At about 4,567 million years old they are the oldest dated solids from the Solar System, and their age is used as its starting point. Amoeboid olivine aggregates formed at slightly lower temperatures. Matrix, the fine dark dust between everything else, can contain tiny presolar grains of diamond, silicon carbide and graphite made in older stars, as well as organic matter.

Groups, clans and isotopes

There are 14 well-established chondrite groups and one grouplet, gathered into three main classes: ordinary (H, L, LL), carbonaceous (CI, CM, CO, CV, CK, CR, CH, CB) and enstatite (EH, EL), plus the separate R group and the K grouplet. Each group is defined by a combination of chondrule size, the proportion of matrix and metal, bulk chemistry and oxygen isotopes. Plotted on a three-isotope diagram, ordinary chondrites lie just above the line made by Earth rocks, enstatite chondrites on it, R chondrites well above it and most carbonaceous chondrites below it.

How to recognise a chondrite

Look for small rounded chondrules on a broken or cut face, often easier to see with a hand lens after the surface is gently wetted. Most chondrites contain enough metal to pull firmly on a magnet hung from a thread, and a cut face shows bright metal flecks. Heated chondrites of types 5 and 6 can have chondrules so blurred they are hard to spot, while carbonaceous chondrites may be dark, soft and only weakly magnetic.

How a chondrite is labelled

• Group letters give the chemical family: H, L or LL (ordinary), CI, CM, CV and so on (carbonaceous), EH or EL (enstatite), R or K.
• Petrologic type, 1 to 7: types 1 and 2 were altered by water (1 most altered); type 3 is the most pristine; types 4 to 6 were increasingly heated, blurring the chondrules; type 7 is close to melting.
• Shock stage, S1 to S6: from unshocked (S1) to very strongly shocked, with melt veins and high-pressure minerals (S6).
• Weathering grade, W0 to W6, for finds: how much of the metal and sulphide has rusted on Earth. Antarctic meteorites use letters instead: A (minor), B (moderate) or C (severe) rust, with 'e' for visible evaporite crusts.
• Example: 'L6 S4 W2' is a heated L chondrite, moderately strongly shocked, with moderate weathering.

Groups

Ordinary chondrites

The everyday meteorite: four in every five falls. Three groups, H, L and LL, from at least three asteroids.

40,278 in the catalogue

Carbonaceous chondrites

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

1,616 in the catalogue

Enstatite chondrites

Chondrites formed with almost no oxygen to spare, full of rare minerals unknown on Earth's surface.

533 in the catalogue

R chondrites (rumurutiites)

Highly oxidised chondrites with almost no metal.

135 in the catalogue

K chondrites (Kakangari grouplet)

A tiny grouplet with a mix of features found in no other chondrite group.

3 in the catalogue

Famous examples

• Allende CV3, Mexico, 1969 · 2 t
• Murchison CM2, Australia, 1969 · 100 kg
• Chelyabinsk LL5, Russia, 2013 · 100 kg
• Jilin H5, China, 1976 · 4 t
• Holbrook L/LL6, United States, 1912 · 220 kg
• Ensisheim LL6, France, 1491 · 127 kg

Related groups

• Ordinary chondrites The everyday meteorite: four in every five falls. Three groups, H, L and LL, from at least three asteroids.
• Carbonaceous chondrites Rare, often dark chondrites rich in carbon, water-bearing minerals and organic compounds, including the building blocks of life.
• Enstatite chondrites Chondrites formed with almost no oxygen to spare, full of rare minerals unknown on Earth's surface.
• R chondrites (rumurutiites) Highly oxidised chondrites with almost no metal.
• K chondrites (Kakangari grouplet) A tiny grouplet with a mix of features found in no other chondrite group.
• Primitive achondrites Rocks that began to melt but were never fully processed: halfway between chondrites and igneous rocks.

Photos shared under Creative Commons licences (Wikimedia Commons).