Howardites
Group · Stony meteorites › Chondrites › Carbonaceous chondrites

CI chondrites

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

CI chondrite
Ivuna, CI1, Tanzania
Classification
Stony meteorites › Chondrites › Carbonaceous chondrites › CI chondrites
Share of witnessed falls
about 0.5%
In our catalogue
9 meteorites, 5 seen to fall
Heaviest
Orgueil, 14 kg
Earliest recorded fall
Alais, 1806
Most recent fall
Revelstoke, 1965
Type specimen
Ivuna, fell in Tanzania in 1938

CI chondrites, named after Ivuna in Tanzania, match the composition of the Sun more closely than any other rock, apart from the most volatile elements like hydrogen and helium. Scientists use them as the standard for the Solar System's chemistry.

Paradoxically, they have no chondrules: water on their parent asteroid altered them completely into clays, magnetite and carbonates (petrologic type 1). They contain up to about 20% water and are very fragile, so they weather away quickly on Earth. Only a handful are known; Orgueil, which fell in France in 1864, is the largest.

Samples from asteroids Ryugu and Bennu are very similar to CI chondrites.

Mineralogy and chemistry

A CI chondrite is a soft, black, fine-grained rock made mostly of phyllosilicates, the clay minerals serpentine and saponite, with grains of magnetite, iron sulphides, carbonates such as dolomite and breunnerite, and veins of sulphates. Carbon makes up about 3.5% by weight. Bulk densities are low, around 1.6 to 2.1 grams per cubic centimetre, because the rock is porous and water-rich. Rare surviving grains of olivine and pyroxene show it once held the usual chondrite ingredients.

In bulk chemistry CI chondrites match the Sun's atmosphere for almost every element. The exceptions are the most volatile ones, such as hydrogen, helium, carbon, nitrogen and the noble gases, which the Sun holds in far greater proportions. This is why tables of 'cosmic abundances' are based on them. Their oxygen isotopes lie slightly above the terrestrial line, unlike most carbonaceous chondrites.

Parent body and formation

CI chondrites accreted ice as well as rock. When aluminium-26 warmed the parent body, the ice melted and the water altered almost everything into clays, at temperatures below about 150 °C. Carbonates from the alteration date to within a few million years of the Solar System's start. The parent body must have formed far from the Sun, probably beyond Jupiter, where ice was stable.

The samples from asteroids Ryugu and Bennu are closely similar to CI chondrites, so dark, water-rich asteroids like these are their probable source. Telescope spectra and the samples suggest such bodies are common, even though CI meteorites are rare on Earth, perhaps because they break up easily in the atmosphere.

Abundance and famous examples

Only five CI falls are known: Alais (France, 1806), Orgueil (France, 1864), Tonk (India, 1911), Ivuna (Tanzania, 1938) and Revelstoke (Canada, 1965). A few more have been found in Antarctica. Orgueil, at about 14 kg, supplies most CI material for research. In 1964 a piece of it was found to have been doctored a century earlier with plant seeds and coal dust glued inside, a hoax to suggest life on the meteorite. Alais was the first meteorite in which organic matter was noticed, by the chemist Jöns Jacob Berzelius in 1834.

How to recognise one

CI chondrites are black, soft enough to leave a mark on paper, and so light that they seem barely heavier than charcoal. They show no chondrules to the eye, are only weakly magnetic, and crumble quickly in damp air as their salts take up water. Any suspected CI fall should be kept dry and sealed and reported at once.

Famous examples

Related groups

See all 9 CI chondrites

Photos shared under Creative Commons licences (Wikimedia Commons).