Carbonaceous chondrites

Not all are black or carbon-rich

The historical family name should not be read as one uniform visible property. Group and petrologic state are needed to explain an individual meteorite’s composition.

A family of very different groups

CI chondrites have virtually no recognisable chondrules and consist mainly of strongly aqueously altered fine matrix. CM chondrites are also commonly hydrated and known for complex organic compounds.

CO and CV chondrites generally show clearer chondrules; CV material may contain large pale CAIs. CR, CH and CB groups have their own proportions of metal, chondrules and matrix. The group designation therefore matters more than the broad word “carbonaceous”.

Water, carbon and organic matter

Carbon does not occur as one uniform substance, but in organic matter, carbonates and graphite among other forms. CI and CM chondrites in particular may contain hydrated minerals produced when original rock reacted with liquid water on the parent body.

Organic molecules and amino acids are scientifically important but are not evidence of extraterrestrial life. They show that abiotic organic chemistry produced diverse compounds early in the Solar System.

Why chondrules do not look the same everywhere

It is not correct to say that all carbonaceous chondrites are “full of chondrules”. CO, CV and some CR material may look chondrule-rich; CI have no normal chondrules, while chondrules in matrix-rich or altered CM material may be small or difficult to recognise.

Identification therefore uses the whole texture — chondrule size, matrix abundance, CAIs, metal, mineralogy and chemical-isotopic data — rather than one visible feature.