CI chondrite
CI chondrites are rare, very crumbly carbonaceous chondrites whose chemistry closely matches that of the Sun.
The CI chondrites are named after Ivuna, which fell in Tanzania in 1938. Although they are called chondrites, they contain hardly any chondrules: water flowing through their parent body altered them almost completely into clay-like minerals called phyllosilicates, along with magnetite, carbonates and sulfates. They are all petrologic type 1, written CI1.
Their great importance is chemical. Apart from the most volatile elements, such as hydrogen, carbon, nitrogen, oxygen and the noble gases, and a few others like lithium, CI chondrites have almost exactly the same proportions of elements as the Sun's visible surface. That makes them a reference standard for the composition of the Solar System. Only a handful of CI falls are known. The best studied is Orgueil, which fell in southern France on 14 May 1864. Samples returned from the asteroid Ryugu in 2020 proved to be very similar to CI chondrites.
CI chondrites are black, soft and porous, and some fall apart in water. They also react with Earth's air and moisture, so curators keep them in dry, sealed conditions.
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