CO chondrite
CO chondrites are carbonaceous chondrites packed with very small chondrules in a modest amount of matrix.
Tiny chondrules are the hallmark of the CO chondrites, a group named after Ornans, which fell in eastern France in 1868. These chondrules are typically only around 0.15 millimetres across, closely packed with small CAIs and amoeboid olivine aggregates. Matrix makes up a smaller share than in CM chondrites, and they contain little water. Oxygen isotopes and chondrule features suggest the CO and CM groups are related, and some scientists group them together as a CM–CO clan.
COs are almost all petrologic type 3, and they form a beautiful sequence of subtypes from about 3.0 to 3.8. This makes them ideal for studying how gentle heating on a parent body gradually changes minerals, for example how iron diffuses into olivine and how presolar grains are destroyed. The least heated members, mostly found in Antarctica, preserve some of the most pristine material from the early Solar System. Other well-known falls include Kainsaz (Russia, 1937) and Lancé (France, 1872).
In hand specimen, CO chondrites are dark grey to black and look finely speckled, as the chondrules are too small to pick out easily without a hand lens or microscope.
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