CM chondrite
CM chondrites are water-altered carbonaceous chondrites that contain small chondrules and rich organic chemistry.
The CM chondrites, named after Mighei, which fell in Ukraine in 1889, are the most commonly seen carbonaceous chondrite falls. Water once flowed through their parent asteroid and turned much of the original material into clay minerals, but not all of it: small chondrules and inclusions survive in a dark, fine-grained matrix. Most are petrologic type 2, written CM2.
CM chondrites are famous for their organic matter. Murchison, which fell in Australia in 1969, has yielded dozens of amino acids and many other carbon compounds formed without life. More recently, Winchcombe fell in Gloucestershire on 28 February 2021; quickly collected and carefully curated, it gave scientists an almost uncontaminated CM sample with water whose chemistry closely resembles Earth's oceans. Aguas Zarcas, which fell in Costa Rica in 2019, is another well-studied fall.
A CM stone is dark grey to black inside, with tiny pale chondrules and inclusions, and many are breccias of differently altered fragments. Fresh CM falls are delicate and should be handled with clean gloves or foil and kept dry, ideally after contacting a museum straight away.
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