Carbonates
Minerals containing carbon and oxygen, such as calcite, that form in meteorites when water flows through rock.
Carbonates are minerals built around the carbonate group, one carbon atom bonded to three oxygen atoms, combined with metals such as calcium, magnesium, iron and manganese. Familiar examples are calcite and dolomite. In meteorites they occur as small grains and veins, chiefly in carbonaceous chondrites of the CI and CM groups, where they precipitated from water circulating inside the parent asteroid.
Carbonates are valuable clocks and thermometers. Measurements using the manganese–chromium system suggest that carbonates in some CI and CM chondrites formed within a few million years of the Solar System's first solids, showing how early water was active. Their isotopes record the temperature and composition of that water. On Mars, the meteorite ALH 84001 contains orange carbonate globules about 3.9 billion years old, evidence of ancient water near the Martian surface. Carbonates are also abundant in samples brought back from the asteroid Bennu.
Carbonates in carbonaceous chondrites are usually tiny white or pale grains visible only under a microscope. Be careful with desert finds: carbonate crusts called caliche can form on meteorites during weathering on Earth, so not every carbonate in a meteorite came from space.
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