Calcite
Calcite is calcium carbonate, found in meteorites both from water on asteroids and from weathering on Earth.
Calcite is calcium carbonate, CaCO3, the mineral of limestone and chalk. It fizzes in weak acid, which is a classic test for it.
In CM and CI chondrites, calcite and other carbonates formed when carbon dioxide-bearing water flowed through the parent asteroid. Their oxygen and carbon isotopes act as thermometers, showing that the fluids were typically between roughly 0 and 80 °C. Some of these carbonates have been dated with the manganese–chromium method to within about 4 to 5 million years of the Solar System's start.
Calcite also forms on Earth. Meteorites found in hot deserts often have white calcite, or caliche, cementing their cracks and coating their undersides, deposited from evaporating groundwater. This terrestrial calcite can confuse chemical studies, so scientists must separate home-grown minerals from those formed in space. Never use acid to test a meteorite: it damages the specimen and is unsafe.
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