Gypsum
Gypsum is a calcium sulfate mineral seen as veins in some CI chondrites, partly formed after they reached Earth.
Gypsum is hydrated calcium sulfate, the mineral used to make plaster. It is soft enough to scratch with a fingernail.
In CI chondrites such as Orgueil, white veins of sulfate minerals including gypsum cross the black rock. For a long time these were taken as proof that water flowed through the parent asteroid. Later studies, including the pristine samples of Ryugu, which contain almost no sulfate veins, suggested that much of the sulfate in CI meteorites grew after they arrived on Earth. Moisture in the air oxidised their sulfides, and the resulting sulfate crystallised as veins and crusts during decades of museum storage.
Gypsum and related salts are also common in hot-desert finds, where they form as evaporating groundwater leaves minerals behind in cracks. Gypsum in a meteorite is therefore a warning to look carefully at whether a feature is extraterrestrial or a product of life on Earth's surface.
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