Calcite is the mineral with the formula CaCO₃ that makes up most limestone.
Calcite is a widespread carbonate mineral and the stable form of calcium carbonate at Earth’s surface. It commonly forms through biological activity, as marine organisms build shells and skeletons from calcium carbonate that later accumulate as sediment. Groundwater can also precipitate calcite in caves, producing stalactites and stalagmites.
Limestone is a sedimentary rock composed mainly of calcite, although it can contain fossils, clay, quartz, and other minerals. Calcite also occurs in marble, which forms when limestone is metamorphosed. A related calcium-carbonate mineral, aragonite, has the same chemical formula but a different crystal structure and is less stable at ordinary surface conditions.
Calcite is distinctive because it reacts readily with dilute hydrochloric acid, fizzing as carbon dioxide is released. This test helps distinguish many calcite-rich rocks from similar-looking materials.