How is calcium carbonate involved in the formation of limestone?

Calcium carbonate (${\text{CaCO}}_3$) is the primary constituent of limestone, typically occurring in the form of the minerals calcite or aragonite. Its involvement in the formation of limestone occurs through two main processes:12

• Biogenic Precipitation: Many limestone formations originate from lime-secreting organisms, such as corals, mollusks, and foraminifera, which extract calcium carbonate from seawater to build their skeletons and shells. When these organisms die, their remains accumulate, and other organisms or chemical processes bind and cement these fragments together into a rigid sedimentary rock structure.13
• Chemical Precipitation: Limestone can also form through the direct precipitation of calcium carbonate from water. For example, when water containing dissolved calcium and bicarbonate ions liberates carbon dioxide, calcium carbonate is deposited as a solid. This process is responsible for the creation of speleothems (such as stalactites and stalagmites) in caves. Additionally, some limestones are formed through the mechanical transport and deposition of preexisting carbonate materials.241