Mineral carbonation is the sequestration of carbon dioxide by forming stable metal carbonate salts such as calcium and magnesium carbonates. Sequestration can be performed either below ground (in situ) or above ground (ex situ) using excavated minerals or metal salts. Natural minerals containing calcium or magnesium oxides or silicates such as wollastonite, olivine, and serpentine will absorb carbon dioxide to form carbonates. Ammonia or other bases will form carbonates.