A metallic element of the alkaline-earth group; never found in nature uncombined, occurs abundantly as limestone (CaCO₃), gypsum (CaSO₄. 2H₂O), and fluorite (CaF₂). 473 Symbol, Ca. Used as a reducing agent, deoxidizer, desulfurizer, or decarburizer for alloys; as quicklime (CaO), it is the great cheap base of the chemical industry with countless uses.