The name traditionally applied to three hydrated magnesium silicate minerals, antigorite, chrysotile, and lizardite. All have similar chemical compositions (ideally ) but with three different but closely related layered crystal structures. Serpentine also has been used as a group name for minerals with the same layered structures but with a variety of compositions. The general formula is , where M may be magnesium (Mg), ferrous iron (Fe²⁺), ferric iron (Fe³⁺), aluminum (Al), nickel (Ni), manganese (Mn), cobalt (Co), chromium (Cr), zinc (Zn), or lithium (Li); and T may be silicon (Si), Al, Fe³⁺, or boron (B).