Element of large ionic radius and with a valency of 1 or 2 (e.g. Rb²⁺, Pb²⁺, and Ba²⁺), which during igneous fractionation tends to be concentrated in silicic melts, and from which LILs are incorporated mainly into potassium silicates such as the alkali feldspars and micas. See also fractional crystallization; incompatible elements.