Most rare-earth elements exist in the trivalent state. Europium, however, can exist as the Eu²⁺ ion and it can proxy for calcium in plagioclase feldspar during igneous fractionation. Crystallization of calcic plagioclase thus depletes residual magmas of europium, relative to the other rare-earth elements. The europium anomaly can therefore be used as an indication of the amount of fractionation a magma has undergone. Efforts to explain the marked europium anomaly discovered in lunar basalts led to much debate about the geochemistry of the Moon, with suggestions that the europium might have been lost by volatization, retained selectively in the lunar interior, or, more probably, that melting and differentiation caused it to be depleted in the basalts but retained in the highlands.