In petrology and geochemistry, an incompatible element is an element that is unsuitable in size and/or charge to the cation sites of the minerals, and is defined by the partition coefficient between rock-forming minerals and melt being much smaller than 1.
During the fractional crystallization of magma and magma generation by the partial melting of the Earth's mantle and crust, elements that have difficulty in entering cation sites of the minerals are concentrated in the melt phase of magma (liquid phase).
Two groups of incompatible elements that have difficulty entering the solid phase are known by acronyms. One group includes elements having large ionic radius, such as potassium, rubidium, caesium, strontium, barium (called LILE, or large-ion lithophile elements), and the other group includes elements of large ionic valences (or high charges), such as zirconium, niobium, hafnium, rare earth elements (REE), thorium, uranium and tantalum (called HFSE, or high field strength elements).
Another way to classify incompatible elements is by mass: light rare earth elements (LREE) are La - Sm, and heavy rare earth elements (HREE) are Eu - Lu. Rocks or magmas that are rich, or only slightly depleted, in light rare earth elements are referred to as fertile, and those with strong depletions in LREE are referred to as depleted.