Incompatible element

Incompatible element is a term used in petrology and geochemistry.

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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). 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.

Two groups of 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, rare earth elements, thorium, and uranium (called LILE, or large-ion lithophile elements), and the other group includes elements of large ionic valences such as zirconium, niobium, hafnium, and tantalum (called HFSE, or high field strength elements).