Elements that, owing to their size, charge, or valency requirements, are difficult to substitute into the crystal structure of a rock-forming mineral (e.g. the boron ion is very small and the tungsten ion may have a +6 charge). This results in their being preferentially introduced into a magma on partial melting and less likely to crystallize out of it. During the crystallization of igneous rocks, incompatible elements (e.g. Sn, Li, Rb, Sr, and rare-earth elements) are often concentrated into pegmatitic or hydrothermal fluids. During the formation of the Earth’s crust the incompatible elements have been transferred through magmatic processes to the crust from the mantle, which has consequently become depleted in these elements.