What are the high field strength and large ion lithophile (HFS or HFSE & LIL or LILE) elements?

Question

There are two groups of elements that are frequently mentioned when discussing incompatible trace elements. They are the high field strength elements (HFS or HFSE) and the large ion lithophile elements (LIL or LILE).What are these groups? How are they used in geochemistry and petrology?

Answer 1

This question relates to the ability of elements to enter a solid or liquid phase during a partial melting of minerals. If (for example) you crystallize K-feldspar from a melt, it will incorporate all of the elements in its chemical structure KAlSi₃O₈ as well as other elements such as Rb which can substitute into the lattice structure of the mineral. The ability of a cation to substitute for another cation in a lattice structure is determined by its size (ionic radii) and charge. Because both K and Rb are in the same group of the periodic table, Rb can readily substitute for K in K-feldspar. However, an element such as Sr will not substitute as readily because of its charge. This is the concept of compatibility.

HFSEs and LILEs are both groups of elements which do not readily enter, or tend to be incompatible with, the solid phase of a mineral. HFSEs are characterized by having large ionic radii and LILEs are characterized by their high charge. Because they do not compose a large percent, by volume, of the mineral they may be found in and they are unlikely to incorporate at other stages of a rocks "life," they can be precisely measured. They are often used to date rocks via radiogenic isotope geochemistry or to understand the overall composition of a magma from which a crystal cooled.

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