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high-field-strength elements (HFS) Elements of high valency (greater than 2), e.g. Sn, W, and U, which are not readily incorporated into the lattices of common rock-forming silicate minerals. During crystallization of igneous rocks they are generally incorporated into accessory phases (see ACCESSORY MINERAL), e.g. zircon and monazite, or else continually concentrated into residual pegmatitic or hydrothermal fluids, from which they may be precipitated in economic concentrations.