Crust-forming (igneous differentiation) processes preferentially concentrate incompatible elements — those with ionic radii and charges that do not fit well into the crystal lattices of common mantle minerals — into the residual melt during partial melting and fractional crystallization, progressively enriching the resulting crust in these elements; the lanthanide (rare earth) group of metals is a classic example of strongly incompatible elements that become concentrated in the crust through these processes.