Chemical Differentiation
Note that although bulk materials differentiate outward or inward according to their density, the elements that are chemically bound in them fractionate according to their chemical affinities, "carried along" by more abundant materials that they're associated with. For instance, although the rare element uranium is very dense as a pure element, it is chemically more compatible as a trace element in the Earth's light, silicate-rich crust than in the dense metallic core.
Read more about this topic: Planetary Differentiation
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—Henry Brooks Adams (1838–1918)