geochemistry
The reason the Earth is not chemically uniform is a matter of fit: whether a particular ion suits a particular site in a particular crystal, under particular conditions. Everything else follows from that.
About: Element partitioning · Fractional crystallisation · Partial melting · Geochemical differentiation
When a crystal and a liquid coexist, elements divide between them. Knowing how they divide is a prerequisite for understanding magma crystallisation and planetary differentiation alike.[1]
What decides the division is how well a cation’s charge and radius fit the lattice site available to it — and how elastically that site can accommodate a poor fit. The partitioning of any series of isovalent cations can be rationalised from the size and elasticity of the site, with site elasticity varying linearly with the cation’s formal charge.[1]
An element taken readily into a crystal is called compatible in that phase; one left in the melt is called incompatible. Both words describe a behaviour under stated conditions.[1]
Change the host mineral, the pressure, the temperature or the oxidation state and the behaviour can change. Cerium and europium make the point: when cerium goes to Ce4+ or europium to Eu2+, their charge and radius change, and they separate from the trivalent rare earths they otherwise travel with.[1,2,3]
A melt does not have the composition of its source. Partitioning divides elements between the liquid and the residue, so the melt is enriched in what the crystals reject and the residue in what they keep.[1]
Remove crystals as they form and the remaining liquid drifts further still: elements that entered the crystals are depleted, and those excluded accumulate.[1]
Repeat that across four and a half billion years, in melting, in crystallisation and in fluid transport, and the differences accumulate into reservoirs of contrasting composition. That is the answer to why the Earth is chemically heterogeneous.[1]
Partitioning explains why elements concentrate. It does not by itself say that any concentration is large enough to matter economically, and Lithoen does not let the two be confused.[1]
Often said
“Some elements are simply incompatible elements.”
What the evidence supports
Compatibility describes how an ion behaves in a particular phase under particular conditions, not a permanent property of an element. It is set by the fit between the cation and the lattice site on offer, so changing the host mineral or the conditions can change it.
Supporting records: partitioning-not-a-label · partitioning-controls
Often said
“A magma has the composition of the rock it melted from.”
What the evidence supports
Partial melting divides elements between melt and residue according to how each partitions, so an incomplete melt cannot have the composition of its source.
Supporting records: melt-differs-from-source · partitioning-controls
Bracketed references link to the structured entity records whose claims support each paragraph. Last updated 2026-09-06.