Earth system
Geochemistry
Why the Earth is not chemically uniform. Elements divide between coexisting phases according to how well they fit, and four and a half billion years of that dividing has produced reservoirs of sharply contrasting composition.
Whether an element enters a crystal or stays in the melt is decided by how well its charge and radius fit the lattice site on offer, and by how elastically that site can absorb a poor fit. Compatibility is therefore a property of a cation in a site under conditions — never a permanent label on an element.[1, 2]
Because elements divide unequally, a melt never has the composition of its source, and removing crystals as they form drives the residual liquid further still. Repeat that across Earth history and the differences accumulate into reservoirs.[1, 2, 3]
Those reservoirs must be named before any number attached to them means anything. Bulk Earth includes the core; bulk silicate Earth excludes it; the crust is a differentiated skin; the upper continental crust is only the sampled part of it.[1, 2, 3]
And a concentration is not a resource. The rare earths are more abundant in the crust than copper, lead, silver or gold; what is scarce is the geology that concentrates them into something recoverable.[1, 2]
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Element partitioning
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.
The geochemical reservoirs of Earth
A number for “the Earth” is not a number at all until it says which Earth. Bulk Earth, bulk silicate Earth, the crust and the upper continental crust are four different volumes with four different compositions.
The rare-earth elements
They are not rare. Cerium is more common in the crust than copper or lead, and every one of them except promethium is more abundant than gold. What is rare is the geology that concentrates them.
Weathering geochemistry
Chemical weathering is a sorting machine. It dissolves some elements, oxidises others, and leaves behind whatever the water could not carry — concentrating elements without any melt or ore fluid being involved.
Processes
How elements are moved between phases and reservoirs — the mechanisms that make heterogeneity.
Geological process
Element partitioning
The division of an element between coexisting phases — crystal and melt, mineral and fluid. It is the mechanism by which Earth becomes chemically heterogeneous, and it is a property of a cation in a particular site under particular conditions, not a fixed label on an element.
Geological process
Fractional crystallisation
Progressive removal of crystals from a cooling magma, so that the remaining liquid drifts away from the composition it started with. Elements excluded from the crystals accumulate in what is left.
Geological process
Partial melting
The production of liquid from only part of a solid rock, leaving a residue behind. Almost all magma on Earth is generated this way; the conditions that trigger it differ from setting to setting.
Geological process
Geochemical differentiation
The cumulative effect of partitioning across Earth history: the separation of a once more uniform body into reservoirs of contrasting composition.
Geological process
Redox processes
Transfer of electrons between species. Oxidation state governs an element’s charge, its ionic radius, its solubility and which minerals will accept it — so redox controls where elements go.
Geological process
Hydrothermal alteration
Chemical change of rock by hot aqueous fluid. Fluids dissolve elements at one place and precipitate them at another, which is how elements are transported and concentrated without melting anything.
Geological process
Solid solution
Continuous substitution of one ion for another in a crystal structure. It is why a mineral formula is a description of a range rather than a fixed recipe.
Reservoirs
Volumes of Earth treated as compositional units. Naming the reservoir is what turns a number into a datum.
Geochemical reservoir
Bulk Earth
The composition of the entire planet, core included. It is not measured; it is estimated from meteorites and from models of how Earth accreted, and it differs sharply from anything that can be sampled.
Geochemical reservoir
Bulk silicate Earth
The whole planet MINUS the core: mantle plus crust taken together. The distinction from bulk Earth is not pedantry — the core holds most of Earth’s iron and its siderophile elements, so the two compositions are entirely different.
Geochemical reservoir
Average crust
Earth’s crust treated as a single averaged composition, as crustal abundance compilations use the term. It does not distinguish continental from oceanic crust, and abundance figures quoted against it should not be read as applying to either specifically.
Geochemical reservoir
Continental crust (reservoir)
The continental crust treated as a compositional unit. Distinct from the Phase 1 EarthLayer record, which describes it as a physical layer: this record is what a composition can be attributed to.
Geochemical reservoir
Upper continental crust
The uppermost part of the continental crust — the part actually exposed to sampling, weathering and erosion. Compositional estimates for it differ substantially from estimates for the crust as a whole, and the two must never be quoted interchangeably.
Geochemical reservoir
Oceanic crust (reservoir)
The oceanic crust treated as a compositional unit: young, basaltic, and continuously created and destroyed, which makes its composition quite unlike that of the continents.
Geochemical reservoir
Mantle (reservoir)
The mantle treated as a compositional unit. It is not uniform, and treating it as one reservoir is a simplification that becomes untenable as soon as isotopic data are considered.
Geochemical reservoir
Core (reservoir)
Earth’s metallic core treated as a compositional unit. Nothing from it has ever been sampled; its composition is inferred from density, from seismology, and from meteorites.
Geochemical reservoir
Seawater
The ocean treated as a geochemical reservoir. Element concentrations in it are governed by solubility and residence time rather than by crustal abundance.
Geochemical reservoir
Soils of the conterminous United States
The soil reservoir characterised by the USGS national soil geochemical survey. Named as a reservoir because a soil concentration means nothing without knowing which soils, sampled at what depth, by what protocol.
The rare earths
A worked example of the whole chain: electron configuration produces a size gradient, the gradient allows fractionation, and abundance still does not imply a resource.
Chemical element
Lanthanum
The first of the lanthanides and the lightest rare-earth element.
Chemical element
Cerium
The most abundant rare-earth element, and unusual among them for adopting a tetravalent state as well as the trivalent one.
Chemical element
Neodymium
A light rare-earth element whose isotopic composition is used to trace the evolution of mantle and crustal reservoirs.
Chemical element
Lutetium
The last and smallest of the lanthanides, and the end point of the lanthanide contraction.
Chemical element
Yttrium
Not a lanthanide, but grouped with the rare-earth elements because its charge and ionic radius make it behave like one.
Surface geochemistry
Concentration without melting: reactions that sort elements by solubility at the surface.
Geological process
Chemical weathering
The alteration of minerals by reaction with water, dissolved carbon dioxide and oxygen. It dissolves some elements, oxidises others, and leaves a residue enriched in whatever is least soluble.
Geological process
Hydrolysis
The reaction in which slightly acidic water breaks down silicate minerals, removing soluble cations and leaving clay. It is the principal chemical weathering reaction of the crust.
Geological process
Oxidation
The loss of electrons by an element on exposure to oxygen. In rocks it converts ferrous iron to ferric, which is far less soluble, and fixes it as oxide.
Geological process
Carbonation
Reaction of carbon dioxide dissolved in water with minerals, particularly carbonates, which it dissolves. It is the link between the atmosphere and the rock record of carbon.
Mineral
Kaolinite
A clay mineral produced by the hydrolysis of feldspar. It is the residue of intense chemical weathering, retaining aluminium and silicon after the soluble cations have gone.
Mineral
Hematite
The ferric iron oxide, and the mineral in which iron released by oxidation is most commonly fixed.
Hub last updated 2026-09-06. Every entry above is a structured record; the bracketed references link to the entities whose claims support each statement.