Abundance of Elements
While the International Union of Pure and Applied Chemistry recognizes 118 chemical elements, only those with atomic numbers up to 98 are found in nature; any element heavier than californium (Z=98) is strictly a laboratory product and plays no role in earthly geology. [1] Even among the 98 naturally occurring elements, 15 are geologically ephemeral - they exist only in vanishingly small quantities as short-lived radioactive products of nuclear decay and are continuously made and destroyed, never accumulating enough to become essential constituents of any mineral. [1] After excluding those 15, and further excluding the noble gases that are chemically inert, only 83 elements are genuinely available to make minerals. [1]
The 15 Geologically Ephemeral Elements
The 15 ephemeral elements are technetium (Tc, Z = 43), promethium (Pm, Z = 61), polonium (Po, Z = 84), astatine (At, Z = 85), radon (Rn, Z = 86), francium (Fr, Z = 87), radium (Ra, Z = 88), actinium (Ac, Z = 89), protactinium (Pa, Z = 91), neptunium (Np, Z = 93), plutonium (Pu, Z = 94), americium (Am, Z = 95), curium (Cm, Z = 96), berkelium (Bk, Z = 97), and californium (Cf, Z = 98). These elements occur only as transient intermediates in the radioactive decay series of long-lived isotopes. They are continuously produced by neutron capture or by the decay of uranium and thorium, and then just as continuously destroyed by their own radioactive decay. [1] Some are present only in almost unimaginable trace amounts. The total quantity of naturally occurring astatine in the Earth’s crust at any given moment has been estimated at about 30 grams. [1]
Noble Gases and Heavy Elements
The five non-reactive noble gases - helium, neon, argon, krypton, and xenon - cannot serve as essential structural elements in any mineral. [1] Small amounts of noble gas atoms are present in many minerals because they are generated in place by radioactive decay or become trapped in the structure during growth, but that is incidental incorporation, not chemical bonding.
Lead (Pb, Z = 82) marks the practical upper boundary of geochemically common elements: it is the element with the highest atomic number that possesses stable isotopes. [1] Among elements heavier than lead, only bismuth (Bi, Z = 83), thorium (Th, Z = 90), and uranium (U, Z = 92) have isotopes with half-lives long enough to persist through geological time and become normal constituents of minerals. [1] Bismuth’s most abundant isotope, 209Bi, was long considered stable but has since been found to have a half-life of 1.9 × 1019 years - effectively stable on any geological timescale. [1]
The Eight Most Abundant Elements in the Crust
Of the 83 available elements, only eight occur in substantial amounts in the Earth’s crust. [1] These eight - O, Si, Al, Fe, Ca, Na, K, and Mg - collectively account for nearly the entire mass of the crust and are the building blocks from which most common minerals are made. [1]
| Element | Oxidation State | Crust (wt%) | Crust (atom%) | Crust (vol%) | Oceanic Crust (wt%) | Whole Earth (wt%) |
|---|---|---|---|---|---|---|
| O | -2 | 46.6 [1] | 62.5 | 91.7 | 40.9 | 29.5 |
| Si | +4 | 27.7 [1] | 21.2 | 0.2 | 23.1 | 15.2 |
| Al | +3 | 8.1 [1] | 6.5 | 0.5 | 8.5 | 1.1 |
| Fe | +2/+3 | 5.0 [1] | 1.9 | 0.5 | 8.2 | 34.6 |
| Ca | +2 | 3.6 [1] | 1.9 | 1.5 | 8.1 | 1.1 |
| Na | +1 | 2.8 [1] | 2.6 | 2.2 | 2.1 | 0.6 |
| K | +1 | 2.6 [1] | 1.4 | 3.1 | 1.3 | 0.1 |
| Mg | +2 | 2.1 [1] | 1.8 | 0.4 | 4.6 | 12.7 |
Several patterns in this table are worth unpacking. Oxygen dominates the crust by weight at 46.6%, but its dominance in volumetric terms (91.7%) is even more striking. Oxygen ions are large: they occupy nearly all of the volume of most mineral structures, with the smaller cations fitting into the spaces between them. Silicon ranks second by weight at 27.7% but contributes only 0.2% of crustal volume - the Si4+ ion is tiny, buried inside oxygen coordination polyhedra. Together, O and Si account for 74% of crustal mass, which explains why silicate minerals dominate the crust so overwhelmingly.
The oceanic crust is appreciably richer in iron and magnesium than the continental average, reflecting its basaltic composition. [1] The whole-Earth composition differs radically from the crust: iron constitutes 34.6% of the Earth by weight once the iron-rich core is included, compared to only 5.0% in the crust. The whole Earth is similarly much richer in magnesium and poorer in silicon, aluminium, potassium, sodium, and calcium than the crust - a consequence of the density-driven separation of the iron-nickel core and the silicate mantle and crust during early Earth differentiation. [1]
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References & Citations
- 1.Introduction to Mineralogy Nesse
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