The abundance of the chemical elements is a quantification of the occurrence of the chemical elements relative to all other elements in a given environment, for example, the Earth’s crust. Abundance can be reported in one of three differ- ent ways: by the mass fraction (the same as weight fraction); by the mole fraction (fraction of atoms by numerical count, or sometimes fraction of molecules in gases); or by the volume fraction. Volume fraction is a common abun- dance measure in mixed gases such as planetary atmospheres and is similar in value to molecular mole fraction for gas mixtures at relatively low densities and pressures and ideal gas mixtures.
The abundance of chemical elements in the universe is dominated by the large amounts of hydrogen and helium, which were formed in the Big Bang. Remaining elements, making up only about 2% of the universe, were mostly formed in supernovae and certain red giant stars. Lithium, beryllium, and boron are rare since, even though they are formed through nuclear fusion, they are subsequently destroyed by other reactions in the stars. The elements from car- bon to iron are relatively more abundant in the universe due to the ease of forming them in supernova nucleosynthesis.
Elements of higher atomic number than iron (element 26) become increasingly rarer in the universe, since they increas- ingly absorb stellar energy in their production. As well, elements with even atomic numbers are usually more common than their neighbors in the periodic table, due to favorable energetics of formation.
The abundance of elements in the Sun and outer planets is similar to that in the universe as a whole. As a result of solar heating, the elements of Earth and the inner rocky planets of the Solar System have experienced an additional depletion of volatile hydrogen, helium, neon, nitrogen, and carbon (which volatilizes as methane). The crust, mantle, and core of the Earth exhibit evidence of chemical segregation plus some sequestration by density. Lighter silicates of aluminum are found in the crust, with more magnesium silicate in the mantle, while metallic iron and nickel compose the core. The abundance of elements in specialized environments, such as the atmosphere, oceans, or even the human body, is principally a product of chemical interactions with the medium in which they reside.
2.3.1 Earth
The Earth formed from the same cloud of matter that formed the Sun, but the planets developed different compositions during the formation and evolution of the solar system. Subsequently, the geological history of the Earth caused parts of the planet to have varying concentrations of the elements. The mass of the Earth is approximately 5.9831024kg. In bulk, by mass, it is composed mostly of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminum (1.4%), with the remaining 1.2% consisting of trace amounts of other elements. The overall composition of the Earth by elemental mass is roughly the same as the gross composition of the solar system, with the most important differences being that Earth is missing a large part of the volatile elements hydrogen, helium, neon, and nitrogen, as well as carbon, which has been lost as volatile hydrocarbons. The remaining elemental composition is roughly characteristic of the “rocky” inner planets that formed in the thermal zone where solar heat drove volatile compounds into space. The Earth retains oxygen as the second-largest component of its mass (and largest atomic fraction), mainly due to this element being retained in silicate minerals that have a very high melting point and low vapor pressure.
2.3.2 Crust
The upper part of the crust, entailing the materials near the Earth’s surface, consists of a relatively large percentage of sedimentary rocks and unconsolidated material. Nevertheless, this sedimentary cover forms only a thin layer on an underlying basement of igneous and metamorphic rocks that are exposed in mountain belts and on the seafloor. It is estimated that the upper 16 km (10 miles) of the crust contains 95% igneous rocks (or their metamorphic equivalents), 4% shale, 0.75% sandstone, and 0.25% limestone. Consequently, the average composition of igneous rocks would closely resemble the average crustal composition. The mass abundance of the nine most abundant elements in the Earth’s crust is about 46% oxygen, 28% silicon, 8.2% aluminum, 5.6% iron, 4.2% calcium, 2.5% sodium, 2.4% magne- sium, 2.0% potassium, and 0.61% titanium. Other elements occur at less than 0.15%. For a complete list, seeTable 2.2.
Many of the elements shown inFig. 2.6are classified into (partially overlapping) categories:
G rock-forming elements (major elements in green field, and minor elements in light green field);
G rare earth elements (lanthanides, La-Lu, and Y; labeled in blue);
G major industrial metals (global production.B33107kg/year; labeled in red);
G precious metals (labeled in purple);
1 2 3 4 5 Average
1 8 Oxygen (O) 466,000 474,000 460,000 467,100 461,000 46,5620
2 14 Silicon (Si) 277,200 277,100 270,000 276,900 282,000 27,6640
3 13 Aluminum (Al) 81,300 82,000 82,000 80,700 82,300 81,660
4 26 Iron (Fe) 50,000 41,000 63,000 50,500 56,300 52,160
5 20 Calcium (Ca) 36,300 41,000 50,000 36,500 41,500 41,060
6 11 Sodium (Na) 28,300 23,000 23,000 27,500 23,600 25,080
7 12 Magnesium (Mg) 20,900 23,000 29,000 20,800 23,300 23,400
8 19 Potassium (K) 25,900 21,000 15,000 25,800 20,900 21,720
9 22 Titanium (Ti) 4400 5600 6600 6200 5600 5680
10 1 Hydrogen (H) 1400 1500 1400 1400 1425
11 15 Phosphorus (P) 1200 1000 1000 1300 1050 1110
12 25 Manganese (Mn) 1000 950 1100 900 950 980
13 9 Fluorine (F) 800 950 540 290 585 633
14 56 Barium (Ba) 500 340 340 500 425 421
15 6 Carbon (C) 300 480 1800 940 200 744
16 38 Strontium (Sr) 370 360 370 367
17 16 Sulfur (S) 500 260 420 520 350 410
18 40 Zirconium (Zr) 190 130 250 165 184
19 23 Vanadium (V) 100 160 190 120 143
20 17 Chlorine (Cl) 500 130 170 450 145 279
21 24 Chromium (Cr) 100 100 140 350 102 158
22 37 Rubidium (Rb) 300 90 60 90 135
23 28 Nickel (Ni) 80 90 190 84 111
24 30 Zinc (Zn) 75 79 70 75
25 29 Copper (Cu) 100 50 68 60 70
26 58 Cerium (Ce) 68 60 66.5 65
27 60 Neodymium (Nd) 38 33 41.5 38
28 57 Lanthanum (La) 32 34 39 35
29 39 Yttrium (Y) 30 29 33 31
30 7 Nitrogen (N) 50 25 20 19 29
31 27 Cobalt (Co) 20 30 25 25
32 3 Lithium (Li) 20 17 20 19
33 41 Niobium (Nb) 20 17 20 19
34 31 Gallium (Ga) 18 19 19 19
35 21 Scandium (Sc) 16 26 22 21
36 82 Lead (Pb) 14 10 14 13
37 62 Samarium (Sm) 7.9 6 7.05 6.98
38 90 Thorium (Th) 12 6 9.6 9.20
39 59 Praseodymium (Pr) 9.5 8.7 9.2 9.13
40 5 Boron (B) 950a 8.7 10 9.35
41 64 Gadolinium (Gd) 7.7 5.2 6.2 6.37
42 66 Dysprosium (Dy) 6 6.2 5.2 5.80
46 55 Cesium (Cs) 3 1.9 3 2.63
47 4 Beryllium (Be) 2.6 1.9 2.8 2.43
48 50 Tin (Sn) 0 2.2 2.2 2.3 1.68
49 63 Europium (Eu) 2.1 1.8 2 1.97
50 92 Uranium (U) 0 1.8 2.7 1.50
51 73 Tantalum (Ta) 2 1.7 2 1.90
52 32 Germanium (Ge) 1.8 1.4 1.5 1.57
53 74 Tungsten (W) 160.6a 1.1 1.25 1.18
54 42 Molybdenum (Mo) 1.5 1.1 1.2 1.27
55 33 Arsenic (As) 1.5 2.1 1.8 1.80
56 67 Holmium (Ho) 1.4 1.2 1.3 1.30
57 65 Terbium (Tb) 1.1 0.94 1.2 1.08
58 69 Thulium (Tm) 0.48 0.45 0.52 0.48
59 35 Bromine (Br) 0.37 3 2.4 1.92
60 81 Thallium (Tl) 0.6 0.53 0.85 0.66
61 71 Lutetium (Lu) 0.5 0.50
62 51 Antimony (Sb) 0.2 0.2 0.2 0.20
63 53 Iodine (I) 0.14 0.49 0.45 0.36
64 48 Cadmium (Cd) 0.11 0.15 0.15 0.14
65 47 Silver (Ag) 0.07 0.08 0.075 0.075
66 80 Mercury (Hg) 0.05 0.067 0.085 0.067
67 34 Selenium (Se) 0.05 0.05 0.05 0.050
68 49 Indium (In) 0.049 0.16 0.25 0.153
69 83 Bismuth (Bi) 0.048 0.025 0.0085 0.027
70 52 Tellurium (Te) 0.005 0.001 0.001 0.002
71 78 Platinum (Pt) 0.003 0.0037 0.005 0.004
72 79 Gold (Au) 0.0011 0.0031 0.004 0.003
73 44 Ruthenium (Ru) 0.001 0.001 0.001 0.001
74 46 Palladium (Pd) 0.0006 0.0063 0.015 0.0073
75 75 Rhenium (Re) 0.0004 0.0026 0.0007 0.0012
76 77 Iridium (Ir) 0.0003 0.0004 0.001 0.0006
77 45 Rhodium (Rh) 0.0002 0.0007 0.001 0.0006
78 76 Osmium (Os) 0.0001 0.0018 0.0015 0.0011
1,Darling (2016); 2,Barbalace (19952019); 3,WebElements (19932019); 4,Israel Science and Technology Directory (19992018); 5,Thomas Jefferson National Accelerator Facility—Office of Science Education.
aDubious.
G the nine rarest “metals”—the six platinum group elements (PGE) plus Au, Re, and Te (a metalloid)—in the yellow field. These are rare in the crust due to being soluble in iron and are therefore strongly concentrated in the Earth’s core. Tellurium is the single most depleted element in the silicate Earth relative to its cosmic abundance, since in addition to being concentrated in dense chalcogenides in the Earth’s core it was severely depleted by preaccretional sorting in the nebula as volatile hydrogen telluride.
There are two breaks where the unstable (radioactive) elements technetium (atomic number 43) and promethium (atomic number 61) would be. These elements both have relatively short half-lives (B4 million years andB18 years, respectively) and are therefore extremely rare, as any primordial initial fractions of these in presolar system materials have long since decayed. These two elements are now only formed naturally through the spontaneous fission of very heavy radioactive elements (e.g., uranium, thorium, or the trace amounts of plutonium that exist in uranium ores), or by the interaction of certain other elements with cosmic rays. There are also breaks in the abundance graph where the noble gases should be, as they are not chemically bound in the Earth’s crust, and they are only formed through decay chains from radioactive elements in the crust and are hence extremely rare there. The eight naturally occurring extremely rare, highly radioactive elements (polonium, astatine, francium, radium, actinium, protactinium, neptunium, and plutonium) are not included in the graph, because any of these elements that were present at the formation of the Earth have decayed away billions of years ago, and their total amount today is negligible and is only produced from the radioactive decay of uranium and thorium.
Oxygen and silicon are the most common elements in the crust. On Earth and in rocky planets in general, silicon and oxygen are far more common than their cosmic abundance. This is due to the fact that they combine with each other to form silicate minerals. As such they are the lightest of all of the 2% “astronomical metals” (i.e., nonhydrogen and helium elements) to form a solid that is refractory to the Sun’s heat, and thus cannot boil away into space. All elements lighter than oxygen have been removed from the crust in this way, as have the heavier chalcogens sulfur, selenium, and tellurium. The Earth’s crust as a result consists almost entirely of silicate, carbonate, oxide, hydroxide, phosphate, and sulfate minerals.
One could imagine the Earth’s crust on an atomic scale to consist in essence of a close packing of oxygen anions with inter- stitial metal ions, primarily silicon. Hence, the minerals referred to as the rock-forming minerals in the crust are, with a few exceptions, members of the silicate, oxide, and carbonate groups. Quantitative chemical analyses of minerals allow for the grouping of elements on the basis of their abundance. When chemical elements occur in large amounts (.1 wt.%), they are considered to be major elements. Minor elements occur in lower concentrations (0.11.0 wt.%), while other elements that occur in minor amounts (,0.1 wt.%) are known as trace elements.
0 10 20 30 40 50 60 70 80 90
10−6 10−3 100 103 106 109
Abundance atoms of element per 106 atoms of Si
Atomic number (Z) H
Li
Be BN C
O
F
Major industrial metals in red Precious metals in purple Rare-earth elements in blue
Na Si
Mg P
S Cl Al
KCa
Sc Ti
VCr Mn Fe Rock-forming elements
Rarest "metals"
CoNi Cu Zn
Ga Ge As
Se Br Rb
Sr
Y Zr
Nb
Mo
Te Pd Ag
Cd SbI In
Sn
Rh Ru
Ba
Cs La
Nd Ce
Pr
Re Tm Ho
Yb Lu
Ir Os Er Hf Gd Eu
PtAu Dy Ta
Tb Sm
Hg W Tl
Pb
Bi Th
U
FIGURE 2.6 Abundance (atom fraction) of the chemical elements in Earth’s upper continental crust as a function of atomic number. The rarest ele- ments in the crust (shown in yellow) are not the heaviest but rather the siderophile (iron-loving) elements. These have been depleted by being relo-
cated deeper into the Earth’s core. Te and Se are concentrated as sulfides in the core and have also been depleted by preaccretional sorting in the nebula that caused them to form volatile hydrogen selenide and hydrogen telluride.
2.3.3 Rare earth elements
“Rare” earth elements can be considered to be a historical misnomer. The persistence of the term shows unfamiliarity rather than true rarity. The more abundant rare earth elements have similar abundances in the crust as commonplace industrial metals such as chromium, nickel, copper, zinc, molybdenum, tin, tungsten, or lead. The two least abundant rare earth elements (thulium and lutetium) are approximately 200 times more common than gold. Still, in contrast to the ordinary base and precious metals, rare earth elements show a very low tendency to become concentrated in exploitable ore deposits. As a result, most of the world’s supply of rare earth elements comes from only a handful of sources. Furthermore, the rare earth metals are all quite chemically similar to each other, and they are thus quite diffi- cult to separate into quantities of the pure elements. Differences in abundances of individual rare earth elements in the upper continental crust of the Earth are due to the superposition of two effects, one nuclear and one geochemical. First, the rare earth elements with even atomic numbers (58Ce,60Nd,. . .) have greater cosmic and terrestrial abundances than the neighboring rare earth elements with odd atomic numbers (57La,59Pr,. . .). Second, the lighter rare earth elements are more incompatible (as they have larger ionic radii) and hence more strongly concentrated in the continental crust than the heavier rare earth elements. In most rare earth elements ore deposits, the first four rare earth elements—lantha- num, cerium, praseodymium, and neodymium—form 80%99% of the total amount of rare earth metal that can be found in the ore.
2.3.4 Mantle
The mass abundance of the eight most abundant elements in the Earth’s mantle is approximately 45% oxygen, 23%
magnesium, 22% silicon, 5.8% iron, 2.3% calcium, 2.2% aluminum, 0.3% sodium, and 0.3% potassium. The mantle differs in elemental composition from the crust in having a higher concentration of magnesium and significantly more iron, while having much less aluminum and sodium. Therefore the upper mantle is dominated by the mineral olivine with lesser amounts of pyroxene and only trace amounts of aluminous minerals, such as feldspar, spinel, and garnet. At greater depth and higher pressure, the mantle transition zone is marked by discontinuities associated with changes in material properties without major changes in overall chemical composition. At about 400 km, olivine, Mg2SiO4, will isochemically transform to a denser structure with closer packing, a spinel structure, named ringwoodite. The intermedi- ate phase in this reaction is another spinel structure, wadsleyite. The lower mantle starts at about 660 km at still higher pressures and silicates undergo another radical mineralogical and structural rearrangement. Silica goes from tetrahedral coordination [4] to octahedral coordination [6]. This depth of about 660 km is also where spinel structures, and other Ca- and Mg-silicates, transform to the perovskite structure together with other oxide structures. In the perovskite struc- ture, six oxygen atoms are grouped around the Mg. Such a structure is in sharp contrast to those of silicate minerals in the crust, where four oxygen atoms surround each Si in tetrahedral coordination.
2.3.5 Core
The transition from the lower mantle to the core is a definite chemical discontinuity. The core is extremely dense and represents 30% of the Earth’s mass but only 17% of its volume. Due to mass segregation, the core of the Earth is believed to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements. The liquid outer core, from 2900 to 5100 km, consists primarily of iron plus about 2% nickel. Its density of 9.9 g/cm3is slightly too low to represent the density of pure iron and incorporation of 9%12% silica, or other light elements, produces a better fit with the known density. The solid inner core from 5100 to 6371 km also con- sists of a Fe-Ni alloy, containing about 20% nickel.