Elemental Abundances in the Earth and Geochemical Classification of the Elements

       Elemental abundances may be expressed in several ways. One is on a number of atoms basis. This assumes that we have, say, one million atoms. Of these one million atoms, so many atoms will be oxygen, so many will be silicon, etc. This is purely a numerical counting system. Another method allows for the weight of each atom. Figures will be reported in weight percent or weight per million grams. If we have a million gram sample, so many grams are oxygen, so many are silicon, etc. This method increases the numbers for heavy elements, while decreasing the numbers for lighter elements. Another method is based on volume. Large atoms (anions and large cations) take up much more volume than small atoms.

SOLID EARTH ELEMENTAL ABUNDANCES

        The two major elements in the universe are hydrogen and helium. Why are these elements scarce on earth? The earth is a differentiated planet. It formed by condensation of gaseous matter around a protosun. The condensate was greatly enriched in elements that condense at higher temperatures than hydrogen and helium, which have the two lowest freezing and melting points of all the elements. Thus, the abundance of elements on earth does not resemble the cosmic abundance scale.

       During the subsequent evolution of the earth, the planet is believed to have remelted due to meteorite impact. This remelting allowed a density separation of the elements. Heavier elements sank to the middle of the earth, while the lighter elements rose to the outer parts. The earth is traditionally divided into the core, the mantle, and the crust. An iron-nickel metal phase is believed to make up the core. The mantle and the crust are dominated by silicate minerals, with those in the mantle being enriched in iron and magnesium (mafic minerals) compared with those in the crust (felsic minerals). We can only directly sample the crust. Thus, the best information about the abundance of elements on earth that we have is for crustal rocks. Even within the crust, differentiation exists. Table 1 shows the relative abundance of major elements in the continental and total crust, expressed as weight percent oxides. Oceanic crustal rocks are enriched in iron, magnesium, and calcium compared with continental crustal rocks. Continental crustal rocks are enriched in silicon, aluminum, and potassium relative to oceanic rocks. Most of these differences can be accounted for by the weight of the elements. Iron, magnesium, and calcium are all heavier than silicon and aluminum. Potassium remains to be explained. Potassium is heavier than silicon or aluminum, so we might expect that it would be concentrated in the mantle. The ion of potassium (K⁺) is very large. It does not fit well into the crystal sites available in the minerals present in the mantle. Thus, by default, it ends up in the crust. It should also be noted that, to a precision of 0.1%, only eleven elements make up all of the earth's crust. Many other elements are present, but only in small amounts. Table 2 shows the effect of different methods of reporting data. The eight most common elements in the earth's crust are tabulated according to atom percent, weight percent, and volume percent. Note that oxygen is more than 90% by volume, more than 60% by number of atoms, but only 46% by weight. Thus the way in which we choose to present data can have a significant impact on our perception of the data. We need to choose carefully how we manipulate data to be sure the point we are making is clear, and that conclusions we draw are real.

 Table 1 Composition of Continental and Total Crust

                                    After Mason and Moore, 1982, p.44

Table 2 Common Chemical Elements in the Earth's Crust

GEOCHEMICAL CLASSIFICATION OF ELEMENTS

       The early earth was probably bombarded by solid planetesimals, primarily chondritic meteorites. Chondritic meteorites may be left over from the protoplanet stage of the solar system. Chondritic meteorites are composed of three different phases, or combinations of these phases. The phases are nickel-iron metal, iron sulfide, and silicates, largely olivine or pyroxene. Various chemical elements distribute themselves among these phases according to their relative affinity for silicate, for sulfide, or for metallic phases. Iron is more abundant than magnesium or silicon in meteorites. Thus, three immiscible phases formed, with iron present in all three phases. Enough oxygen plus sulfur to combine with all of the cations does not exist, so a metallic phase is present. All other cations had to compete with iron for anions. Reactions of two types were possible, with M representing a non-iron cation.

Thus, the free energies of the metallic silicates or sulfides in relation to the free energy of the iron silicate or sulfide determine the distribution of the non-iron cations. Metals more electropositive than iron can displace iron from the silicate phases. Elements, less electropositive than iron, were displaced by iron from the combined phases and were concentrated in the metallic phase. The sulfide phase attracted elements able to form covalent, rather than ionic, bonds with the sulfur.

       Study of the meteorites suggests that element distributions are controlled by the affinity of each element for one of the major phases present. These affinities are the result electronic configurations of the elements, which control their chemical bonding characteristics. These same processes might govern the distribution of elements within the earth.

       V.M. Goldschmidt (1937, 1954) was the first to point out that the primary differentiation of elements was based on geochemistry, not density. He introduced four new terms to describe the chemical affinity of elements in the earth. These are:

Siderophile: Elements concentrated in the metallic phase, along with metallic iron.

Chalcophile: Elements concentrated in the sulfide phase.

Lithophile: Elements concentrated in the silicate phase.

Atmophile: Elements concentrated in the atmosphere.

Goldschmidt's ideas were first proposed in 1922-23, when few quantitative data were available to buttress his arguments. Laboratory experiments on the distribution of elements from a liquid phase to metal, sulfide, and silicate phases had not been done, and were of great difficulty. Thus Goldschmidt based his arguments on meteorites, a sort of "fossilized" experiment. Many meteorites consist of the phases nickel-iron (metal), troilite (sulfide), and silicate, and have probably condensed from a liquid phase. By mechanically separating these phases and measuring the chemistry of each, the affinity of various elements for each phase could be determined. In addition, data from smelting processes, in which metals separate from a melt into either a metallic phase or a slag phase (silicates), or a matte rich in sulfides. Based on these measurements, elements can then be separated into Goldschmidt's categories. Table 3-2 presents the classification of elements in these terms. The chart shows that some elements do not show a clear affinity for one category. Chemical affinity depends on temperature, pressure, and the total chemical environment. Elements that belong to one category in meteorites may belong to another category within the earth, particularly within the crust.

       Examples of mixed preferences include chromium, which is strongly lithophile in the presence of oxygen, but, under the reducing conditions found in meteorites, is chalcophile. Carbon is lithophile or atmophile under oxidizing conditions, but siderophile under reducing conditions. Phosphorous is lithophile under oxidizing conditions, siderophile under reducing conditions. In the crust, insufficient enough iron for nickel and cobalt to combine exists within a metallic phase, so these elements are chalcophile in the crust. Another problem with Goldschmidt's classification arises for trace elements. No exact definition of a trace element has been established but a useful working definition of a trace element is one whose concentration is less than 0.1%. Trace elements may form their own minerals. Then they usually obey the Goldschmidt classification. For example, all Tl minerals are sulfides. Typically, however, trace elements will follow a major element into another mineral, where they replace part of the major element. Thallium usually substitutes for potassium ion. Since potassium is lithophile, this makes thallium seem lithophile.

       Goldschmidt's classification should be treated as a qualitative indicator only. It cannot be used to make detailed predictions of element distribution under all of the various conditions that are found within the earth. The fact that the classification scheme is generally valid may be attributed to the similarity among the electronic configurations of groups of elements. The electronic configurations that may be associated with each category are as follows:

Table 3 Geochemical Classification of Elements

^* Elements are chalcophile and lithophile in the earth's crust.

⁺ Elements are chalcophile in the earth's crust

⁺⁺ Elements are lithophile in the earth's crust

() Elements show affinity for more than one group. Secondary group(s) are shown in parentheses.

After Mason and Moore (1982); Brownlow (1979)

Siderophile: Elements whose valence electrons are not readily available for combination with other elements. Positive charge on the nucleus, at least under certain conditions, exerts a strong attraction on the outer electrons, preventing combination. These elements usually occur in the native state.

Chalcophile: Elements whose valence electrons may be shared, but are not electropositive enough to donate electrons or electronegative enough to accept electrons. Thus, the bonds formed are predominantly covalent. Since sulfur is much less electronegative than oxygen, sulfur is prone to form covalent bonds with these elements. Generally the chalcophile elements have their valence electrons outside a shell of 18 electrons.

Lithophile: Elements that are strongly electropositive or electronegative and thus typically donate or accept electrons, forming ionic bonds. Most silicate minerals have oxygen ions that can form ionic bonds to metal cations. Generally the lithophile elements have their valence electrons outside a shell of eight electrons.

Atmophile: Elements that do not readily combine with other elements, or which form diatomic molecules held together in the solid or liquid states only by very weak Van der Waal forces. All of the inert gases, with completed shells or subshells, fall into this category. It should be noted that oxygen, although classified as a secondarily atmophile element, would not occur in the atmosphere of the earth if the earth were at chemical equilibrium. Oxygen is maintained in the atmosphere only by the continual photosynthesis within the biosphere. Indeed, the presence of oxygen in an atmosphere is often regarded as an indicator of life on the planet.

       Goldschmidt noted one other aspect of atomic behavior that correlates with his classification scheme. If a plot is made of atomic volume versus atomic number, the plot shows various maxima and minima. Siderophile elements occur near the minima. Chalcophile elements occur where the atomic volume is increasing compared with atomic number. Atmophile elements follow the chalcophile and are near the maxima. The lithophile elements are generally on the declining side of the curve.

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Last updated: November 29, 2002