chemical element

Introduction

also called  element,  

      any substance that cannot be decomposed into simpler substances by ordinary chemical processes. Elements are the fundamental materials of which all matter is composed.

      This article considers the origin of the elements and their abundances throughout the universe. The geochemical distribution of these elementary substances in the Earth's crust and interior is treated in some detail, as is their occurrence in the hydrosphere and atmosphere. The article also discusses the periodic law and the tabular arrangement of the elements based on it. For detailed information about the compounds of the elements, see chemical compound.

Ed.

General observations

      At present there are 117 known chemical elements. About 20 percent of them do not exist in nature (or are present only in trace amounts) and are known only because they have been synthetically prepared in the laboratory. Elements can combine with one another to form a wide variety of more complex substances called compounds (chemical compound). The number of possible compounds is almost infinite; perhaps a million are known, and more are being discovered every day. When two or more elements combine to form a compound, they lose their separate identities, and the product has characteristics quite different from those of the constituent elements. The gaseous elements hydrogen and oxygen, for example, with quite different properties, can combine to form the compound water, which has altogether different properties from either oxygen or hydrogen. Water clearly is not an element because it consists of, and actually can be decomposed chemically into, the two substances hydrogen and oxygen; these two substances, however, are elements because they cannot be decomposed into simpler substances by any known chemical process. Most samples of naturally occurring matter are physical mixtures of compounds. Seawater, for example, is a mixture of water and a large number of other compounds, the most common of which is sodium chloride, or table salt. Mixtures differ from compounds in that they can be separated into their component parts by physical processes; for example, the simple process of evaporation separates water from the other compounds in seawater.

Historical development of the concept of element

      The modern concept of an element is unambiguous, depending as it does on the use of chemical and physical processes as a means of discriminating elements from compounds and mixtures. The existence of fundamental substances from which all matter is made, however, has been the basis of much theoretical speculation since the dawn of history. The ancient Greek (ancient Greek civilization) philosophers Thales, Anaximenes, and Heracleitus each suggested that all matter is composed of one essential principle—or element. Thales believed this element to be water; Anaximenes suggested air; and Heracleitus, fire. Another Greek philosopher, Empedocles, expressed a different belief—that all substances are composed of four elements: air, earth, fire, and water. Aristotle agreed and emphasized that these four elements are bearers of fundamental properties, dryness and heat being associated with fire, heat and moisture with air, moisture and cold with water, and cold and dryness with earth. In the thinking of these philosophers all other substances were supposed to be combinations of the four elements, and the properties of substances were thought to reflect their elemental compositions. Thus, Greek thought encompassed the idea that all matter could be understood in terms of elemental qualities; in this sense, the elements themselves were thought of as nonmaterial. The Greek concept of an element, which was accepted for nearly 2,000 years, contained only one aspect of the modern definition—namely, that elements have characteristic properties.

      In the latter part of the Middle Ages, as alchemists (alchemy) became more sophisticated in their knowledge of chemical processes, the Greek concepts of the composition of matter became less satisfactory. Additional elemental qualities were introduced to accommodate newly discovered chemical transformations. Thus, sulfur came to represent the quality of combustibility, mercury that of volatility or fluidity, and salt that of fixity in fire (or incombustibility). These three alchemical elements, or principles, also represented abstractions of properties reflecting the nature of matter, not physical substances.

      The important difference between a mixture and a chemical compound eventually was understood, and in 1661 the English chemist Robert Boyle (Boyle, Robert) recognized the fundamental nature of a chemical element. He argued that the four Greek elements could not be the real chemical elements because they cannot combine to form other substances nor can they be extracted from other substances. Boyle stressed the physical nature of elements and related them to the compounds they formed in the modern operational way.

      In 1789 the French chemist Antoine-Laurent Lavoisier (Lavoisier, Antoine-Laurent) published what might be considered the first list of elemental substances based on Boyle's definition. Lavoisier's list of elements was established on the basis of a careful, quantitative study of decomposition and recombination reactions. Because he could not devise experiments to decompose certain substances, or to form them from known elements, Lavoisier included in his list of elements such substances as lime, alumina, and silica, which now are known to be very stable compounds. That Lavoisier still retained a measure of influence from the ancient Greek concept of the elements is indicated by his inclusion of light and heat (caloric) among the elements.

      Seven substances recognized today as elements (i.e., gold, silver, copper, iron, lead, tin, and mercury) were known to the ancients because they occur in nature in relatively pure form. They are mentioned in the Bible and in an early Hindu medical treatise, the Caraka-saṃhitā. Sixteen other elements were discovered in the second half of the 18th century, when methods of separating elements from their compounds became better understood. Eighty-two more followed, after the introduction of quantitative analytical methods.

The atomic (atomism) nature of the elements

      Paralleling the development of the concept of elements was an understanding of the nature of matter. At various times in history, matter has been considered to be either continuous or discontinuous. Continuous matter is postulated to be homogeneous and divisible without limit, each part exhibiting identical properties regardless of size. This was essentially the point of view taken by Aristotle when he associated his elemental qualities with continuous matter. Discontinuous matter, on the other hand, is conceived of as particulate—that is, divisible only up to a point, the point at which certain basic units called atoms are reached. According to this concept, also known as the atomic hypothesis, subdivision of the basic unit (atom) could give rise only to particles with profoundly different properties. Atoms, then, would be the ultimate carriers of the properties associated with bulk matter.

      The atomic hypothesis is usually credited to the Greek philosopher Democritus, who considered all matter to be composed of atoms of the four elements—earth, air, fire, and water. But Aristotle's concept of continuous matter generally prevailed and influenced thought until experimental findings in the 16th century forced a return to the atomic theory. Two types of experimental evidence gave support to the atomic hypothesis: first, the detailed behaviour of gaseous substances and, second, the quantitative weight relationships observed with a variety of chemical reactions. The English chemist John Dalton was the first to explain the empirically derived laws of chemical combination by postulating the existence of atoms with unique sets of properties. At the time, chemical combining power (valence) and relative atomic weights were the properties of most interest. Subsequently numerous independent experimental verifications of the atomic hypothesis were carried out, and today it is universally accepted. Indeed, in 1969 individual uranium and thorium atoms were actually observed by means of an electron microscope.

The structure of atoms (atom)

      Atoms of elemental substances are themselves complex structures composed of more fundamental particles called protons (proton), neutrons (neutron), and electrons. Experimental evidence indicates that, within an atom, a small nucleus, which generally contains both protons and neutrons, is surrounded by a swarm, or cloud, of electrons. The fundamental properties of these subatomic particles are their weight and electrical charge. Whereas protons carry a positive charge and electrons a negative one, neutrons are electrically neutral. The diameter of an atom (about 10⁻⁸ centimetre) is 10,000 times larger than that of its nucleus. Neutrons and protons, which are collectively called nucleons (nucleon), have relative weights of approximately one atomic mass unit, whereas an electron is only about ¹/₂₀₀₀ as heavy. Because neutrons and protons occur in the nucleus, virtually all of the mass of the atom is concentrated there. The number of protons in the nucleus is equivalent to the atomic number of the element. The total number of protons and neutrons is called the mass number because it equals the relative weight of that atom compared to other atoms. Because the atom itself is electrically neutral, the atomic number represents not only the number of protons, or positive charges, in the nucleus but also the number of electrons, or negative charges, in the extranuclear region of the atom.

      The chemical characteristics of elements are intimately related to the number and arrangement of electrons in their atoms. Thus, elements are completely distinguishable from each other by their atomic numbers. The realization that such is the case leads to another definition of an element, namely, a substance, all atoms of which have the same atomic number.

The existence of isotopes (isotope)

      Careful experimental examination of naturally occurring samples of many pure elements shows that not all the atoms present have the same atomic weight, even though they all have the same atomic number. Such a situation can occur only if the atoms have different numbers of neutrons in their nuclei. Such groups of atoms—with the same atomic number, but with different relative weights—are called isotopes. The number of isotopic forms that a naturally occurring element possesses ranges from one (e.g., fluorine) to as many as ten (e.g., tin); most of the elements have at least two isotopes. The atomic weight of an element is usually determined from large numbers of atoms containing the natural distribution of isotopes, and, therefore, it represents the average isotopic weight of the atoms constituting the sample. More recently, precision mass-spectrometric methods have been used to determine the distribution and weights of isotopes in various naturally occurring samples of elements.

J.J. Lagowski

Origin of the elements

      The fundamental reaction that produces the huge amounts of energy radiated by the Sun and most other stars (star) is the fusion (nuclear fusion) of the lightest element, hydrogen, its nucleus having a single proton, into helium, the second lightest and second most abundant, with a nucleus consisting of two protons and two neutrons. In many stars the production of helium is followed by the fusion of helium into heavier elements, up to iron. The still heavier elements cannot be made in energy-releasing fusion reactions; an input of energy is required to produce them.

      The proportion of different elements within a star—i.e., its chemical composition—is gradually changed by nuclear fusion reactions. This change is initially concentrated in the central regions of the star where it cannot be directly observed, but it alters some observable properties of the star, such as brightness and surface temperature, and these alterations are taken as evidence of what is going on in the interior. Some stars become unstable and discharge some transmuted matter into interstellar space; this leads to a change in the chemical composition of the interstellar medium and of any stars subsequently formed. The main problem concerned with the origin of the chemical elements is to decide to what extent the chemical composition of the stars seen today differs from the initial chemical composition of the universe and to determine where the change in chemical composition has been produced. Reference is made in this article to the chemical composition of the universe, but most of the observations refer to our own and neighbouring galaxies.

Cosmic abundances of the elements

      The relative numbers of atoms of the various elements are usually described as the abundances of the elements. The chief sources of data from which information is gained about present-day abundances of the elements are observations of the chemical composition of stars and gas clouds in the Galaxy, which contains the solar system and part of which is visible to the naked eye as the Milky Way; of neighbouring galaxies; of the Earth, Moon, and meteorites; and of the cosmic rays.

Stars and gas clouds

      Atoms absorb and emit light, and the atoms of each element do so at specific and characteristic wavelengths. A spectroscope (spectroscopy) spreads out these wavelengths of light from any source into a spectrum of bright-coloured lines, a different pattern identifying each element. When light from an unknown source is analyzed in a spectroscope, the different patterns of bright lines in the spectrum reveal which elements emitted the light. Such a pattern is called an emission, or bright-line, spectrum. When light passes through a gas or cloud at a lower temperature than the light source, the gas absorbs at its identifying wavelengths, and a dark-line, or absorption, spectrum will be formed.

      Thus, absorption and emission lines in the spectrum of light from stars yield information concerning the chemical composition of the source of light and of the chemical composition of clouds through which the light has traveled. The absorption lines may be formed either by interstellar clouds or by the cool outer layers of the stars. The chemical composition of a star is obtained by a study of absorption lines formed in its atmosphere.

      The presence of an element can, therefore, be detected easily, but it is more difficult to determine how much of it there is. The intensity of an absorption line depends not only on the total number of atoms of the element in the atmosphere of the star but also on the number of these atoms that are in a state capable of absorbing radiation of the relevant wavelength and the probability of absorption occurring. The absorption probability can, in principle, be measured in the laboratory, but the whole physical structure of the atmosphere must be calculated to determine the number of absorbing atoms. Naturally, it is easier to study the chemical composition of the Sun than of other stars, but, even for the Sun, after many decades of study, there are still significant uncertainties of chemical composition. The spectra of stars differ considerably, and originally it was believed that this indicated a wide variety of chemical composition. Subsequently, it was realized that it is the surface temperature of a star that largely determines which spectral lines are excited and that most stars have similar chemical compositions.

      There are, however, differences in chemical composition among stars, and these differences are important in a study of the origin of the elements. Studies of the processes that operate during stellar evolution enable estimates to be made of the ages of stars. There is, for example, a clear tendency for very old stars to have smaller quantities of elements heavier than helium than do younger stars. This suggests that the Galaxy (Milky Way Galaxy) originally contained little of the so-called heavy elements (elements beyond helium in the periodic table); and the variation of chemical composition with age suggests that heavy elements must have been produced more rapidly in the Galaxy's early history than now. Observations are also beginning to indicate that chemical composition is dependent on position in the Galaxy as well as age, with a higher heavy-element content near the galactic centre.

      In addition to stars, the Galaxy contains interstellar gas and dust. Some of the gas is very cold, but some forms hot clouds, the gaseous nebulae, the chemical composition of which can be studied in some detail. The chemical composition of the gas seems to resemble that of young stars. This is in agreement with the theory that young stars are formed from the interstellar gas.

Cosmic rays (cosmic ray)

      High-energy electrons and atomic nuclei known as cosmic rays reach the Earth from all directions in the Galaxy (see Cosmos: Cosmic rays and magnetic fields (Cosmos)). Their chemical composition can be observed only to a limited extent, but this can give some information about their place of origin and possibly about the origin of the chemical elements.

      The cosmic rays are observed to be proportionately richer in heavy elements than are the stars, and they also contain more of the light elements lithium, beryllium, and boron, which are very rare in stars. One particularly interesting suggestion is that transuranium (transuranium element) nuclei may have been detected in the cosmic rays. Uranium is element 92, the most massive naturally occurring on Earth; 20 elements beyond uranium (called the transuranium series) have been created artificially. All transuranium nuclei are highly unstable, which would seem to indicate that the cosmic rays must have been produced in the not too distant past.

Roger John Tayler

solar system

      Direct observations of chemical composition can be made for the Earth, the Moon, and meteorites, although there are some problems of interpretation. The chemical composition of the Earth's crust, oceans, and atmosphere can be studied, but this is only a minute fraction of the mass of the Earth, and there are many composition differences even within this small sample. Some information about the chemical properties of the Earth's unobserved interior can be obtained by the study of the motion of earthquake waves and by the Earth's magnetic field, which originates in the interior (see below Geochemical distribution of the elements (chemical element)).

       Chemical elementsUntil recently, more was known about element abundances in distant stars than in the Earth's nearest neighbour, the Moon. The lunar landings have provided samples that have been intensively analyzed in many laboratories throughout the world. The data for the Apollo 11 material, collected in the Sea of Tranquility (Mare Tranquillitatis), are given in the Table (Chemical elements). Analyses of Apollo 12 collections are similar for most of the elements. Comparison of the analytical data with those for carbonaceous chondrites (a type of meteorite that provides a good average sample of nonvolatile solar system material) shows that the lunar material has undergone marked geochemical fractionation (segregation of elements). Meteorites (meteorite) suffer from heating in the Earth's atmosphere, so that what is found on Earth is not necessarily the original chemical composition of the meteorites, especially for the volatiles, light gases that are easily lost. When allowance is made for the loss of volatile light gases and for effects of chemical separation, it seems quite possible that the overall chemical composition of the Earth, Moon, Sun, and the meteorites is essentially the same and that they have a common origin.

      If elemental abundances are the same in the Earth and stars, isotopic abundances are likely to be the same. Theories predict the relative production of the different isotopes, and it is desirable to be able to compare these with observation. The study of terrestrial abundances of radioactive elements yields information about the age of the solar system, which is discussed below.

Roger John Tayler Ed.

Summary of observations

      The chemical composition of all objects in the universe is not quite the same, and not all elements can be observed in any one object, even if they are present. Nevertheless, the compositions of many objects are sufficiently similar to make it worthwhile to try to construct a typical table of abundances. Such compilations have been made by several authors and the best known is the work of the American physicists Hans Suess and Harold Urey. Although it dates from 1956, and later compilations differ in some details, its general character is not in dispute.

      The main properties shown in the abundance table are quite clear. hydrogen and helium are much more common than all of the other elements. There is a gradual decline toward higher atomic number with a great underabundance of lithium, beryllium, and boron. There is a significant peak in the region of iron, the element with the highest fractional binding energy, and the decline continues to higher atomic number with some subsidiary peaks. These peaks are associated with nuclei containing 50, 82, or 126 neutrons; the theory of nuclear structure predicts that these nuclei should be particularly stable, and these numbers are known as “magic” numbers (magic number).

Processes producing heavier elements

      As mentioned above, energy can be released by either nuclear fusion or fission reactions and there will be a tendency for material to be gradually converted into elements with maximum binding energy. As observations suggest that hydrogen and helium are much more abundant than other elements, and there is an abundance peak near iron, it is generally supposed that heavy elements have been built up from light elements. In addition, some sites in which element transmutations (transmutation) can occur are known; for example, the interiors of stars tend to get hotter as they evolve, and a succession of nuclear reactions provides the energy that they radiate. Whether or not stars are the site of major nucleosynthesis, some nucleosynthesis certainly occurs there.

      Atomic nuclei interact through two strong forces (strong force). Because they have positive electric charges, they repel one another, but there is also a very short-range strong nuclear interaction that is attractive. This may cause fusion reactions to occur if the nuclei ever approach close enough for it to be operative. To overcome the electrical repulsion, the particles must be moving rapidly, as they will be if the material is at a high temperature. The overcoming of the electrical repulsion leads to what are known as thermonuclear reactions (thermonuclear reaction). Heavy nuclei have higher electric charges than light nuclei, and a higher temperature is required for reactions between them. The rate of thermonuclear reactions depends on density as well as temperature, but the temperature dependence is much more critical.

Reaction stages reflecting increasing temperature

      If one imagines a mixture of light elements gradually heated up, a succession of nuclear reactions occurs that is described below.

hydrogen burning

      Hydrogen is converted into helium by a succession of nuclear reactions that change four protons into a helium nucleus, two positrons, and two neutrinos. (A positron is a particle like an electron but with a positive charge; a neutrino is a particle with no charge and negligible mass.) Two different reaction chains exist. In the proton–proton (proton-proton cycle) chain the helium nucleus is built up directly from protons. In another series of reactions that involve carbon and nitrogen, called the carbon–nitrogen cycle (carbon cycle), the nuclei of carbon and nitrogen are used as catalysts to transform hydrogen into helium; protons are successively added to carbon or nitrogen until a helium nucleus can be emitted by them and the original carbon or nitrogen nucleus reproduced. Both of these reactions occur at temperatures of about 10,000,000 to 20,000,000 K (10,000,000 K is approximately 18,000,000° F).

helium burning

      At temperatures of about 100,000,000 to 200,000,000 K (1 to 2 × 10⁸ K), three helium nuclei can fuse to form carbon. This reaction takes place in the following way: two helium nuclei combine to form an unstable isotope of beryllium, which has an extremely short life; rarely, a third helium nucleus can be added to form carbon before the beryllium decays. Subsequently, a fourth helium nucleus may combine with carbon to give oxygen. The relative amounts of carbon and oxygen produced depend on the temperature and density at which helium is burned.

Carbon and oxygen burning

      At temperatures between 5 × 10⁸ K and 10⁹ K, pairs of carbon and oxygen nuclei can fuse to produce such elements as magnesium, sodium, silicon, and sulfur.

Silicon burning

      Further heating of the material leads to a complicated set of nuclear reactions whereby the elements produced in carbon and oxygen burning are gradually converted into the elements of maximum fractional binding energy; e.g., chromium, manganese, iron, cobalt, and nickel. These reactions have collectively been given the name silicon burning because an important part of the process is the breaking down of silicon nuclei into helium nuclei, which are added in turn to other silicon nuclei to produce the elements noted above.

Reversible nuclear reaction equilibrium

      Finally, at temperatures around 4 × 10⁹ K, an approximation to nuclear statistical equilibrium may be reached. At this stage, although nuclear reactions continue to occur, each nuclear reaction and its inverse occur equally rapidly, and there is no further overall change of chemical composition. Thus, the gradual production of heavy elements by nuclear fusion reactions is balanced by disintegrations, and the buildup process effectively ceases once the material is predominantly in the form of iron and its neighbouring elements of the periodic table. Indeed, if further heating occurs, a conversion of heavy nuclei to light nuclei follows in much the same way as occurs in the ionization of atoms when they are heated up. The elements heavier than iron cannot be produced by fusion reactions between light elements; an input of energy is required to produce them.

neutron capture

      It is believed that these heavier elements, and some isotopes of lighter elements, have been produced by successive capture of neutrons. Two processes of neutron capture may be distinguished: the r -process, rapid neutron capture; and the s -process, slow neutron capture. If neutrons are added to a stable nucleus, it is not long before the product nucleus becomes unstable and the neutron is converted into a proton. Outside a nucleus, a neutron decays into a proton and an electron by a process called beta decay (β-decay). Inside a nucleus it can be stable if the nucleus does not contain too many neutrons. In slow neutron capture, neutrons are added at a rate such that whenever an unstable nucleus is formed, it beta-decays before another neutron can be added. If neutrons can be added more rapidly, as in the r -process, the unstable nuclei formed cannot decay before additional neutrons are added until a nucleus is eventually produced that will not accept a further neutron. This nucleus, however, will eventually be subject to beta decay, thus permitting further neutron capture.

      It can be imagined that neutron capture could proceed at an arbitrary rate, giving a mixture of the two processes, but, when the possible sites where neutron-capture reactions could take place are considered, it appears that a fairly clean-cut division between the two processes can be made. If the neutron capture occurs during a quiet stage of stellar evolution, there will be ample time for beta decays to occur, and an s -process will result. If neutron capture occurs in an explosive situation, the time scale will be so short that the reaction will have to be an r -process. The r -process produces the most neutron-rich isotopes of the heavy elements, while those isotopes produced by the s -process tend to have relatively more protons. The naturally radioactive nuclei are produced by the r -process. The neutron-capture processes appear to give a simple explanation of the magic-number abundance peaks mentioned earlier.

      Two small groups of nuclei are not readily fitted into either the sequence of nuclear fusion reactions or the neutron-capture processes. These are nuclei with very low relative abundances. One group consists of the light-nuclei lithium, beryllium, and boron, together with the heavy stable isotope of hydrogen, deuterium. These nuclei are destroyed by nuclear fusion reactions at temperatures lower than that needed to convert hydrogen into helium, and they are bypassed by the production of carbon from helium. The other group consists of the most proton-rich isotopes of some heavy elements, which cannot be produced by the addition of neutrons. Two rather rare or inefficient processes would suffice to produce these isotopes, but there is no complete agreement about what these processes are. It has been suggested that the heavy, proton-rich isotopes might be produced by a process of proton capture and that lithium, beryllium, and boron have been produced by the breakdown of heavier nuclei. A recent suggestion is that they are produced in interstellar space by collisions between cosmic-ray protons and interstellar carbon, nitrogen, and oxygen.

Regions of element synthesis

      A discussion of how the present chemical composition of the universe has arisen brings to light two distinct questions: what was the initial chemical composition and what alterations have occurred since creation. Ideally, by working backwards, the initial composition can be deduced from the present composition and a life-history, but this approach is overambitious. The initial composition predicted by simple cosmological theory can then be tested for compatibility with present observations. Element production in the universe as a whole can be discussed first; production in stars and other objects in the Galaxy is treated in the sections that follow.

Element production in the universe (Cosmos) as a whole

      Hydrogen and helium are overwhelmingly the most abundant elements in the objects of which there is direct knowledge, and, as some buildup of heavy elements occurs in stars, the working hypothesis is usually adopted that the initially created matter contained only light elements. Theories differ only as to whether the initial composition was pure hydrogen or whether there was also a substantial amount of helium.

      It is usual to try to devise the simplest theory that can account for the small number of reliable observations of agreed cosmological significance. This need not imply that the universe is simple, but, rather, that it is easiest to work out the consequences of simple theories and there is a better chance of discovering contradictions before too much time has been wasted on detail. The simplest assumption, made in several theories, about the initial chemical composition is that it was pure hydrogen, the hope being that it can be shown that all other elements have been synthesized during the lifetime of the universe.

      In the steady-state theory, in which matter is assumed to be continuously created, the initial composition is arbitrary, although supporters of the steady-state theory have always hoped that production of pure hydrogen would give a consistent picture of the universe. In the big-bang (big-bang model) theory, which has received much attention in recent years, there is considerably less freedom of choice. Observations of distant galaxies suggest that the universe is expanding and, if this is so, that galaxies may have been very close together about 10,000,000,000 years ago. In the big-bang theory it is assumed that the universe was created at that time and, in the simplest version, that at its creation the universe was very hot as well as very dense. Nuclear reactions in the early stages of the expansion lead to a rather well-defined initial chemical composition for the universe.

      Although there are more complicated big-bang theories than the one just mentioned, there is only one arbitrary quantity in the simplest theory, and assignment of some arbitrary specified value to any one of the unknowns will determine the others. If, for example, the density of the universe is specified at some temperature (perhaps 10¹⁰ K), the subsequent life history of the universe is, in principle, determined. For a wide range of values of this density, the chemical composition after the initial phase is a mixture of hydrogen and helium, with between 20 percent and 30 percent by mass in the form of helium. Most study in recent years has been concerned with trying to decide whether an initial chemical composition devoid of helium or one with 25 percent helium is most likely to be consistent with present observations.

      There are two particular reasons why interest in the big-bang theory has recently been stimulated. The first is concerned with the observed helium content of objects in the Galaxy. It is not always easy to estimate the helium abundance in a star or gas cloud, but most estimates have indicated helium abundances greater than 25 percent by mass. Such values would fit in well with most of the helium being primeval and a small admixture having been produced in stars in the galactic lifetime. The second reason for interest in the big-bang theory is the discovery that very short radio (radio source) waves, microwaves, are observed to be reaching the Earth from all directions in space. According to the big-bang theory, the universe was filled with radiation in its early stages and most of this radiation has never subsequently been absorbed. As the universe has expanded, the radiation has been shifted toward longer wavelengths by the Doppler effect, a change in wavelength brought about by motion of the source with respect to the observer. As a result of this effect, the radiation created by the big bang would be expected to appear today as microwaves of just the type that have been observed.

      The big-bang theory not only predicts that all objects, except those in which the helium can have been destroyed, should have a minimum of about 25 percent helium but that the microwave radiation should have a particular distribution with frequency known as the Planck form. The theory can be proved false either if objects are discovered with a substantially smaller amount of helium than 25 percent or if the microwave spectrum does not have the Planck form. Several observations have been made which suggest that the theory is false, but to date none of them appears to be conclusive.

Element production in stars

      A substantial amount of nucleosynthesis must have occurred in stars. It was stated above that a succession of nuclear fusion reactions takes place as the temperature of the stellar material rises. Theories of stellar evolution indicate that the internal temperatures of stars first rise during their life history and eventually fall after reaching a maximum value. For very low-mass stars, the maximum temperature may be too low for any significant nuclear reactions to occur, but for stars as massive as the Sun or greater, most of the sequence of nuclear fusion reactions described above can occur. Moreover, a time scale for stellar evolution is derived in theories of stellar evolution that show that stars substantially more massive than the Sun can have completed their active life history in a time short compared with the age of the universe derived from the big-bang cosmological theory.

      This result implies that stars more massive than the Sun, which were formed very early in the life history of the Galaxy, could have produced some of the heavy elements that are seen today but that stars much less massive than the Sun could have played no part in this production. Unless the Galaxy is very much older than is generally believed, such low mass stars, even if formed with the Galaxy, would still be at an early stage in their evolution because changes within them proceed at a relatively slow pace. If there has been substantial heavy-element production in stars, a sufficient fraction of the earliest stars formed must have been relatively massive.

      If substantial nucleosynthesis has occurred in stars, could such a process have produced all of the heavy elements that are observed today and possibly all of the helium inside the stars? A vital point is the following: if the heavy elements produced in stars are to influence what is observed, they must be expelled from the interiors of the stars in which they are produced and incorporated into future generations of stars, in which they can be observed subsequently. Unfortunately, direct knowledge of mass loss from stars is fragmentary; steady loss of mass is observed in some stars, and a few are observed to explode catastrophically, as in the explosion of a supernova. At present it is only possible for a very rough estimate to be made of the rate of exchange of matter between stars and the interstellar medium.

      Supernovae are believed to be stars reaching the end of their evolution, and many astronomers believe that a supernova explosion is the main process whereby heavy elements produced inside stars are returned to the interstellar medium. In addition, because a supernova explosion is the most violent type of event regularly observed in galaxies, it is believed that cosmic rays must also be produced in the explosion. Some rough estimates follow. The mass of the Galaxy is believed to be between 10¹¹ and 2 × 10¹¹ solar masses, and perhaps 2 × 10⁹ solar masses are heavy elements. If these heavy elements were produced steadily in a galactic lifetime of about 10¹⁰ years, one-fifth of a solar mass of heavy elements must have been produced each year. Counts of supernovae in nearby galaxies suggest that there might be one supernova explosion per large galaxy about every 30 years. If all the heavy elements are produced in supernovae, about six solar masses are required from each explosion. Although these numbers are very uncertain, this amount seems too large, but it could be reduced if the frequency of supernovae is very much higher in young galaxies. The possibility remains that a significant quantity of heavy elements may be produced by a very large number of less spectacular stars or by much more massive objects that are mentioned below.

      If there has been a gradual production of heavy elements, recently formed stars should contain more than old stars. It is possible to identify some stars which have formed quite recently. The light output of stars rises as a rather high power of their mass according to a mass–luminosity relation that is valid for the vast majority of stars whose masses are known, while their supply of nuclear energy is only directly proportional to the mass. This means that the more massive stars complete their life history much more rapidly than low-mass stars and that the brightest stars observed today cannot be more than a few million years old at the most. The heavy-element content of the young stars is greater than that of many old stars, perhaps because of a gradual increase in the heavy-element content of the interstellar medium from which stars are formed. Observations show that only the very oldest stars have an extremely small amount of very heavy elements in their visible layers, and it appears that element production must have been much more rapid when the Galaxy was young than it is now. There may indeed have been a much higher frequency of supernovae. Recent observations suggest also that chemical composition is a function of a star's place of origin as well as its age. In particular, the production of heavy elements may have been higher near the centre of the Galaxy than elsewhere (see below Element production in massive objects).

      Although the first nuclear reaction to occur in stars is the conversion of hydrogen into helium, all of the helium that is observed today can hardly have been produced in ordinary stars, the more so if all objects contain more than about 25 percent helium by mass. Considering the relative amounts of helium and heavier elements, observations indicate that the total mass of helium may be ten times greater than that of the heavier elements; if all elements other than hydrogen have been produced in stars, the relative production of helium and heavier elements must have just this value. As stars evolve, however, the conversion of hydrogen into helium is followed by the conversion of helium into heavier elements. At all stages in a star's evolution there will be a region where the temperature is suitable for the conversion of hydrogen into helium, but it appears that there will be only a thin shell of helium separating the regions in which hydrogen has not yet been converted into helium and the region where helium has been burned into heavy elements. The possible chemical composition of a highly evolved star is a series of layers of different chemical composition. The central region would contain elements such as iron and nickel with layers of successively lighter elements surrounding it and the outermost layer containing essentially only hydrogen or hydrogen and helium. A very special type of mass loss would be required to expel 10 times as much helium as heavy elements from these different layers into interstellar space.

      It is also difficult to see how the full amount of helium could have been produced. If a quarter of the galactic mass, originally hydrogen, has been converted into helium, it can be shown that essentially all of the mass must have passed through at least one generation of massive stars. The total energy release under such a circumstance would imply that the Galaxy was very much more luminous in the past—one hundred times more luminous for the first 10 percent of its lifetime, for example.

Element production in massive objects

      Although there is no direct evidence for the existence of stars (star) more than about 50 times as massive as the Sun, there is no obvious reason why much more massive objects should not exist. If they were sufficiently massive, they would not behave as ordinary stars because their gravitational attraction would be so strong that not even the energy released by conversion of hydrogen into helium would prevent such supermassive stars from continuing to collapse rapidly. According to present theoretical ideas, if such a collapse is spherically symmetrical, nothing can prevent the supermassive object from collapsing to an extremely high or infinite density; but, if it is asymmetrical—because it is, for example, rapidly rotating—there is some possibility that the catastrophic collapse, called an implosion, might be followed by explosion. At the high-density, high-temperature phase of such an object, some nucleosynthesis (manufacture of nuclei from smaller nuclei) would occur, primarily of helium but with a small amount of heavier elements according to the arguments given early in this article. Such objects have been suggested as a possible important source of helium.

      There is some observational evidence that explosions on a very much greater scale than single supernovae are occurring in galaxies. In some peculiar galaxies that are strong emitters of radio waves, there is evidence that explosions have thrown a large quantity of gas hundreds of thousands of light-years into intergalactic space. Such galactic explosions may not be related to the theoretical supermassive objects mentioned above, but it is difficult to believe that some nucleosynthesis does not take place during the phases of extreme conditions that must occur in such objects. The suggestion that heavy-element abundances may be higher near the centre of the Galaxy could be related to a past explosion there.

Radioactive chronologies

      Radioactive elements in the Earth, the Moon, and in meteorites can provide useful information about the ages of these objects and about the dates of formation of the heavy elements themselves. The elements uranium and thorium (uranium-thorium-lead dating) gradually decay into lead, different isotopes of lead arising from the various isotopes of uranium and thorium; some isotopes of lead are, however, not produced by any radioactive decay process. When the rocks of the Moon or the Earth's crust or the meteorites solidified, further chemical separation of the radioactive elements and their decay products was prevented. By studying the relative amounts of the radioactive isotopes and their decay products, it is possible to obtain an estimate of when the rocks solidified. Estimates can also be made using radioactive isotopes other than uranium and thorium.

      The results of these discussions indicate that the meteorites, or at least the parent body of the meteorites, solidified between 4.5 × 10⁹ and 4.6 × 10⁹ years ago. It is possible to speak with such confidence of this age because two isotopes of uranium and one of thorium have very different decay times that bracket that value. There is no unique age for the rocks of the Earth's crust because there has been considerable volcanic activity during the Earth's history and rocks have solidified at all stages. All indications are that the oldest rocks have ages of the same order as the ages of the parent bodies of the meteorites. Only a very small region of the Moon's (Moon) surface has been studied so far, but it has been found to have very old rocks of age up to about 4.5 × 10⁹ years. No conclusions can be drawn about the date of solidification of the Moon from these few observations, as nothing is known about its past geological history, but they are certainly not inconsistent with the view that the Earth, the Moon, and meteorites have a similar age and origin.

      It has also been found possible to obtain information about the time of formation of the radioactive (radioactive isotope) elements. Assuming that both radioactive nuclei and their stable neighbours are produced by the neutron-capture process discussed earlier, theory predicts a relative production rate for all of the nuclei. The radioactive nuclei can be divided into three groups: short-lived, medium-lived, and long-lived, where short-lived means considerably less than the believed age of the universe and long-lived means comparable with that age. If radioactive nuclei are produced and decay steadily, then at some point in time the total amount of a short-lived isotope reaches a steady value. In meteorites, one can study the decay products of such short-lived nuclei and can discover their abundance when the meteorites were formed. This amount is lower than the expected value, suggesting that nucleosynthesis ceased in the solar system material about 2 × 10⁸ years before the meteorites and planets solidified.

      Study of the decay products of nuclei with medium decay rates indicates that their abundance is higher than if nucleosynthesis has occurred at a constant rate throughout galactic history. This suggests that the solar system material was significantly enriched in heavy elements shortly before the cessation of nucleosynthesis—that is, before the Sun and planets were formed. Finally, the very long-lived isotopes give information about the total time scale of nucleosynthesis that is not inconsistent with the galactic age estimated by other methods.

      Although there is not unanimous agreement concerning these results, it appears that it is, in principle, possible to obtain a considerable amount of information about the past rate of nucleosynthesis and possibly about the types of objects in which it has occurred. In particular, it may eventually be possible to decide whether most element production has occurred in a large number of supernovae or in a much smaller number of massive objects.

Roger John Tayler

Geochemical distribution of the elements

      Knowledge of the geochemical distribution of elements involves elucidation of the relative and absolute abundances of the chemical elements in the Earth and in its various parts—the crust, interior, atmosphere, and hydrosphere. This comprises a major part of the science of geochemistry, which is the study of the distribution of the chemical elements in space and time and the laws governing this distribution. Basic knowledge in this area was largely accumulated during the 19th century. As noted above, the concept of a limited number of chemical elements had been established by 1800, and the appearance of the periodic table, in 1869, provided a new insight into the limitations on the number of elements. Concurrent with these advances in chemical understanding, from about 1850 onward there was a steadily increasing output of analytical data on the Earth's rocks, minerals, and waters, mainly from laboratories in Europe and North America. The output from North America was materially increased following the establishment of the United States Geological Survey in 1879 and the appointment of Frank W. Clarke as chief chemist in 1884.

      Clarke's name will always be linked with the study of the geochemical distribution of the elements—indeed, the term clarke was proposed as the unit for the average percentage of an element in the Earth's crust by Soviet scientists and has been generally adopted. In 1889 Clarke wrote the first of his many publications on the geochemical distribution of the elements. He assembled many chemical analyses of rocks from different continents, calculated average values, and showed that the overall chemical compositions of continental areas are remarkably similar. By combining these averages he obtained values for the abundances of the commoner elements in the continental crust of the Earth, values that have not been materially changed in spite of the vast increase of available data since that time. He also estimated abundances for many of the less common elements; these estimates were based in many instances on very limited and imprecise data and subsequently have been improved.

      A further development of great significance was the assemblage of comprehensive data on the abundances of individual elements in terrestrial materials and in the Cosmos (based on solar and meteorite abundances) by the Norwegian geochemist Victor Moritz Goldschmidt (Goldschmidt, Victor Moritz) during the 1930s. Goldschmidt's tables provided the basis for modern research on the geochemical distribution of the elements, and his compilation of data on cosmic abundances was the key to later theories on element synthesis in stars and supernovae.

      Goldschmidt also contributed to the understanding of elemental distribution within the Earth through his geochemical classification of the elements into lithophile, siderophile, chalcophile, and atmophile. Lithophile elements are those with a strong affinity for oxygen; they are concentrated in the crust or lithosphere as silicate and oxide minerals. Siderophile elements are principally metals that alloy readily with iron; Goldschmidt explained their scarcity in the Earth's crust by their concentration in the nickel–iron core (the siderosphere). Chalcophile elements are those with strong affinity for sulfur; they occur mainly as sulfides. And atmophile elements are gases, such as nitrogen, argon, and other rare gases, which are unreactive and hence accumulate in the atmosphere. (Goldschmidt also proposed a group of biophile elements, for those that concentrate in living matter—essentially carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorus.)

Terrestrial distribution

      The study of earthquake waves passing through the body of the Earth has shown that the interior is not uniform; it consists of distinct shells separated by concentric discontinuities at which the velocities of the passing waves change. The two major discontinuities that are universally recognized are the Mohorovičić Discontinuity, which divides the Earth's crust from its underlying mantle, and the Wiechert–Gutenberg Discontinuity, which separates the mantle from the core. The latter discontinuity exists at a depth of 2,900 kilometres (1,800 miles); it is marked by a sudden increase in density, from about 5.7 at the base of the mantle to 9.7 at the top of the core. The only reasonable interpretation of this discontinuity is that the mantle consists of silicates and oxides of the common elements (largely magnesium and iron), and the core consists of metallic iron alloyed with minor amounts of other elements (analogous to the nickel-iron in meteorites). The Mohorovičić Discontinuity varies in depth from place to place; it averages about 33 kilometres (20 miles) below the continents and about 8 kilometres (5 miles) below the bottom of the deep oceans. It too is marked by a density increase from crust to mantle—a comparatively small one, from about 3 to 3.3.

      To the three spherical divisions—crust, mantle, and core—two more should be added: the hydrosphere, which is the discontinuous shell of fresh and salt water, on and within the crust; and the atmosphere, the ocean of air that surrounds the Earth, gradually thinning into the vacuum of outer space.

The Earth's core

      The evidence for the composition of the core is all indirect because no means have yet been devised for directly sampling the deep interior of the Earth. The moment of inertia of the Earth indicates that there is a concentration of mass around the centre, and seismic data have shown that below the Wiechert–Gutenberg Discontinuity the density of the material is high, ranging upwards from 9.7. The only heavy element with high cosmic abundance is iron, and because an iron–nickel alloy is an important meteorite component, it is reasonable to conclude that the Earth's core consists largely of metallic iron with a minor admixture of other elements. This conclusion is supported by geophysical evidence that indicates that the mean atomic number of the material of the core is about 22. The atomic number of iron is 26, so this implies that the core also contains elements of lower atomic number. Sulfur, with atomic number 16, and carbon, 6, are relatively abundant in meteoritic matter, and the presence of minor amounts of these elements in the core would effectively reduce the mean atomic number. Some authorities have advocated silicon (atomic number 14) as the major alloying component in the core, but this seems less likely; if silicon were the sole alloying element, then the core would have to contain more than 30 percent silicon in order to reduce its mean atomic number to 22. In addition, free silicon requires extremely reducing conditions (lack of oxygen), and the presence of ferrous iron in the mantle is inconsistent with this requirement.

      It is not possible to give definite figures for the abundances of the elements in the Earth's core. It is certainly made up largely of metallic iron, however, probably with some nickel, a little cobalt, and appreciable amounts of such lighter elements as carbon and sulfur.

The Earth's mantle

      The mantle comprises that part of the Earth between the Mohorovičić and the Wiechert–Gutenberg discontinuities. It makes up 83 percent of the volume of the Earth and 67 percent of its mass and is thus of decisive importance in determining the bulk composition of the planet. In estimating elemental abundances in the mantle, however, the same difficulty as with the core arises: direct sampling is not feasible. Much more geophysical data are available for the mantle, however, and some volcanic eruptions have brought rock fragments to the surface that have certainly been derived from this zone. The most remarkable of these materials are the diamond-bearing (diamond) inclusions found in the famous pipes, or volcanic necks, that are mined in South Africa and Siberia. The presence of diamond, the high-pressure form of carbon, implies a depth of origin of at least 100 kilometres (62 miles), but these inclusions are rare. The common type of mantle-derived inclusion is peridotite, a silicate rock consisting largely of olivine, (Mg,Fe)₂SiO₄, with minor amounts of orthopyroxene, (Mg,Fe)SiO₃, and diopside, CaMg(Si₂O₆).

      Geophysical information indicates that below a depth of about 1,000 kilometres (620 miles), the mantle behaves as an essentially homogeneous material, but above this level its physical properties are more varied, and there is evidence for second-order discontinuities. This region above 1,000 kilometres is frequently referred to as the upper mantle, and in recent years has been the object of a concentrated research effort by geologists and geophysicists all over the world. The significance of the upper mantle is that processes originating there have dramatic effects on the surface—in the form of volcanic eruptions and some earthquakes—and less dramatic but equally important effects within the crust, such as the introduction and concentration of some elements, possibly leading to the formation of ore deposits. Increased knowledge of the upper mantle thus has both scientific and economic appeal.

      Geophysical data on the properties of the upper mantle suggest that it must consist essentially of magnesium-iron silicates, probably largely olivine in the region immediately below the crust. Olivine is not stable under very high pressures, however; it is converted to a different phase of about 10 percent higher density and with a structure like the mineral oxide spinel (MgA1₂O₄). This conversion would occur in the mantle at depths of around 400 kilometres, and a second-order discontinuity at that depth can plausibly be ascribed to this conversion. Pyroxenes also undergo transformations to phases of greater density at the high pressures within the mantle. Thus the mantle, although composed of material of familiar chemical composition, consists, in its lower part at least, of different minerals than those in the upper part.

      Many estimates of the composition of the upper mantle have been made in recent years. On the whole, the similarities are more important than the differences. All agree that the principal components are oxides of silicon, magnesium, and iron. The differences are mainly in the minor components such as aluminum oxide, calcium oxide, and the alkalies, and are determined largely by theoretical considerations and the weight given to specific aspects of the geophysical and geochemical data.

      Although fairly reliable estimates exist for the abundances of the major elements in the mantle, little is known of minor and trace elements. Knowledge of the crystal structure of possible mantle minerals indicates that many minor and trace elements will not be readily incorporated, however. They are therefore likely to concentrate in liquid material in the mantle and be carried upward in solution, eventually being transported into the crust. It is thus probable that the mantle is relatively depleted, and the crust relatively enriched, in minor and trace elements. This is certainly true for uranium and thorium, because the amount of these elements in the crust is almost sufficient to account for the total amount of heat flowing out of the Earth.

The Earth's crust

      The crust is a comparatively thin shell on the surface of the Earth and makes up less than 1 percent of its total mass. Its geochemical significance is only marginally related to its bulk, however. It has been subjected to extensive investigations, and it provides the raw materials on which civilization depends. It is the most diverse of the geospheres, being a complex mosaic of many rock types—igneous, sedimentary, and metamorphic—each with a wide variety of chemical and mineralogical compositions. The surface is veneered with soils, related in composition to the rocks from which they formed, but with important modifications because of the smaller grain size, the presence of organic matter, and an intricate complex of living organisms. Ultimately, man's welfare and indeed his survival depends on the wise utilization of the materials in the crust. Modern civilization has been erected upon the exploitation of fuels and ore deposits, which are simply geochemical concentrations of useful elements.

Igneous rocks (igneous rock)

      Clarke estimated that 95 percent of crustal rocks are of igneous origin (formed from molten silicate masses, or magmas). Sedimentary rocks occur as a thin veneer on an igneous or metamorphic basement, except where locally thickened in mountain belts. The primordial rocks of the crust must have been essentially igneous, and the first sedimentary rocks were derived from them by processes of weathering and erosion. Metamorphic rocks are formed from both sedimentary and igneous rocks by transformations due to heat and pressure at depth in the crust; unless very intense, these transformations do not totally obliterate the primary igneous or sedimentary features.

Major components

      Igneous rocks show a wide range of composition; the principal component, silica (SiO₂), ranges from about 35 percent to 80 percent among the commoner igneous rocks, and other components also show a wide variation. They thus illustrate some quite extensive geochemical fractionations of the elements, the fractionations that may have economic significance if they bring about the findings of workable ore deposits.

       Chemical elementsIn 1924 a comprehensive review of igneous rock composition based on compilation of over 5,000 superior analyses was published. This was in many respects the ultimate refinement of Clarke's initial review of 1889. It confirmed that the averages of analyses from different continental areas are essentially identical. It also revealed significant geochemical differences between the continental and oceanic crusts. The average of igneous rock analyses from the oceanic islands is notably lower in silica and alkalies, and higher in magnesium and calcium oxides, than the continental averages. This is simply a reflection of the fact that most oceanic islands, such as Hawaii, consist almost entirely of basalts (averaging about 50 percent silica), whereas continental areas include large granitic masses, with silica contents around 70 percent. In terms of volumes, igneous rocks consist predominantly of two great types, granitic and basaltic. The former essentially are confined to the continents and the latter occur in both continents and ocean basins. The other types of igneous rocks, while many and varied, are quantitatively insignificant and hardly affect the averages. Thus, for the major elements, the average of over 5,000 analyses of igneous rocks is not significantly different from the simple average of two individual rocks, a granite (G-1) and a basalt (W-1). This can be seen from the Table (Chemical elements), by comparing G-1 and W-1 values with those in the column headed “Earth's crust.”

       Chemical elements Chemical elementsThe figures for the specific granitic (G-1) and basaltic (W-1) rocks are included in the Table (Chemical elements) because they have been analyzed for practically all the elements in different geochemical laboratories throughout the world. The rock G-1 was a granite from Westerly, Rhode Island, and W-1 was a basaltic rock (specifically a diabase) from Centerville, Virginia. Several hundred kilograms of each of these rocks were crushed to a fine powder in the laboratories of the U.S. Geological Survey and samples distributed to analytical laboratories throughout the world, in order to obtain as many analyses as possible. The results were then critically examined, and the figures given in the Table (Chemical elements) are considered to be the best available in terms of accuracy and precision. G-1 and W-1 are undoubtedly the most thoroughly analyzed rocks and now serve as basic geochemical standards. It must be borne in mind that they are individual rocks, however, and cannot be considered to be averages of all granites and all basalts. Indeed, it is clear that G-1 is unusually rich in some trace elements (e.g., thorium).

      Perhaps the most significant feature of the composition of the Earth's crust is that it is dominated by comparatively few elements. Only eight—oxygen, silicon, aluminum, iron, calcium, magnesium, sodium, and potassium—are present in amounts greater than 1 percent, and these eight make up almost 99 percent of the whole. Of these, oxygen comprises almost 50 percent by weight. The dominance of oxygen is even more marked when weight percentages are converted to atomic percentages, as follows: oxygen (weight percentage, 46.6; atomic percentage, 62.2), silicon (27.7; 21.2), aluminum (8.13; 6.47), iron (5.00; 1.92), calcium (3.63; 1.94), magnesium (2.09; 1.84), sodium (2.83; 2.64), potassium (2.59; 1.42). This comparison, of course, merely emphasizes the fact that the crust consists almost entirely of oxygen compounds, mostly silicates and aluminosilicates of iron, calcium, magnesium, and the alkali metals. As Goldschmidt remarked, the lithosphere may well be called the oxysphere. Clarke and his collaborators calculated that the average mineralogical composition of igneous rocks is: quartz 12.0 percent, feldspars 59.5 percent, pyroxene and hornblende 16.8 percent; biotite 3.8 percent, titanium minerals 1.5 percent, apatite 0.6 percent, and other accessory minerals 5.8 percent.

Elements of minor and trace abundance

  Chemical elementsSo far only the major elements have been discussed, those present in amounts greater than 1 percent by weight. Consideration must now be given to the minor and trace elements. The data are given in the Table (Chemical elements), and are presented diagrammatically in theFigure—.

      A cursory examination immediately reveals some intriguing features in the abundance pattern. The predominance of the even-numbered elements over the neighbouring odd-numbered ones is still apparent but not so regular as in the cosmic abundances. Chemical fractionations taking place during the evolution of igneous rocks from primordial matter have clearly modified this basic relationship. The odd–even relationship is most prominent in the rare-earth elements (atomic numbers 57–71), which are chemically so similar that they are little fractionated by geochemical processes.

      A particularly noteworthy feature is that some unfamiliar elements, such as rubidium, are relatively abundant, whereas others, such as most of the industrial metals except iron and aluminum, are actually of very low abundance. Thus boron, familiar to every homemaker in the form of borax cleansers and boric-acid antiseptics and well-known in the ancient world, is an element of extremely low abundance, much lower, for example, than zirconium, and lower even than hafnium, an element first discovered as late as 1923. Mercury, another element known to the ancient Greeks and Romans, is also of extremely low abundance. On the other hand, the so-called rare-earth elements are not extremely rare. Evidently one must make a clear distinction between abundance and availability. Some elements may be of low abundance but are readily available, because geochemical processes result in their concentration in specific deposits which can be commercially exploited. Other elements that are relatively abundant may be widely dispersed in small amounts and seldom or never occur in concentrated large deposits. A typical example is titanium (titanium processing), which is present in practically all igneous rocks in amounts ranging up to several percent, but which, in spite of its useful properties as a metal, is still not widely used. This is partly because of its dispersed nature and partly because of the technical difficulties in extracting the element from the minerals in which it occurs.

      The distribution of minor and trace elements in igneous rocks, for those of lithophile affinity, is largely controlled by their ionic radii or size. Minor and trace elements with radii similar to those of major elements can substitute for these elements in the common minerals of the igneous rocks. The crystal structures of these minerals act as sorting mechanisms, accepting those atoms of appropriate size and rejecting others. Thus rubidium, with a radius of 1.47Å (one angstrom [Å] = 10⁻⁸ centimetres) is incorporated in potassium feldspar, KAlSi₃O₈, because its radius is close to that of potassium (1.33Å). The next higher alkali element, cesium, with a considerably larger radius (1.67Å), is not accepted into the feldspar structure; it remains in the igneous liquid during the crystallization of the major minerals until its concentration increases to such an extent that it can form the independent mineral pollucite (CsAlSi₂O₆).

      The factor of ionic size, coupled with geochemical affinity (lithophile, chalcophile, or siderophile) is a key to the distinction between abundance and availability. Elements that are similar in size and geochemical affinity to major elements are dispersed in small amounts in common minerals; i.e., rubidium in potassium feldspar, gallium in aluminum minerals, and germanium in silicate minerals. Elements that do not readily enter the common minerals of igneous rocks remain in the residual melt as crystallization proceeds. Fractional crystallization of magmas (igneous melts) normally results in a residual liquid of granitic composition. Under suitable conditions this residual liquid solidifies as a coarse-grained rock known as a pegmatite. Pegmatites are famous for their content of rare and unusual minerals, which contain many of the minor and trace elements. They are the commercial sources of lithium, beryllium, scandium, yttrium, the rare earths, cesium, niobium, and tantalum, all elements that concentrate in the residual liquid because of their specific geochemical properties.

      Chalcophile elements are all of rather low abundance, and the minerals that they form, mainly sulfides and some arsenides, are not stable at the high temperatures of igneous crystallization. Sometimes these elements are found in granites and pegmatites—molybdenite (MoS₂) is a typical example. More frequently they are removed from the crystallizing magma as hot aqueous solutions and are deposited as metalliferous veins in the surrounding rock. Sometimes they may reach the surface as components in thermal springs; mercury has been deposited (as native mercury and as cinnabar, HgS) by some of these springs, occasionally in sufficient amounts for profitable mining.

Sedimentary rocks (sedimentary rock)

      The decomposition of pre-existing rocks by weathering, the transportation and deposition of the weathering products as sediments, and the eventual formation of sedimentary rocks might be expected to produce a gross mixture of materials, thereby working against further geochemical differentiation of the elements. This is not the case; sedimentary processes frequently produce remarkable concentrations of the elements, leading to almost pure deposits of certain minerals. Some sandstones, e.g., contain over 99 percent quartz and some limestones over 99 percent calcium carbonate. The ultimate is reached in salt deposits, with extensive beds of anhydrite (CaSO₄), gypsum (CaSO₄ · 2H₂O), halite (NaCl), and other compounds. Goldschmidt compared the sedimentary process with a quantitative chemical analysis, involving the successive separation of specific elements or groups of elements.

      Quartz (SiO₂) is highly resistant to weathering and accumulates as deposits of sand. When consolidated these deposits form sandstones, an important group of sedimentary rocks. Under special conditions almost any mineral may be deposited in sand-sized grains, but most minerals are eventually decomposed by weathering. A few resistant ones may survive and be sufficiently concentrated to form economic deposits known as placers; the most familiar are probably the gold-bearing sands, important sources of this element, but sand deposits may have economic concentrations of zirconium (as the mineral zircon, ZrSiO₄), titanium (as rutile, TiO₂, and ilmenite, FeTiO₃), tin (as cassiterite, SnO₂), and others.

      The aluminosilicates of igneous rocks, mainly the feldspars, (K,Na)AlSi₃O₈ and (Na,Ca)(Al,Si)₄O₈, are relatively easily decomposed by weathering. The alkali elements and calcium are largely carried away in solution, whereas the aluminum and silicon are quickly redeposited as insoluble clay minerals. When consolidated, these minerals form shales and mudstones. The ferromagnesian minerals undergo a more complex decomposition, sometimes leading to the deposition of iron-rich sediments consisting largely of hydrated ferric oxide; such sediments are valuable iron ores in many countries.

      Calcium is carried away in solution mainly as calcium bicarbonate, Ca(HCO₃)₂. Most of it eventually reaches the sea, where it is utilized by a vast variety of organisms as skeletal material in the form of calcite and aragonite (polymorphs—different forms—of CaCO₃). Accumulation of skeletal materials after death of the organisms has formed extensive deposits of limestone throughout geological time. Magnesium in seawater can react with calcium carbonate to form dolomite, CaMg(CO₃)₂, and in this way some magnesium is removed from solution and deposited in sediments.

       Chemical elementsMuch of the magnesium, however, remains in seawater, which is essentially a dilute solution of magnesium, calcium, sodium, and potassium chlorides and sulfates, with many other elements in small amounts (see Table (Chemical elements)). Under special geological circumstances bodies of seawater can be cut off from the open ocean, and under arid conditions the water will evaporate and extensive salt beds be deposited. Such conditions have occurred in different regions throughout geological time, and the resulting salt deposits are economically important as sources of sodium, potassium, calcium, magnesium, chlorine, and sulfur.

       Chemical elementsThe three major groups of sedimentary rocks are sandstones, shales, and the carbonate rocks (limestones and dolomites). Much less geochemical research has been devoted to sedimentary rocks than to igneous rocks, and the data for their contents of minor and trace elements are therefore less extensive. The figures in the Table (Chemical elements) show that the minor and trace elements generally tend to be more concentrated in shales than in the sandstones and carbonate rocks.

      The problem of arriving at an average composition for all sedimentary rocks is still largely unresolved, largely because of uncertainty in the relative amounts of shales, sandstones, and carbonate rocks. From geochemical arguments Clarke estimated the relative percentages of these three groups as 80:15:5, respectively. Actual measurements of sedimentary rocks suggest that these figures overestimate the amount of shales and underestimate that of limestones, however. Thus, a compilation of the recorded amounts of shales, sandstones, and limestones in more than 213,000 metres (700,000 feet) of sedimentary rock formations gave relative percentages of 46:32:22, respectively. The identification of a formation as a limestone, a sandstone, or a shale, however, is likely to be gross; shales usually contain considerable sand, sandstones may carry much clay, and the term limestone is applied to many rocks with 50 percent or less of carbonate. It does appear that limestones are more prominent in the geological record than might be expected from geochemical calculations, however; this probably reflects the fact that shallow-water environments are the great places of carbonate deposition, whereas the ocean deeps are the repository primarily of clay-rich sediments.

Metamorphic rocks (metamorphic rock)

      Comparatively few investigations have been made of the elemental composition of metamorphic rocks. Many of these rocks retain the geochemical features of their parent igneous or sedimentary materials, and their bulk composition has been little changed despite complete recrystallization and the production of new minerals and structures in some instances. Some metamorphic rocks, however, have been markedly modified by the removal of some components and the addition of others.

       Chemical elementsThe Geological Survey of Canada has performed a comprehensive study of a large area of the Canadian Shield, a region of complex geology largely made up of metamorphic rocks. From a collection of more than 8,000 bedrock samples, the average abundances of all the major elements and a number of minor and trace elements were determined; the figures are given in the Table (Chemical elements). As might be anticipated, the average composition is not very different from the average composition of igneous rocks. It does show a somewhat higher silicon content, probably reflecting a preponderance of granitic over basaltic rocks and a relative abundance of quartz-rich sedimentary rocks in the original makeup of the Canadian Shield. The general validity of these abundance figures for metamorphic rocks has been confirmed by a similar study of the average composition of metamorphic rocks in the former Soviet Union, which has given closely comparable results.

Ore deposits

      An ore deposit, in its simplest terms, is a portion of the Earth's crust from which some industrial raw material can be extracted at a profit. As such, its characteristics are as much economic as geochemical. Nevertheless, its formation required the operation of geochemical processes to produce the concentration of a specific element or elements in a particular place. Economics decide whether this concentration is rated as an ore deposit or merely as a deposit of scientific interest. The economics may change with time, depending upon price, availability of transportation, cost of labour, and other factors.

       Concentration factors for ore bodies of common metalsSome general principles can, however, be enunciated. Proceeding from the average abundance of an element in the crust, and the minimum abundance that can be profitably exploited under normal circumstances, a factor of enrichment necessary to produce an ore deposit can be derived (see Table (Concentration factors for ore bodies of common metals)). The economic control is immediately evident in the approximate relation between the factor of enrichment and the price of the product sought. The most extreme example of this is in diamond mining, where the product sought may be present in the rock mined in as low a concentration as 1 part in 50,000,000. Ease of extraction, of course, plays an important role in this. Diamonds are readily separated from the great mass of waste rock by a relatively simple and inexpensive process. Magnesium is commercially extracted from seawater, where its concentration is 0.13 percent, rather than from the common rock dunite, where its concentration is about 25 percent, because of the ready availability of seawater and the cheapness of the extraction process.

      Ore deposits may be found in all types of rocks—igneous, sedimentary, and metamorphic—and seawater is also a significant source of such elements as sodium, chlorine, magnesium, and bromine. There are many processes of geochemical enrichment leading to the formation of ore deposits, and they are often the end result of a complex series of such processes acting over a long period of time. The economic importance of ore deposits has ensured their thorough study by all techniques of geological and geochemical research, but much controversy still exists regarding the origin of many of the more complex deposits.

      The most readily understood ore deposits are those of sedimentary origin. They have been formed at the surface of the Earth by processes that can usually be observed operating at the present time and that can readily be simulated in the laboratory. Salt deposits are one kind whose origin is clearly amenable to such an approach. As long ago as 1849 an Italian scientist initiated laboratory studies on the evaporation of seawater and elucidated the sequence of crystallization of the different salts. Comparison of the results with the mineralogy of salt deposits revealed gross similarities but also important differences; these differences can be explained by a variety of mild metamorphic reactions resulting from burial of these deposits under overlying sediments.

      Some sedimentary deposits are not readily explicable by such an approach, however. The most extensive and economically important are the vast Precambrian iron ore deposits, which are a major source for the hundreds of millions of tons of steel produced annually. They occur on all the continents (except perhaps Antarctica) and are uniformly of great age (about 1,900,000,000 years or older). Probably the most extensive and best exposed of these are in the Hamersley Range of Western Australia, where individual beds of iron ore are continuous over hundreds of square miles in a horizontally bedded sequence of iron ore and quartzite thousands of feet thick. The conditions that gave rise to these deposits were apparently unique to this early period in Earth history, because similar deposits are not known in younger geological formations. It has been argued that the explanation lies in an oxygen-free reducing atmosphere in early geological times, under which iron could readily be transported in solution as ferrous compounds to the ocean or large lakes, where deposition eventually took place, perhaps through the agency of primitive organisms. As soon as free oxygen appeared in the atmosphere, 1,000,000,000 to 2,000,000,000 years ago, the geochemical cycle for iron was profoundly modified, and this type of transportation and deposition ended forever.

      Processes other than fractional crystallization from igneous melts also give rise to magmatic ore deposits. Economic deposits of the oxide mineral chromite ([Fe,Mg] [Cr,Al]₂O₄), for example, occur almost entirely as bands or lenses in magnesium-rich igneous rocks. Chromite evidently crystallizes early from a magma, and, being of higher density than the liquid, it sinks to the bottom of the magma chamber and becomes concentrated as almost pure bodies of this mineral. Some accessory minerals of igneous rocks are important sources of metallic elements, but the rocks cannot be mined directly because the grade is too low. If these minerals are chemically and mechanically resistant, weathering and transportation may eventually concentrate them into workable deposits. A large proportion of the world's zirconium, hafnium, rare earths, and thorium, and some iron and titanium, come from such deposits in river and beach sands.

      A large number of important ore deposits occur in metamorphic rocks. The ultimate origin of these deposits is frequently obscured by the complex processes they have undergone. If it can be established that the enclosing metamorphic rocks were of sedimentary origin, the question then arises whether the ore material was deposited along with the sediments or was introduced by circulating solutions during the metamorphism or possibly at some later time. The answer is seldom clear-cut, and such deposits continue to excite lively controversy among geochemists and economic geologists.

mineral fuels

      The mineral fuels—coal, petroleum, and natural gas—may be described as a special type of economic deposit. Geochemically they represent the concentration of carbon and hydrogen by processes that were initially biological in nature. Coal is essentially the product of accumulation of land plants in large amounts, and petroleum and natural gas are the products of marine organisms (although the origin of some petroleum and natural gas under nonmarine conditions cannot be entirely excluded). The origin of petroleum and natural gas presents a more difficult problem than coal because they are fluids and thus are free to migrate from their place of origin.

      The formation of coal is a relatively straightforward geochemical process that can readily be traced through its successive stages. The first requirement is a geological one—the rapid accumulation of plant material under conditions that inhibit its decomposition, followed by its burial under inorganic sediments such as shales and sandstones. The great coal-forming period in the Northern Hemisphere followed the Devonian Period (345,000,000 to 395,000,000 years ago), when abundant land plants first appeared, and has been named the Carboniferous Period (280,000,000 to 345,000,000 years ago). During this period, large areas in North America and Europe were evidently low-lying swamps that supported a lush vegetation. This vegetation died, accumulated in successive layers, and was partly decomposed by bacteria and other organisms to form peat. Burial of peat deposits under inorganic sediments brought an end to the period of bacterial decomposition, and the further changes to coal were essentially a mild metamorphism caused by an increase in temperature and pressure.

      Chemically, this mild metamorphism was in large part the expulsion of carbon dioxide and water from the coal-forming substance. The main trend in the change from peat through lignite to bituminous coal and anthracite is the decrease in oxygen content and the increase in carbon. If carried to its ultimate conclusion the product would be pure carbon in the form of graphite. This occurs comparatively rarely, but evidence for it is the presence of small amounts of graphite in many metamorphic rocks.

      Coal also contains inorganic material that appears as ash when it is burned, and some coal ashes show a remarkable concentration of unusual elements. This was demonstrated by Goldschmidt in 1933, when he found appreciable amounts of germanium in some coal ashes. The Hartley Seam of the Durham Coalfield in England contains so much germanium that the ash has a brilliant yellow colour because of the presence of the oxide (GeO₂).

      The source of these minor and trace elements and their mode of incorporation in the coal are still not fully understood. There are three possibilities: (1) these elements were taken up by the plants during growth; (2) they were carried into the coal swamp as a component of the inorganic sediments; or (3) they were absorbed during or after the coal-forming processes from circulating solutions. The first possibility is not favoured, because growing plants seldom incorporate appreciable amounts of nonorganic elements. The second possibility also is unlikely, because there is no correlation, or rather an inverse correlation, between ash content and trace element concentrations. This leaves the third possibility as the most likely one. The presence of a large amount of carbonaceous matter means that the coal-forming environment is a highly reducing one, which will favour the precipitation of some elements; the presence of hydrogen sulfide and sulfide ions will cause the precipitation of chalcophile elements (with affinity for sulfur); and complex organic compounds are noted for their absorptive, or chelating, capacity for metallic ions. Thus several individual reactions are potentially available for the fixation of foreign elements in the coal substance.

      As pointed out above, the origin of petroleum is not as readily elucidated as the origin of coal, because petroleum can migrate from the region in which it was formed. Indeed, the very occurrence of a commercial oil field probably implies the concentration of the petroleum from a large volume of source rocks into a relatively much smaller reservoir.

      The fact that petroleum is almost always found in marine sedimentary rocks has long been a basic argument in favour of a marine origin for this material. It is certainly true that some oil has been found in igneous and metamorphic rocks, but migration from a sedimentary source bed is a reasonable explanation for these occurrences. Proof of a marine origin has been forthcoming in recent years by the sensitive analyses of recent marine sediments, which show that they contain small amounts of petroleum hydrocarbons, evidently generated either directly by marine organisms or by their subsequent decomposition.

      Natural petroleum is a complex mixture of hundreds of different hydrocarbons, but its bulk composition is remarkably constant, about 85 percent carbon and 15 percent hydrogen. It may include small amounts of organic compounds containing oxygen, sulfur, and nitrogen. Its content of other elements is exceedingly small. Petroleum ash, unlike coal ash, is not noted for its trace element content. Some petroleum ash contains appreciable amounts of vanadium, however, and has been utilized as a source of this element. A class of nitrogen-bearing organic compounds known as porphyrins include a metal atom in their molecular structure; usually this atom is iron, but other elements in this region of the periodic table, especially vanadium, nickel, and copper, may play a similar role. The vanadium content of some petroleum ash probably originates as a vanadium porphyrin in some of the organisms involved in petroleum formation.

      Petroleum is always accompanied by natural gas, but many natural gas fields have no petroleum associated with them. This can probably be ascribed to greater possibility for migration for a gas as compared to a liquid. It is also possible that some natural gas is generated from coal deposits. Natural gas consists largely of methane, but small amounts of more complex hydrocarbons may be present, and it may also contain unwanted components such as nitrogen, carbon dioxide, and hydrogen sulfide. Natural gas containing hydrogen sulfide is known as “sour gas” and for long was an undesirable material because of the noxious nature of this compound; recently, however, it has been found profitable to extract the hydrogen sulfide by converting it to sulfur and then utilize the hydrocarbons.

      Natural gas is the sole source of one element, helium, the industrial demand for which has steadily increased in recent years. Comparatively few occurrences of natural gas contain sufficient helium for the extraction to be commercially profitable. Currently, the Western world's need for helium is largely met by its extraction from wells in western Texas. The explanation for this local concentration of helium in some natural gas fields is still a matter for discussion; the only reasonable source is from the disintegration of radioactive elements in the crust, but the mechanism of concentration remains something of an enigma.

Soils (soil)

      Soil is a thin veneer that forms a discontinuous cover on the land areas of the Earth. Its volume and its mass are small in comparison to the major geospheres, but it is of vast importance to man. Superficially it might be considered merely as comminuted (pulverized) and decomposed bedrock; however, this viewpoint takes into account only its inorganic components and completely neglects the complex of organic compounds, living organisms, water, and included gases that gives the soil its characteristic properties and its value as the abode of life. Comminuted bedrock alone is not soil in any real sense; the Antarctic continent, where not ice-covered, has a surface of comminuted bedrock, as does the Moon, but neither material can properly be termed soil.

      Soils result from the weathering of rocks, and hence their composition might be expected to reflect the composition of the rocks from which they were formed. This is true only in a very broad sense, however. Environmental factors play an important part in soil formation. The same parent rock may give rise to very different soils under different conditions. climate, topography, vegetation, biological activity, and time are all important factors in determining the nature of a soil. Climate is probably the most important of these, as can be demonstrated by contrasting the soils developed on the same rock type under tropical and temperate conditions. In general, the soil in the humid tropics will be different in texture and composition and much less fertile, as a result of the intense leaching brought about by high rainfall, high temperatures, and the almost complete removal of organic matter by microorganisms.

      The complex of inorganic compounds, organic compounds, water, and air that makes up the soil is in a continual state of change. Water tends to dissolve and remove the relatively soluble elements such as calcium, magnesium, sodium, and potassium, and the comparatively insoluble elements—aluminum, iron, and silicon—are thereby relatively enriched in the soil. The enrichment of iron is frequently manifested by a red-brown or yellow-brown colour caused by an accumulation of iron oxides. The most reactive part of the soil is the complex of clay minerals and organic matter, which is largely responsible for its agronomic characteristics. True soil does not exist without the presence of colloidal and organic matter. The relative absence of soils in desert areas reflects the fact that chemical and physical weathering of rocks alone does not necessarily result in soil formation. Most soil processes are directly or indirectly biological in nature. Organisms and organic compounds produced by their vital activities or their decomposition are effective agents for dissolving and extracting many elements from the inorganic constituents of the soil, thereby making them available for plant growth.

      Although soils do differ in composition, the range of variation in the major elements is rather small. Minor and trace elements may show considerably greater variability. The importance of certain trace elements in the soil for the healthy growth of plants, and through the plants, of the animals that graze on them, has become increasingly apparent in recent years. Most soils contain these trace elements in sufficient amounts, but when deficiencies are present, puzzling diseases appear, which in the past have rendered large areas of otherwise suitable land unavailable for farming. On a large area in the North Island of New Zealand, for example, although it grew satisfactory pasture, sheep and cattle failed to thrive and eventually died if not removed. As a result, much of this area was given over to afforestation. It was eventually discovered that cobalt, in the amount of a few parts per million, would completely eliminate the disease when applied in fertilizer or administered directly to the animals. The ultimate explanation is the need of animals (but not plants) for vitamin B₁₂, which contains an atom of cobalt in its structure.

      Occasionally, an excess of a specific element may have a deleterious effect on plant growth. Most obvious, of course, are the alkaline or saline soils of desert and coastal areas on which only an impoverished vegetation exists. Magnesium-rich soils are notably infertile; such soils develop on areas of ultrabasic igneous rocks consisting largely of olivine, (Mg,Fe)₂SiO₄, and the boundaries of these areas can frequently be readily mapped from aerial photographs by the marked change in vegetation. Sometimes plants take up available trace elements in amounts deleterious to animals grazing on them. A well-known example is Astragalus racemosus (locoweed), which in some areas of the western U.S. contains sufficient selenium to be poisonous to grazing animals.

      The possible correlation between soil geochemistry and the geographical distribution of disease is thus a field of extreme significance which as yet has been insufficiently studied. The problem is a complex one, in large part because of the difficulty in isolating the numerous factors involved.

The hydrosphere

      The hydrosphere is the discontinuous shell of water—fresh, salt, and solid—on the surface of the Earth. As such it comprises the oceans and the connecting seas and inlets, the lakes, rivers, and streams, the groundwater that feeds them, and the snow and ice cover of high altitudes and high latitudes. The mass of the ocean waters far outweighs the other parts of the hydrosphere. Goldschmidt estimated that there are 273 litres of water in all its forms for every square centimetre of the Earth's surface made up as follows:

       Chemical elements seawater thus makes up over 98 percent of the total mass of the hydrosphere, and its composition (see Table (Chemical elements)) essentially can be taken as giving the average composition of the hydrosphere.

Composition of seawater

      Research during the past century has demonstrated that the composition of seawater is essentially uniform and that the relative proportions of the various ions are practically constant. In the open ocean the salinity (approximately the total weight of dissolved solids per kilogram) averages about 35 parts per thousand, but may rise to 40 parts per thousand in regions such as the Red Sea and the Persian Gulf, where rainfall and inflow are low and evaporation high. Sodium chloride is the dominant compound of the salts in solution and comprises about three-quarters of the whole; the remainder consists largely of chlorides and sulfates of magnesium, calcium, and potassium.

       Chemical elementsThough many data on minor and trace elements in seawater (Table (Chemical elements)) are available, the interpretation of these data is subject to some uncertainties. The concentrations of these elements are probably more variable than for the major elements and may depend to some degree on the sampling location. This is particularly true for the elements that are utilized by marine organisms. Phosphorus is a good example; it is markedly depleted in surface and near-surface waters by biological activity, but it enriches the deeper parts of the ocean through the dissolution of dead organisms. Silica is brought into the ocean in large amounts in solution in river water, but most of it is soon removed to become the skeletal material of diatoms, radiolaria, and sponges.

      Seawater also contains dissolved gases in variable amounts. Seawater of normal salinity at 0° C (32° F) in equilibrium with the atmosphere will contain about eight millilitres per litre of dissolved oxygen and 14 millilitres per litre of dissolved nitrogen. Dissolved nitrogen is essentially an inert constituent, but dissolved oxygen plays a fundamental role in the growth and decay of organisms and so varies greatly in concentration from place to place. In stagnant regions that are rich in decaying organic matter the water may be completely depleted in free oxygen, and a considerable concentration of hydrogen sulfide may be present. Much of the Black Sea below a depth of a few hundred metres is in this condition.

      Another dissolved gas of prime importance for biological activity is carbon dioxide. The conditions controlling its concentration are quite complex, however, because in solution it can be present as free carbon dioxide, as undissociated carbonic acid, as carbonate ions, and as bicarbonate ions. The concentration of these ions will also be affected by biological activity and by the precipitation or dissolution of calcium carbonate.

Circulation of water through the hydrosphere

      The circulation of water through the hydrosphere is controlled in large part by the reservoir effect of the oceans. Evaporation from the ocean surface is precipitated as rainfall. Of that falling on the land, some is directly re-evaporated, some is absorbed into the reservoir of groundwater, and some flows off directly into rivers and streams. The total annual rainfall on the Earth is estimated to be 123 × 10¹⁸ grams, and the total annual runoff to the oceans 32 × 10¹⁸ grams.

      Even the purest rainfall contains some material in solution, not only dissolved gases but also nonvolatile material. Rainfall near seacoasts always contains some sodium chloride and small amounts of other marine salts, the concentration of which falls off generally with distance from the ocean. Rainfall in industrial regions may of course contain a variety of pollutants; in many areas it is essentially a dilute sulfuric acid solution. Such material may also be carried far beyond the place of origin; acid rainfall in the Scandinavian countries probably originates in part from England and Germany.

      The runoff from the land contains additional material in solution, picked up during its circulation through the crustal rocks. River water averages about 120 parts per million dissolved solids, but the range is great, from about 10 parts per million up to several thousand parts per million. Commonly, the range is from 50 to 200 parts per million; contents greater than 200 parts per million are usually the result of human activities or of drainage from soils containing soluble salts, as in desert regions.

      With an average content of 120 parts per million dissolved matter, the rivers of the world deliver 3.9 × 10⁹ tons of material in solution to the sea each year. The average concentration of the important constituents (in parts per million) is: bicarbonate, 58.4; sulfate, 11.2; chloride, 7.8; nitrate, 1.0; calcium, 15.0; magnesium, 4.1; sodium, 6.3; potassium, 2.3; iron, 0.67; and silica, 13.1. Although these ten constituents account for most of the dissolved material, many other elements have been detected in river and lake waters.

Geochemical balance of seawater over time

      The 3.9 × 10⁹ tons carried annually in solution to the oceans are but a small fraction of the total amount of material in solution in the oceans. Nevertheless, when integrated over the whole of geological time, more than 4 × 10⁹ years, it greatly exceeds the present material in solution. Some of the material, especially sodium chloride, is of course cyclical, being circulated from the oceans to the land as aerosols and incorporated in marine sedimentary rocks and ultimately in large part being returned to the oceans in runoff.

       Geochemical balance of some elements in seawaterGoldschmidt made an interesting calculation on the geochemical balance in seawater. From the amount and composition of sedimentary rocks he estimated that erosion during geological time had amounted to about 160 kilograms of igneous rock per square centimetre of the Earth's surface. Combining this figure with the amount of seawater per square centimetre, 273 kilograms, he derived a figure of 600 grams of igneous rock eroded per kilogram of seawater. Assuming this 600 grams had gone fully into solution (obviously a gross simplification but a limiting one), he drew up a balance sheet between the amounts of different elements potentially supplied to the oceans and the amounts actually present. Some of these figures are presented in the Table (Geochemical balance of some elements in seawater). Despite the imperfections of the method, the results are certainly significant in a qualitative sense. Some elements—chlorine, bromine, boron, and sulfur—are present in seawater in amounts far in excess of those that can have been derived by erosion. The source of these “superabundant” elements has probably been volcanism and related magmatic activity. Halides, sulfates, and borates are deposited by volcanic gases and carried in solution in hot springs. The relative depletion of fluorine with respect to chlorine in seawater can be ascribed to the precipitation of highly insoluble fluorine-bearing compounds, mainly apatite (calcium fluophosphate). Sodium clearly remains in solution to a much greater extent than potassium; the latter element reacts with sedimentary materials to form insoluble potassium-bearing silicates such as illite and glauconite, which have no sodium-bearing analogs. Calcium is removed from solution much more effectively than strontium, evidently because it is utilized by organisms. Goldschmidt pointed out that many highly poisonous elements, such as arsenic and selenium, have been potentially supplied in dangerous amounts. Their concentration remains very low, however, presumably because of efficient processes of removal as insoluble compounds. Adsorption on colloidal particles of clay and iron oxides is a likely process.

      The geological and geochemical evidence indicates that the ocean waters are, and have been for a long time, in a steady state of essentially unchanging composition. The addition of material by runoff from the land is adjusted by reactions within the ocean waters or between the ocean waters and sedimentary materials whereby the concentrations of the individual elements remain essentially constant. How far back in geological time this steady state has persisted remains something of an open question. The existence of most forms of marine life from the Cambrian to the present indicates a uniformity of marine conditions over the past 600,000,000 years; how far back into the Precambrian this uniformity extended is more difficult to elucidate. The earlier discussion of Precambrian iron formations suggested the possibility of very different atmospheric composition some 2,000,000,000 years ago, and the considerable interdependence of atmospheric and oceanic composition indicates that this might have resulted in marked geochemical differences in the ocean waters.

The atmosphere

      The atmosphere is the most homogeneous and thus the most easily studied of the geospheres. Its mass is readily determined from the product of the average height of the mercury barometer in centimetres, the density of mercury (13.6 grams per cubic centimetre), and the area of the Earth (5.1 × 10¹⁸ square centimetres). Recent calculations give 51.17 × 10²⁰ grams for its total mass.

Composition

       Average Composition of the Atmosphere Average Composition of the AtmosphereThe composition is also relatively simple, although a considerable number of gases may be present in small amounts (Table (Average Composition of the Atmosphere)). Almost 99 percent consists of oxygen and nitrogen, with argon making up most of the remainder. Carbon dioxide, essential for plant life, is present in an extremely small amount. Some gases not listed in the Table (Average Composition of the Atmosphere) may be present as local or even regional pollutants—city dwellers are becoming increasingly aware of oxides of sulfur as atmospheric pollutants, and the scientific study of smog is largely concerned with reactions taking place between hydrocarbons, oxides of nitrogen, oxygen and ozone.

      The atmosphere gradually thins out into the vacuum of outer space, and its upper limit can conveniently be placed at about 600 kilometres. An important zone in the stratosphere is known as the ozonosphere, a diffuse layer characterized by an increase in the concentration of ozone, O₃. This zone is highly important for life on Earth because it absorbs most of the ultraviolet radiation from the Sun; if this penetrated to the Earth's surface it would act as a potent sterilizer, fatal for most forms of life. It also helps to maintain a more uniform surface temperature by reducing the loss of heat by radiation to space—the so-called greenhouse effect.

Geochemical history

      The geochemical history of the atmosphere has been a complex one. Scientists agree that the present atmosphere is quite different from the original one. It is certainly quite different from those of the other planets. It is reasonable to conclude that this reflects, in part at least, the Earth as the abode of life. The Earth's atmosphere differs from those of its neighbours in the solar system probably in large part through the action of photosynthesis, a complex biological process which was probably preceded by a lengthy period of organic evolution.

      The nature of the Earth's primitive atmosphere is still a subject of some speculation. Some scientists, reasoning by analogy with the larger planets such as Jupiter, have argued for an original atmosphere consisting largely of methane and ammonia. Others have considered that present-day volcanic gases may indicate the nature of the primitive atmosphere, in which case it contained carbon dioxide, possibly carbon monoxide, nitrogen, and water vapour. In either case, free oxygen was absent. If the evolution of the atmosphere is traced backward in time through the geological record, then extensive terrestrial photosynthesis is indicated by an abundance of land plants in Devonian times, about 400,000,000 years ago. Marine photosynthesis, however, is much older, since practically all the major groups of marine organisms were established by the beginning of the Cambrian period, some 540,000,000 years ago. As discussed earlier, the extensive Precambrian iron formations suggest an oxygen-free atmosphere which was terminated about 2,000,000,000 years ago. This evidence has been translated into estimates of oxygen content of the atmosphere of about 1 percent of the present level 2,000,000,000 years ago, about 10 percent of the present level at the beginning of the Cambrian Period, and essentially the present content by Devonian times.

      Although it is not yet possible to know the quantitative composition of the primitive atmosphere, the geochemical processes that have operated to modify its composition during geological time can be evaluated. These processes can be summarized as a series of gains and losses. Additions to the atmosphere comprise: (1) gases released by igneous activity; (2) oxygen and hydrogen produced by the photochemical dissociation of water vapour; (3) oxygen produced by photosynthesis; (4) helium produced by the radioactive breakdown of uranium and thorium; and (5) argon produced by the radioactive breakdown of potassium. Atmospheric losses include: (1) oxygen removal by oxidation of ferrous to ferric iron, sulfur compounds to sulfates, hydrogen to water, and similar reactions; (2) carbon dioxide removed by the formation of coal, petroleum, and the death and burial of organisms; (3) carbon dioxide removed by the formation of calcium and magnesium carbonates; (4) nitrogen removed by the formation of oxides of nitrogen in the air and by the action of nitrifying bacteria in the soil; and (5) hydrogen and helium by escape from the Earth's gravitational field.

      Photosynthesis has certainly been the most significant process in controlling atmospheric composition during much of geological time. Through this process carbon dioxide and water are converted to carbohydrate, with the accompanying release of oxygen. Much of this carbohydrate is consumed by animals and reconverted to carbon dioxide and water by respiration, and oxidative decay leads to the same result. Some, however, is incorporated into sediments; part may go to form exploitable deposits of coal and petroleum, but most of it remains as disseminated carbonaceous material; the average carbon content of sedimentary rocks is about 0.4 percent.

      Quantitatively, more significant amounts of carbon dioxide have been removed from the atmosphere in the form of limestone and dolomite. Most of this removal has been effected by marine organisms, especially algae and corals, but direct inorganic precipitation may occur, especially in warm tropical waters. Judging from the vast deposits of limestone and dolomite throughout the sedimentary record, this process has operated with a remarkable degree of uniformity throughout geological time. Extensive limestone formations are possibly less common in older Precambrian rocks, indicating a slower beginning of carbonate precipitation. It is truly remarkable that so much carbonate rock has been deposited during geological time, with the carbonate being ultimately derived from an atmosphere which may never have contained a much higher concentration of carbon dioxide than is present today. It has been pointed out that reactions like the decomposition of calcium silicate—CaSiO₃ + CO₂ = CaCO₃ + SiO₂, in which CaSiO₃ is calcium silicate, CO₂ is carbon dioxide, CaCO₃ is calcium carbonate, and SiO₂ is silicon dioxide, which tends to go toward the right at ordinary temperatures—will act as buffering mechanisms to keep the carbon dioxide concentration of the atmosphere at a continuously low level.

      If the carbon dioxide concentration has remained essentially constant, and yet this compound has been continuously extracted to form carbonates and organic compounds, then clearly a balancing source of “new” carbon dioxide is required. This evidently has been provided by volcanism and other igneous activity. The Earth is steadily being degassed, in the sense that gaseous compounds contained in the mantle are escaping to the surface. The presence of carbon dioxide in the mantle has been demonstrated by the presence of microscopic inclusions of liquid carbon dioxide in the minerals of the peridotite xenoliths (rocks contained within other rocks) brought up in some volcanoes. Along with carbon dioxide, much water and small amounts of other volatiles are being added continuously from sources in the mantle. Ultimately, the hydrosphere, as well as the atmosphere, is the product of the degassing of the Earth's interior.

      Of the remaining atmospheric gases, argon presents some intriguing features. Argon is by far the most abundant of the inert gases on Earth, whereas in the universe as a whole it is much less abundant than either helium or neon. In addition, its isotopic composition is quite distinct, consisting almost entirely of argon-40, whereas in the Cosmos argon-36 is the most abundant isotope. The reason for these anomalies is that atmospheric argon is almost entirely radiogenic, the product of the decay of the potassium-40 isotope of potassium.

      Similarly, the helium in the atmosphere is probably entirely the product of the radioactive decay of uranium and thorium. Actually, the atmosphere contains only about 10 percent of the total amount of helium generated from these sources during geological time. Some of this helium remains occluded in the rocks where it was formed, some has escaped from the upper atmosphere. Helium (and hydrogen), consisting of light atoms, can escape from the gravitational field of the Earth, whereas heavier gases cannot. A minor source of atmospheric oxygen throughout geological time is probably the photochemical decomposition of water vapour in the upper atmosphere, with the subsequent loss of the hydrogen to outer space.

      Some oxygen has been removed from the atmosphere by oxidative reactions, of which the most significant has been the conversion of ferrous to ferric iron. In igneous rocks the average ferrous-to-ferric iron ratio (FeO/Fe₂O₃) is greater than unity, whereas in sedimentary rocks the proportion is reversed, ferric iron being dominant over ferrous iron. Other oxidative reactions are the conversion of manganous compounds to manganese dioxide and of hydrogen sulfide to free sulfur and sulfate. Nitrogen is almost inert geochemically, but a little is fixed as oxides of nitrogen by lightning, and somewhat more by the action of nitrifying bacteria in the soil. Most of this nitrogen is ultimately returned to the atmosphere by the decay of the organisms. Oxides of nitrogen formed in the atmosphere are removed in rain as nitrite and nitrate. Nitrogen does not accumulate in the soil, however, except perhaps under extremely arid conditions, as in the deserts of northern Chile, the locale of the unique nitrate deposits.

The geochemical cycle

Early history of the Earth

      The preceding discussion has shown that at the time of formation of the Earth the chemical elements already had been considerably fractionated: the universe (Cosmos) consists almost entirely of hydrogen and helium, probably with less than 1 percent of the heavier elements. The Earth, on the other hand, consists almost entirely of the heavier elements. Helium is a very rare element on Earth, so rare in fact that it was first discovered as an unidentified line in the Sun's spectrum in 1868, some 30 years before it was detected on Earth. Hydrogen is moderately abundant on Earth, largely because it combines with oxygen to form water, whereas helium is an inert element.

      The rarity of helium and the other inert gases neon, krypton, and xenon on Earth is good evidence that the Earth formed by the accretion of small solid objects, or planetesimals (planetesimal). (Argon is a special case, since most of the Earth's argon has been formed within the planet by the radioactive decay of potassium.) These planetesimals had no atmosphere, and the atmosphere of the Earth has been derived by the outgassing of combined and occluded gases within these planetesimals. This process has operated throughout geological history and is probably still continuing; volcanic activity not only brings up solid material from the Earth's interior but also large amounts of gases, principally water vapour, carbon monoxide and dioxide, and nitrogen. The oxygen in the present atmosphere is almost entirely the product of photosynthesis, whereby carbon dioxide and water are converted to carbohydrate and free oxygen.

      Direct information on the composition of the Earth's crust is available in the form of thousands of analyses of individual rocks, the average of which provides a reasonably precise estimate of the bulk composition. For the mantle and the core the information is indirect and thus much less precise. The origin of the Earth by the accretion of planetesimals is a well-founded hypothesis, however, and meteorites (meteorite) are probably examples of planetesimals that have survived from the preplanetary stage of the solar system. It thus seems likely that the Earth formed by the accretion of solid bodies with the average composition of stony meteorites. The accretion process, however, led to massive segregation of the elements. Much of the iron was reduced to the metallic state and sank to the centre to form the core, carrying with it the major part of the siderophile elements. Lithophile elements, those with a greater affinity for oxygen than iron, combined as oxide compounds, mostly silicates, and provided material for the mantle and crust. Chalcophile elements would tend to form sulfides; however, few sulfides are stable at the high temperatures of the Earth's interior, so the fate of the chalcophile elements during the early history of the Earth is somewhat uncertain.

      This primary geochemical differentiation of the Earth can be interpreted in terms of the system iron–magnesium–silicon–oxygen–sulfur, because these five elements make up about 95 percent of the Earth. There was insufficient oxygen to combine with the major metallic elements iron, magnesium, and silicon; because magnesium and silicon have a greater affinity for oxygen than iron, these elements combined completely with oxygen, and the remaining oxygen combined with part of the iron, leaving the remainder as the metal iron and iron sulfide. As indicated above, the metal sank to form the core, carrying with it the major part of the siderophile elements. The iron sulfide phase probably incorporated much of the chalcophile elements; it does not seem to have formed a distinct shell within the Earth and probably remains primarily in disseminated form through the mantle and the core.

      This primary geochemical differentiation was largely controlled by two independent factors, the geochemical affinity of the individual elements and the density of the principal phases. It is significant that the density of the individual elements is irrelevant in this context. Uranium and thorium, for example, are very heavy elements; nevertheless, they are concentrated in the crust, not in the core, because of their lithophile character (affinity for oxygen).

      The separation of crust from mantle has involved rather different geochemical processes than those of the segregation of the core. The core represents the physical separation of a high-density metallic liquid immiscible with the silicate material of the mantle and crust, whereas the separation of crust from mantle has resulted from more subtle geochemical processes. The mantle is made up very largely of the magnesium-iron silicates olivine and pyroxene (or their high-pressure equivalents). As discussed earlier, these minerals act as sorting mechanisms for other elements, accepting those of ionic size comparable to magnesium and iron and rejecting those much larger or smaller. The crust differs from the mantle in being enriched in those elements that do not readily enter the crystal structures of olivine and pyroxene. The common elements include the alkali metals (except lithium), aluminum, and calcium to some degree. These are combined in the crust in the form of the feldspar minerals, and one of the principal differences between crust and mantle is the dominance of feldspar in the former and its absence in the latter.

Results of magmatism, sedimentation, and metamorphism

      The differentiation of the crust from the mantle and core permits consideration of the outer part of the Earth as a distinct geochemical system. The distribution and migration of the elements within the crust (and in the hydrosphere and atmosphere) are the results of processes of magmatism, sedimentation, and metamorphism. The fate of an element during magmatic crystallization is primarily a function of its ionic size. If of appropriate size it enters the structure of one or other of the major minerals; otherwise, it tends to remain in the magmatic liquid until its concentration increases sufficiently for it to form specific minerals in which it is a major component. Volatile elements and compounds in the magma (principally water, carbon dioxide, nitrogen, and sulfur compounds) escape and are added to the atmosphere and hydrosphere.

      Sedimentary processes lead to further geochemical differentiation. Liquid water is the medium in which these processes largely operate, and the solution chemistry of the individual elements is the key to their geochemical behaviour. The atmosphere and hydrosphere interact importantly with the lithosphere. Water and carbon dioxide are incorporated in sedimentary minerals; soluble ions, especially sodium, are added to the hydrosphere. Processes involving living organisms are intimately associated with sedimentation; indeed, some abundant and widespread sedimentary rocks, such as coal and limestone, are essentially biological in origin. These biological processes thus have had significant geochemical results. Photosynthesis also has been responsible for the present composition of the atmosphere.

       metamorphism tends in some degree to reverse the geochemical differentiation of the elements produced by igneous and sedimentary processes. In general, metamorphism promotes a more uniform distribution of the elements and tends to erase compositional differences within large rock masses. Unlimited metamorphism would result in a situation in which the whole lithosphere reached a uniform composition; such a trend is exemplified by the comparatively monotonous chemical and mineralogical composition of many extensive areas of metamorphic rocks.

 This overall picture of the migration of the elements in the outer part of the Earth is the basis for the concept of the geochemical cycle (Figure—>). The geochemical cycle may be considered as beginning with the crystallization of a magma, which may produce a variety of igneous rocks and which adds volatile materials to the atmosphere and hydrosphere. Weathering decomposes the igneous rocks, and erosion eventually transports the decomposed rock to the oceans, partly as solid matter to form sedimentary rocks and partly as material in solution. Crustal movements may transport sedimentary and igneous rocks to deeper levels in the crust, where the combined effects of heat and pressure induce metamorphism. If the temperature rises above the melting point of some of the rock material, magma is regenerated, and the cycle is complete. Such a geochemical cycle seldom goes to completion; at some stage it may be indefinitely halted or its direction reversed. Nor is the cycle a closed one, either materially or energetically. The crust receives “primary” magma generated in the upper mantle, bringing with it additional material and energy in the form of heat. The surface of the Earth receives an increment of meteoritic matter from outer space, insignificant in amount but nevertheless detectable in deep-sea deposits, where the rate of sedimentation is very low.

      The geochemical cycle provides a useful conceptual framework for considering the course followed by an individual element in proceeding through the different stages. For a specific element, a complete understanding of its behaviour throughout the cycle is one of the major objectives of geochemical research. An element may tend to concentrate in a specific type of deposit at a particular stage in the cycle, or it may remain dispersed throughout the entire cycle. The geochemical cycles of a few elements have been elucidated in considerable detail, but for many of them knowledge is still fragmentary.

Brian H. Mason Ed.

Additional Reading

General works

Overviews are provided by Mary Elvira Weeks, Discovery of the Elements, 7th ed. rev. by H.J. Leicester (1968), a description of the events, both human and technical, surrounding the discovery of each of the elements and the implications on the ideas of the time; D.N. Trifonov and V.D. Trifonov, Chemical Elements: How They Were Discovered, trans. from Russian (1982); Esmarch S. Gilreath, Fundamental Concepts of Inorganic Chemistry (1958), a short, technical description of ideas concerning atomic structure, periodic relationships, and radioactivity; and Eduard Farber, The Evolution of Chemistry, 2nd ed. (1969), a history of ideas, methods, and materials from a chemist's viewpoint.The following works may be consulted for information on individual elements and groups of elements: P.A. Cox, The Elements (1989); R.T. Sanderson, Chemical Periodicity (1960), chapter 5, which provides a clearly written, nontechnical discussion of the physical properties of the elements individually up to xenon, followed by a discussion of the various groups and the trends to be observed in these groups; John Emsley, The Elements, 2nd ed. (1991); Clifford A. Hampel (ed.), The Encyclopedia of the Chemical Elements (1968); Allen J. Bard (ed.), Encyclopedia of Electrochemistry of the Elements (1973– ); E.I. Hamilton, The Chemical Elements and Man: Measurements, Perspectives, Applications (1979); N.N. Greenwood and A. Earnshaw, Chemistry of the Elements (1984); J.W. Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, 16 vol. (1922–37), and supplements; Therald Moeller, Inorganic Chemistry: An Advanced Textbook (1952), with discussions of the physical forms and properties of the elements in the various periodic groups, as well as comments on their preparation; M. Cannon Sneed, J. Lewis Maynard, and Robert C. Brasted (eds.), Comprehensive Inorganic Chemistry, 8 vol. (1953–61); John C. Bailar, Jr., et al. (eds.), Comprehensive Inorganic Chemistry, 5 vol. (1973); F. Albert Cotton and Geoffrey Wilkinson, Advanced Inorganic Chemistry, 5th ed. (1988); and A.G. Massey, Main Group Chemistry (1990).Two important comprehensive reference works are Herman F. Mark et al. (eds.), Encyclopedia of Chemical Technology, 3rd ed., 31 vol. (1978–84), formerly known as Kirk-Othmer Encyclopedia of Chemical Technology, with a 4th edition begun in 1991, covering commercial preparations of elements and important compounds; and Gmelins Handbuch der anorganischen Chemie, 8th ed. (1924– ), arranged by element, with articles in German and English—since 1981 most of the articles have appeared in English, and the volumes now have English titles: Gmelin Handbook of Inorganic Chemistry (1981–89) and Gmelin Handbook of Inorganic and Organometallic Chemistry (1990– ).J.J. Lagowski Ed.

Origin of the elements

Studies on the topic include Donald D. Clayton, Principles of Stellar Evolution and Nucleosynthesis (1968, reprinted 1983); Claus E. Rolfs and William S. Rodney, Cauldrons in the Cosmos (1988); Virginia Trimble, “The Origin and Abundances of the Chemical Elements,” Reviews of Modern Physics, 47(4):877–976 (October 1975), and “The Origin and Abundances of the Chemical Elements Revisited,” The Astronomy and Astrophysics Review, 3(1):1–46 (1991); G.J. Mathews (ed.), Origin and Distribution of the Elements (1988), a collection of symposium papers; G.J. Wasserburg, “Short-Lived Nuclei in the Early Solar System,” in David C. Black and Mildred Shapley Matthews (eds.), Protostars & Planets II (1985), pp. 703–754; D.L. Lambert, “The p-Nuclei: Abundances and Origins,” The Astronomy and Astrophysics Review, 3:201–256 (1992).

Geochemical distribution of the elements

Introductory works include V.M. Goldschmidt, Geochemistry, ed. by Alex Muir (1954, reissued 1970), an authoritative, comprehensive account of the whole field of geochemistry; Brian Mason and Carleton B. Moore, Principles of Geochemistry, 4th ed. (1982), a standard introductory text; K.H. Wedepohl (ed.), Handbook of Geochemistry, 2 vol. in 6 (1969–78), reviewing each element in detail; Paul Henderson, Inorganic Geochemistry (1982); and John A. Tossell and David J. Vaughan, Theoretical Geochemistry: Applications of Quantum Mechanics in the Earth and Mineral Sciences (1992). The following works provide more specialized coverage on the Earth and solar system: Heinrich D. Holland, The Chemistry of the Atmosphere and Oceans (1978); Stuart Ross Taylor and Scott M. McLennan, The Continental Crust: Its Composition and Evolution (1985), which provides data on the oceanic crust as well; H. Wänke, G. Dreibus, and E. Jagoutz, “Mantle Chemistry and Accretion History of the Earth,” in A. Kröner, G.N. Hanson, and A.M. Goodwin (eds.), Archaean Geochemistry (1984), pp. 1–24; Horton E. Newsom and John H. Jones (eds.), Origin of the Earth (1990); Grant Heiken, David Vaniman, and Bevan M. French (eds.), Lunar Sourcebook (1991); John F. Kerridge and Mildred Shapley Matthews (eds.), Meteorites and the Early Solar System (1988); Stuart Ross Taylor, Solar System Evolution: A New Perspective (1992); and E. Anders and N. Grevesse, “Abundances of the Elements: Meteoritic and Solar,” Geochimica et cosmochimica acta, 53(1):197–214 (January 1989). Ed.

* * *