For geochemistry, it is important to find out the principle of the distribution of chemical elements in the earth's crust. Why are some of them often found in nature, others are much rarer, and still others are "museum rarities"?
A powerful tool for explaining many geochemical phenomena is the Periodic Law of D.I. Mendeleev. In particular, it can be used to investigate the distribution of chemical elements in the earth's crust.
For the first time, the relationship between the geochemical properties of elements and their position in the Periodic Table of Chemical Elements was shown by D.I. Mendeleev, V.I. Vernadsky and A.E. Fersman.
Rules (laws) of geochemistry
Mendeleev's rule
In 1869, while working on the periodic law, D.I. Mendeleev formulated the rule: Elements with low atomic weight are generally more common than elements with high atomic weight.» (See Appendix 1, Periodic Table of Chemical Elements). Later, with the disclosure of the structure of the atom, it was shown that for chemical elements with a small atomic mass, the number of protons is approximately equal to the number of neutrons in the nuclei of their atoms, that is, the ratio of these two quantities is equal to or close to unity: for oxygen = 1.0; for aluminum
For less common elements, neutrons predominate in the nuclei of atoms and the ratio of their number to the number of protons is significantly greater than one: for radium; for uranium = 1.59.
Further development of the "Mendeleev's rule" was found in the works of the Danish physicist Niels Bohr and the Russian chemist, academician of the USSR Academy of Sciences Viktor Ivanovich Spitsyn.
 | Viktor Ivanovich Spitsyn (1902-1988) |
Oddo rule
In 1914, the Italian chemist Giuseppe Oddo formulated another rule: The atomic weights of the most common elements are expressed in multiples of four, or deviate little from such numbers.". Later, this rule received some interpretation in the light of new data on the structure of atoms: a nuclear structure consisting of two protons and two neutrons has a special strength.
Harkins' rule
In 1917, the American physical chemist William Draper Harkins (Harkins) drew attention to the fact that chemical elements with even atomic (ordinal) numbers are distributed in nature several times more than their neighboring elements with odd numbers. Calculations confirmed the observation: of the first 28 elements of the periodic system, 14 even ones make up to 86%, and odd ones - only 13.6% of the mass of the earth's crust.
In this case, the explanation may be the fact that chemical elements with odd atomic numbers contain particles that are not bound into helions, and therefore are less stable.
There are many exceptions to Harkins' rule: for example, even noble gases are extremely rare, and odd aluminum Al overtakes even magnesium Mg in distribution. However, there are suggestions that this rule applies not so much to the earth's crust, but to the entire globe. Although there are no reliable data on the composition of the deep layers of the globe, some information suggests that the amount of magnesium in the whole globe is twice that of aluminum. The amount of helium He in outer space is many times greater than its terrestrial reserves. This is perhaps the most common chemical element in the universe.
Fersman's rule
A.E. Fersman clearly showed the dependence of the abundance of chemical elements in the earth's crust on their atomic (ordinal) number. This dependence becomes especially obvious if you build a graph in coordinates: atomic number - logarithm of the atomic clarke. The graph shows a clear trend: atomic clarks decrease with increasing atomic numbers of chemical elements.
Rice. . The prevalence of chemical elements in the earth's crust
Rice. 5. The prevalence of chemical elements in the universe
(log C are logarithms of atomic clarkes according to Fersman)
(data on the number of atoms refer to 10 6 silicon atoms)
Solid curve - even Z values,
dashed - odd Z values
However, there are some deviations from this rule: some of the chemical elements significantly exceed the expected abundance values (oxygen O, silicon Si, calcium Ca, iron Fe, barium Ba), while others (lithium Li, beryllium Be, boron B) are much less common, than would be expected from Fersman's rule. Such chemical elements are called respectively redundant and scarce.
The formulation of the basic law of geochemistry is given on p.
The chemical composition of the earth's crust was determined from the analysis of numerous samples of rocks and minerals that come to the surface of the earth during mountain-building processes, as well as taken from mine workings and deep boreholes.
At present, the earth's crust has been studied to a depth of 15-20 km. It consists of chemical elements that are part of the rocks.
The most widespread in the earth's crust are 46 elements, of which 8 make up 97.2-98.8% of its mass, 2 (oxygen and silicon) - 75% of the mass of the Earth.
The first 13 elements (with the exception of titanium), which are most often found in the earth's crust, are part of the organic matter of plants, participate in all vital processes and play an important role in soil fertility. A large number of elements involved in chemical reactions in the bowels of the Earth leads to the formation of a wide variety of compounds. Chemical elements, which are most in the lithosphere, are part of many minerals (they mainly consist of different rocks).
Separate chemical elements are distributed in the geospheres as follows: oxygen and hydrogen fill the hydrosphere; oxygen, hydrogen and carbon form the basis of the biosphere; oxygen, hydrogen, silicon and aluminum are the main components of clays and sands or weathering products (they mostly make up the upper part of the Earth's crust).
Chemical elements in nature are found in a variety of compounds called minerals. These are homogeneous chemicals of the earth's crust, which were formed as a result of complex physicochemical or biochemical processes, for example, rock salt (NaCl), gypsum (CaS04 * 2H20), orthoclase (K2Al2Si6016).
In nature, chemical elements take an unequal part in the formation of different minerals. For example, silicon (Si) is found in over 600 minerals and is also very common in the form of oxides. Sulfur forms up to 600 compounds, calcium-300, magnesium -200, manganese-150, boron - 80, potassium - up to 75, only 10 lithium compounds are known, and even less iodine.
Among the best known minerals in the earth's crust is dominated by a large group of feldspars with three main elements - K, Na and Ca. In soil-forming rocks and their weathering products, feldspars occupy the main position. Feldspars gradually weather (decompose) and enrich the soil with K, Na, Ca, Mg, Fe and other ash substances, as well as trace elements.
Clarke number- numbers expressing the average content of chemical elements in the earth's crust, hydrosphere, Earth, cosmic bodies, geochemical or cosmochemical systems, etc., in relation to the total mass of this system. Expressed in % or g/kg.
Types of clarks
There are weight (in %, in g/t or in g/g) and atomic (in % of the number of atoms) clarks. Generalization of data on the chemical composition of various rocks that make up the earth's crust, taking into account their distribution to depths of 16 km, was first made by the American scientist F. W. Clark (1889). The numbers obtained by him for the percentage of chemical elements in the composition of the earth's crust, later somewhat refined by A. E. Fersman, at the suggestion of the latter were called Clark numbers or clarks.
The structure of the molecule. Electrical, optical, magnetic and other properties of molecules are associated with wave functions and energies of various states of molecules. Information about the states of molecules and the probability of transition between them is provided by molecular spectra.
The vibration frequencies in the spectra are determined by the masses of the atoms, their arrangement, and the dynamics of interatomic interactions. The frequencies in the spectra depend on the moments of inertia of molecules, the determination of which from spectroscopic data makes it possible to obtain exact values of interatomic distances in a molecule. The total number of lines and bands in the vibrational spectrum of a molecule depends on its symmetry.
Electronic transitions in molecules characterize the structure of their electron shells and the state of chemical bonds. The spectra of molecules that have a greater number of bonds are characterized by long-wavelength absorption bands that fall into the visible region. Substances that are built from such molecules are characterized by color; such substances include all organic dyes.
Ions. As a result of electron transitions, ions are formed - atoms or groups of atoms in which the number of electrons is not equal to the number of protons. If an ion contains more negatively charged particles than positively charged ones, then such an ion is called negative. Otherwise, the ion is called positive. Ions are very common in substances, for example, they are in all metals without exception. The reason is that one or more electrons from each metal atom are separated and move inside the metal, forming the so-called electron gas. It is because of the loss of electrons, that is, negative particles, that metal atoms become positive ions. This is true for metals in any state - solid, liquid or gaseous.
The crystal lattice models the arrangement of positive ions inside the crystal of a homogeneous metallic substance.
It is known that in the solid state all metals are crystals. The ions of all metals are arranged in an orderly manner, forming a crystal lattice. In molten and vaporized (gaseous) metals, there is no ordered arrangement of ions, but the electron gas still remains between the ions.
Isotopes- varieties of atoms (and nuclei) of a chemical element that have the same atomic (ordinal) number, but different mass numbers. The name is due to the fact that all isotopes of one atom are placed in the same place (in one cell) of the periodic table. The chemical properties of an atom depend on the structure of the electron shell, which, in turn, is determined mainly by the charge of the nucleus Z (that is, the number of protons in it), and almost do not depend on its mass number A (that is, the total number of protons Z and neutrons N) . All isotopes of the same element have the same nuclear charge, differing only in the number of neutrons. Usually, an isotope is denoted by the symbol of the chemical element to which it belongs, with the addition of an upper left index indicating the mass number. You can also write the name of the element with a hyphenated mass number. Some isotopes have traditional proper names (for example, deuterium, actinon).
Until now, speaking about the atomic theory, about how substances that are completely different from each other are obtained from several types of atoms connected to each other in a different order, we have never asked the “childish” question - where did the atoms themselves come from? Why are there a lot of atoms of some elements, and very few of others, and they are very unevenly distributed. For example, just one element (oxygen) makes up half of the earth's crust. Three elements (oxygen, silicon and aluminum) in total already account for 85%, and if we add iron, potassium, sodium, potassium, magnesium and titanium to them, we will get 99.5% of the earth's crust. The share of several dozen other elements accounts for only 0.5%. The rarest metal on Earth is rhenium, and there is not so much gold with platinum, it is not for nothing that they are so expensive. And here is another example: there are about a thousand times more iron atoms in the earth's crust than copper atoms, a thousand times more copper atoms than silver atoms, and a hundred times more silver than rhenium atoms.
The elements on the Sun are distributed in a completely different way: there is the most hydrogen (70%) and helium (28%), and only 2% of all other elements. If we take the entire visible Universe, then there is even more hydrogen in it. Why is that? In ancient times and in the Middle Ages, questions about the origin of atoms were not asked, because they believed that they always existed in an unchanged form and quantity (and according to the biblical tradition, they were created by God on one day of creation). And even when the atomistic theory won and chemistry began to develop rapidly, and D. I. Mendeleev created his famous system of elements, the question of the origin of atoms continued to be considered frivolous. Of course, occasionally one of the scientists mustered up the courage and proposed his theory. As already mentioned. In 1815, William Prout suggested that all elements originated from atoms of the lightest element, hydrogen. As Prout wrote, hydrogen is the same "first matter" of the ancient Greek philosophers. which by "condensing" gave all the other elements.
In the 20th century, through the efforts of astronomers and theoretical physicists, a scientific theory of the origin of atoms was created, which in general terms answered the question of the origin of chemical elements. In a very simplified way, this theory looks like this. At first, all matter was concentrated at one point with an incredibly high density (K) * "g / cm") and temperature (1027 K). These numbers are so large that there are no names for them. Approximately 10 billion years ago, as a result of the so-called Big Bang, this super-dense and super-hot spot began to expand rapidly. Physicists have a fairly good idea of how events developed 0.01 seconds after the explosion. The theory of what happened before was developed much worse, because in the then-existing clot of matter, the now known physical laws were poorly observed (and the sooner, the worse). Moreover, the question of what happened before the Big Bang was essentially not even considered, since then there was no time itself! After all, if there is no material world, that is, no events, then where does time come from? Who or what will count it? So, the matter began to rapidly scatter and cool down. The lower the temperature, the more opportunities for the formation of various structures (for example, at room temperature, millions of different organic compounds can exist, at +500 ° C - only a few, and above +1000 ° C, probably, no organic substances can exist, - All of them break down into their component parts at high temperatures. According to scientists, 3 minutes after the explosion, when the temperature dropped to a billion degrees, the process of nucleosynthesis began (this word comes from the Latin nucleus - “core” and the Greek “synthesis” - “connection, combination”), i.e. the connection process protons and neutrons into the nuclei of various elements. In addition to protons - hydrogen nuclei, helium nuclei also appeared; these nuclei could not yet add electrons and form agoms due to too high a temperature. The Primary Universe consisted of hydrogen (about 75%) and helium, with a small amount of the next largest element, lithium (its core has three protons). This composition has not changed for about 500 thousand years. The universe continued to expand, cool, and become increasingly rarefied. When the temperature dropped to +3000 "C. the electrons got the opportunity to combine with the nuclei, which led to the formation of stable hydrogen and helium atoms.
It would seem that the Universe, consisting of hydrogen and helium, should continue to expand and cool to infinity. But then there would be not only other elements, but also galaxies, stars, and also us. The forces of universal gravitation (gravity) counteracted the infinite expansion of the Universe. The gravitational contraction of matter in different parts of the rarefied Universe was accompanied by repeated strong heating - the stage of mass formation of stars began, which lasted about 100 million years. In those regions of space consisting of gas and dust, where the temperature reached 10 million degrees, the process of thermonuclear fusion of helium began by fusion of hydrogen nuclei. These nuclear reactions were accompanied by the release of a huge amount of energy that was radiated into the surrounding space: this is how a new star lit up. As long as there was enough hydrogen in it, radiation that "pressed from within" counteracted the compression of the star under the influence of gravity. Our Sun also shines due to the "burning" of hydrogen. This process is very slow, since the rapprochement of two positively charged protons is prevented by the Coulomb repulsion force. So our luminary is destined for many more years of life.
When the supply of hydrogen fuel comes to an end, the synthesis of helium gradually stops, and with it the powerful radiation fades. The forces of gravity again compress the star, the temperature rises and it becomes possible for helium nuclei to merge with each other to form carbon nuclei (6 protons) and oxygen (8 protons in the nucleus). These nuclear processes are also accompanied by the release of energy. But sooner or later helium stocks will come to an end. And then comes the third stage of compression of the star by the forces of gravity. And then everything depends on the mass of the star at this stage. If the mass is not very large (like our Sun), then the effect of the increase in temperature during the compression of the star will not be sufficient for carbon and oxygen to enter into further nuclear fusion reactions; such a star becomes a so-called white dwarf. Heavier elements are "manufactured" in stars that astronomers call red giants - their mass is several times that of the Sun. In these stars, the reactions of synthesis of heavier elements from carbon and oxygen take place. As astronomers figuratively express themselves, stars are nuclear fires, the ashes of which are heavy chemical elements.
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The energy released at this stage of the life of a star greatly "inflates" the outer layers of the red giant; if our Sun were such a star. The earth would be inside this giant ball - the prospect for everything on the earth is not the most pleasant. Stellar wind.
"breathing" from the surface of red giants, brings to outer space the chemical elements synthesized by these stars, which form nebulae (many of them are visible through a telescope). Red giants live relatively short lives - hundreds of times less than the Sun. If the mass of such a star exceeds the mass of the Sun by 10 times, then conditions arise (temperature of the order of a billion degrees) for the synthesis of elements up to iron. Yalro iron is the most stable of all cores. This means that the reactions of synthesis of elements that are lighter than iron proceed with the release of energy, while the synthesis of heavier elements requires energy. With the expenditure of energy, reactions of the decomposition of iron into lighter elements also occur. Therefore, in stars that have reached the "iron" stage of development, dramatic processes take place: instead of releasing energy, it is absorbed, which is accompanied by a rapid decrease in temperature and compression to a very small volume; astronomers call this process gravitational collapse (from the Latin word collapsus - “weakened, fallen”; it’s not for nothing that doctors call a sudden drop in blood pressure, which is very dangerous for humans). During the gravitational collapse, a huge number of neutrons are formed, which, due to the absence of a charge, easily penetrate into the nuclei of all available elements. Nuclei supersaturated with neutrons undergo a special transformation (called beta decay), during which a proton is formed from a neutron; as a result, the next element is obtained from the nucleus of this element, in the nucleus of which there is already one more proton. Scientists have learned to reproduce such processes in terrestrial conditions; a well-known example is the synthesis of the plutonium-239 isotope, when, when natural uranium (92 protons, 146 neutrons) is irradiated with neutrons, its nucleus captures one neutron and an artificial element neptunium (93 protons, 146 neutrons) is formed, and from it the same deadly plutonium ( 94 protons, 145 neutrons), which is used in atomic bombs. In stars that undergo gravitational collapse, as a result of neutron capture and subsequent beta decays, hundreds of different nuclei of all possible isotopes of chemical elements are formed. The collapse of a star ends with a grandiose explosion, accompanied by the ejection of a huge mass of matter into outer space - a supernova is formed. The ejected substance, containing all the elements from the periodic table (and our body contains those same atoms!), Scatters around at a speed of up to 10,000 km / s. and a small remnant of the dead star's matter shrinks (collaises) to form a superdense neutron star or even a black hole. Occasionally, such stars flare up in our sky, and if the outbreak is not too far away, the supernova outshines all other stars in brightness. And no wonder: the brightness of a supernova can exceed the brightness of an entire galaxy consisting of a billion stars! One of these " new "stars, according to the Chinese chronicles, flared up in 1054. Now in this place is the famous Crab nebula in the constellation Taurus, and in its center there is a rapidly rotating (30 revolutions per second!) Neutron star. Fortunately (for us , and not for the synthesis of new elements), such stars have flared so far only in distant galaxies ...
As a result of the "burning" of stars and the explosion of supernovae, all known chemical elements turned out to be in outer space. The remnants of supernovae in the form of expanding nebulae, “heated up” by radioactive transformations, collide with each other, condense into dense formations, from which new generation stars arise under the influence of gravitational forces. These stars (including our Sun) from the very beginning of their existence contain an admixture of heavy elements in their composition; the same elements are contained in the gas and dust clouds surrounding these stars, from which the planets are formed. So the elements that make up all the things around us, including our body, were born as a result of grandiose cosmic processes ...
Why are some elements formed a lot, and others - a little? It turns out that in the process of nucleosynthesis, nuclei consisting of a small even number of schutons and neutrons are most likely to be formed. Heavy nuclei, "overflowing" with protons and neutrons, are less stable and there are fewer of them in the Universe. There is a general rule: the greater the charge of the nucleus, the heavier it is, the less such nuclei in the Universe. However, this rule is not always followed. For example, there are few light nuclei of lithium (3 protons, 3 neutrons) and boron (5 protons and 5 or 6 neutrons) in the earth's crust. It is assumed that for a number of reasons these nuclei cannot be formed in the interiors of stars, but under the action of cosmic rays they “break off” from heavier nuclei accumulated in interstellar space. Thus, the ratio of various elements on Earth is an echo of the turbulent processes in space that took place billions of years ago, at the later stages of the development of the Universe.
Elemental Composition of Living Matter and OM of Combustible Fossils
Combustible fossils contain in their composition the same elements as the substance of living organisms, so the elements - carbon, hydrogen, oxygen, nitrogen, sulfur and phosphorus called or biogenic, or biophilic, or organogenic.
Hydrogen, carbon, oxygen and nitrogen account for over 99% both the mass and the number of atoms that make up all living organisms. In addition to them, in significant quantities in living organisms, another eye can be concentrated.
lo 20-22 chemical elements. 12 elements make up 99.29%, the rest 0.71%
Space abundance: H, He, C, N.
Up to 50% - C, up to 20% - O, up to 8% - H, 10-15% - N, 2-6% - P, 1% - S, 1% - K, ½% - Mg and Ca, 0 .2% - Fe, in trace amounts - Na, Mn, Cu, Zn.
The structure of the atom, isotopes, distribution of hydrogen, oxygen, sulfur and nitrogen in the earth's crust
HYDROGEN - the main element of the cosmos, the most common element of the universe . Chem e-t 1 group, atomic number 1, atomic mass 1.0079. In modern editions of the periodic table, H is also placed in group VII above F, since some properties of H are similar to the properties of halogens. Three H isotopes are known. Two stable ones are protium 1 H - P (99.985%), deuterium 2 H - D (0.015%), and one radioactive is tritium 3 H - T, T 1/2 = 12.262 years. One more is artificially obtained - the fourth extremely unstable isotope - 4 H. In the separation of P and D under natural conditions, evaporation plays the main role, however, the mass of the world's oceans is so large that the deuterium content in it changes slightly. In tropical countries, the content of deuterium in precipitation is higher than in the polar zone. In the free state, H is a colorless gas, odorless and tasteless, the lightest of all gases, 14.4 times lighter than air. H becomes liquid at -252.6°C, solid at -259.1°C. H is an excellent reducing agent. It burns in O with a non-luminous flame, forming water. In the earth's crust, H is much smaller than in stars and on the Sun. Its weight clarke in the earth's crust is 1%. In natural chemical compounds, H forms ionic, covalent and hydrogen bonds . Hydrogen bonds play an important role in biopolymers (carbohydrates, alcohols, proteins, nucleic acids), determine the properties and structure of kerogen geopolymers and GI molecules. Under certain conditions, the H atom is able to combine simultaneously with two other atoms. As a rule, it forms a strong covalent bond with one of them, and a weak one with the other, which is called hydrogen bond.
OXYGEN - The most common element of the earth's crust, it is 49.13% by weight. O has serial number 8, is in period 2, group VI, atomic mass 15.9994. Three stable isotopes of O are known - 16 O (99.759%), 17 O (0.0371%), 18 O (0.2039%). There are no long-lived radioactive isotopes of O. Artificial radioactive isotope 15 O (T 1/2 = 122 seconds). The isotope ratio 18 O/16 O is used for geological reconstructions, which in natural objects varies by 10% from 1/475 to 1/525. The polar ices have the lowest isotopic coefficient, the highest - CO 2 of the atmosphere. When comparing the isotopic composition, the value is used d 18 O, which is calculated by the formula: d 18 O‰= . Per standard the average ratio of these isotopes in ocean water is taken. Variations in the isotopic composition of O in gp, water are determined by the temperature at which the process of formation of specific minerals proceeds. The lower T, the more intensive isotope fractionation will be. It is believed that the O isotope composition of the ocean has not changed over the past 500 million years. The main factor determining the isotopic shift (variations in the isotopic composition in nature) is the kinetic effect determined by the reaction temperature. O under normal conditions, the gas is invisible, tasteless, odorless. In reactions with the overwhelming majority of atoms, O acts as oxidizer. Only in the reaction with F is the oxidizing agent F. O exists in biallotropic modifications . First - molecular oxygen - O 2 The second modification is ozone - O 3, arr under the action of electrical discharges in air and pure O, in radioactive processes, by the action of ultraviolet rays on ordinary O. In nature About 3 formed constantly under the influence of UV rays in the upper atmosphere. At an altitude of about 30-50 km there is an "ozone screen" that traps the bulk of UV rays, protecting the organisms of the biosphere from the harmful effects of these rays. At low concentrations, About 3 pleasant, refreshing smell, but if in the air more than 1% O 3 it is highly toxic .
NITROGEN - concentrated in the biosphere: it prevails in the atmosphere (75.31% by weight, 78.7% by volume), and in the earth's crust it weight clark - 0.045%. Chemical element of group V, 2 periods atomic number 7, atomic mass 14.0067. Three N isotopes are known - two stable 14 N (99.635%) and 15 N (0.365%) and radioactive 13 N, T 1/2 = 10.08 min. General scatter of ratio values 15 N/ 14 N small . The oils are enriched in the 15 N isotope, while the accompanying natural gases are depleted in it. Oil shale is also enriched in heavy isotope N 2 colorless gas, tasteless and odorless. N unlike O does not support breathing, the mixture N with O is most acceptable for the breath of most of the inhabitants of our planet. N is chemically inactive. It is part of the GI of all organisms. The low chemical activity of nitrogen is determined by the structure of its molecule. Like most gases, except for inert ones, the molecule N consists of two atoms. In the formation of a bond between them, 3 valence electrons of the outer shell of each atom participate, forming triple covalent chemical bond , which gives the most stable of all known diatomic molecules. "Formal" valency from -3 to +5, "true" valence 3. Forming strong covalent bonds with O, H and C, it is part of the complex ions: -, -, +, which give easily soluble salts.
SULFUR - e-t ZK, in the mantle (ultrabasic rocks) it is 5 times less than in the lithosphere. Clark in ZK - 0,1%. Chemical element group VI, 3 periods, atomic number 16, atomic mass 32.06. Highly electronegative el-t, exhibits non-metallic properties. In hydrogen and oxygen compounds, it is in the composition of various ions. Arr acid and salt. Many sulfur-containing salts are sparingly soluble in water. S can have valences: (-2), (0), (+4), (+6), of which the first and last are the most characteristic. Both ionic and covalent bonds are characteristic. The main value for natural processes is the complex ion - 2 S - non-metal, chemically active element. Only with Au and Pt S does not interact. Of the inorganic compounds, in addition to sulfates, sulfides and H2SO4, oxides of SO 2 - a gas that strongly pollutes the atmosphere, and SO 3 (solid), as well as hydrogen sulfide, are common on Earth. Elementary S is characterized by three allotropic varieties : S rhombic (most stable), S monoclinic (cyclic molecule - eight-membered ring S 8) and plastic S 6 are linear chains of six atoms. 4 stable isotopes of S are known in nature: 32S (95.02%), 34S (4.21%), 33S (0.75%), 36S (0.02%). Artificial radioactive isotope 35 S c T 1/2 = 8.72 days. S is accepted as standard. troilite(FeS) from the Canyon Diablo meteorite (32 S/ 34 S= 22.22) Oxidation and reduction reactions can cause isotopic exchange, which is expressed in an isotopic shift. In nature, it is bacterial, but thermal is also possible. In nature, to date, there has been a clear division of the S of the earth's crust into 2 groups - biogenic sulfides and gases enriched in the light isotope 32 S, and sulfates, included in the salts of oceanic water of ancient evaporites, gypsum containing 34 S. The gases associated with oil deposits vary in isotopic composition and differ markedly from oils.