Geochemistry and the Biosphere. Vladimir I. Vernadsky
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Earth’s crust – includes the two last columns of D. I. Mendeleyev’s Periodic Table of elements; gold can be included here too. These elements give an almost infinite number of compounds in our laboratories and this is their difference from the rare gases. But their compounds are almost absent in the Earth’s crust. The minerals corresponding to them, mainly alloys, existing because of a complicated pneumolythic and magmatic process, or (for gold) because of abyssal hydrothermal processes in thermodynamic conditions that are distinctly different from those of the biosphere, change very little or not at all in the course of geological time. This stability, as well as that of the rare gases, is not complete. For some small part of their terrestrial mass, very slow chemical reactions must exist that change them, and these reactions are not well studied.
For instance, in the biosphere, oxygen compounds of palladium emerge. For palladium and for the nuggets of platinum and gold, there are numerous phenomena of weathering connected with the re-crystallization and change of the chemical composition of the alloy. For gold, their phenomena are connected with decomposition of telluride compounds. But these slow and local chemical reactions do not change the general character of the group – its terrestrial chemical inertness. It is furthermore characteristic of the whole group that these elements are only slightly affected by the aquatic structure of the Earth. They find themselves in a dispersed state in water solutions or connected with phenomena of sorption.
The third group of cyclical or organogenic elements is the largest in mass. It includes the greatest number of chemical elements and makes up almost the entire Earth’s crust. It is characterized by numerous reversible chemical processes. The geochemical history of these elements may be expressed by cycles. Each element gives compounds characteristic of a certain geosphere; these compounds are constantly being renewed. After more or less long and complicated changes, an element returns to its initial compound and begins a new cycle. This character of terrestrial chemical reactions was noticed for oxygen in the second half of the eighteenth century; the great scientists of that time, who had discovered the terrestrial gases and their properties, foresaw these characteristic chemical cycles.
I think that Dr. J. Pringle, the President of the Royal Society in London, was the first to express these notions in 1773, in his speech about J. Priestley. He defined the general features of the great equilibrium of vegetative green chlorophyll matter, together with animal matter, in relation to free oxygen and carbon dioxide. In 1842, two French scientists – J. B. Dumas and J. Boussingault – gave a clear picture of these cycles, and in the 1850s C. G. H. Bischof, J. Liebich and K. Moor transferred these notions to the rest of the matter of the Earth’s crust. Since then, science has collected a great quantity of empirical facts confirming these generalizations. These facts were not coordinated though, and are in a state of almost complete chaos. The importance of living matter for these cycles is being confirmed. This importance is observed not only for organogenic elements, such as C, O, H, N, P, and S, but also for metals such as Fe, Cu, Si, V, Mn, etc., and for all the chemical elements of this group, as we shall see.
The elements of this group are part of cycles that are characterized by chemical compounds, molecules, or crystals. These cycles are reversible only for the main part of the atoms involved, some of the elements inevitably and continually leave the cycle. This is natural; that is, the cyclical process is not completely reversible. Among such ways of leaving the cycle, the most significant dispersal of an element is its exit in the form of free atoms. In this way the element may leave the cycle forever. Still, it is clear that even if future discoveries more or less alter our present-day ideas, they will not deny the main empirical generalization regarding the prevalent significance of chemical compounds and reversible cycles in the history of the main mass of the Earth’s crust. The cyclic elements are included and play an important role in the aquatic apparatus of the Earth’s crust: they are included in water solutions (ions), and make up minerals formed by water. Only zirconium and hafnium seem to stand aside in this respect. Zr and Hf do not enter living matter, and germanium has not yet been found in it either, but judged by its aquatic history, it surely will be.
In the next group, that of dispersed elements, free atoms prevail. They cover a small part of matter, and they also have their cycles, which renew constantly. Not always though are they expressed by chemical compounds, by molecules; their compounds decompose more or less completely in one area of these cycles and renew under different thermodynamic conditions in another area. All the dispersed elements are characterized by the absence or rareness of chemical compounds, not only in certain areas of the Earth’s crust, but in the Earth’s crust as a whole.
There are two cases that are distinctly different from each other. Some of the elements, such as Li, Sc, Rb, Y, Cs, Nb, Ta and maybe In, form chemical compounds only in deep zones of the Earth’s crust. Their minerals are located in the surface area in the biosphere, but the new compounds of these elements – new minerals – are not formed here; the elements do not form vadose19 minerals. Instead, the elements are dispersed throughout the surrounding substance as “traces,” as analysts say, and have seemingly nothing to do with the mountain rocks they are found in.
The second case is that of iodine and bromine. They enter compounds with other elements only in the biosphere, which means that all their minerals are of vadose nature. If we try to reconstruct their history and find out their origin, we shall make sure that the sources of iodine and bromine are water solutions, and that living matter has extracted and concentrated them from those very solutions. In the depths of the crust we find iodine and bromine only dispersed as traces in minerals or in rocks – both metamorphic and plutonic – without any apparent relation to their chemical composition. Our knowledge is not sufficient to fully discover the history of gallium, but apparently it belongs to the second group as well. At the present time, its compounds are not known. The maximum content of gallium in a mineral – germanite – does not exceed 7 × 10-10 % of metal, and in micas its content reaches the same order.
All these are minerals of the deep regions of the Earth’s crust. Hence, the cyclic processes corresponding to these elements are specific; the elements give chemical compounds and free atoms in turn. But the majority of them do not enter compounds at all. They are constantly dispersed everywhere in the matter of our surroundings, apparently in the state of free atoms. They appear to be in a state close to that of rare gases, outside chemical reactions in the parts of the planet accessible to our investigation. The fact that all these elements belong to one and the same group, to that of atoms with uneven atomic numbers, evidently shows that the structure of these atoms has peculiar characteristics connected with this way of spreading.
This phenomenon deserves much more attention than is usually paid to it. Such a state of chemical elements can bring about processes of great cosmic importance. If it is the common property of elements with uneven atomic numbers, it can explain the prevalence of their antipodes – even elements – in the Earth’s crust and meteorites. All the uneven elements, except for Sc, Nb, and Ta, take part in the aquatic regime of the planet by being there in a dispersed state. Some of them, such as Li, Br, and I, are concentrated by living matter; Sc, Ga, Y, Nb, In, and Ta are concentrated by organisms that have not yet been studied.
The fifth group of elements includes very radioactive elements: the families of uranium, actinouranium and thorium. Here the incomplete reversibility of processes is quite evident. In general, uranium and thorium make up compounds included in reversible cycles, the closed cycles, which are analogous to the cyclic processes of the cyclic elements. But part of their atoms is lost in the course of the cyclic processes and does not return; it gets decomposed, changes and gives birth to other elements, two of which, helium and lead isotopes, belong to the groups of rare gases and cyclic elements, which are quite different chemical groups.
Now it is becoming clear that radioactive decomposition is characteristic not only of heavy atoms, but of light atoms as well. In 1907, Campbell discovered two radioactive elements with beta-radiation: potassium (from the group of cyclic elements) and rubidium (from the group of dispersed elements). In the case of rubidium, atoms of strontium must appear (belonging to a different geochemical group), and in the case of potassium, atoms of calcium (belonging to the same group) and of argon. Twenty-five years later, another period of discoveries began, in which von Hevesy and Pahl discovered the radioactivity of samarium, belonging to the group of rare Earths; it transforms to neodim