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 through alpha-radiation. We seem to be on the verge of great discoveries.
It is quite probable that frailty is a property of all elements. Even if these probabilities become scientifically proven facts, it will not affect the specific position of the group of radioactive elements in the system of classification. Decomposition of elements in this group is quantitatively incomparable with its possible manifestation in all other elements. Weak radioactive elements, in their geochemical manifestation, can be united into one group with strong radioactive ones as little as ferromagnetic elements can be united with the usual paramagnetic ones in case of magnetic properties.
The last group – that of rare Earths – must here and in the Periodic Table of chemical elements be presented as a special group. I think it consists of 15 elements that correspond to atomic numbers 57 to 71 without a break. Scandium and yttrium are sometimes included into this group although they do not really belong there. As we have seen, they belong to the group of dispersed elements. Concerning scandium, this seems certain to me from the chemical point of view. As for yttrium, some chemists, for instance R. Vogel, have come to the conclusion that it should be separated from the rare Earths for purely chemical reasons.
From the geochemical point of view, the main characteristic feature of these elements is the complete absence of their vadose compounds (compounds that have appeared in the biosphere). But their history in the biosphere is not quite clear as yet. It is evident that some of them get dispersed in it: for instance, gadolinium, samarium, europium, and neodymium. They, as well as cerium and lanthanum, enter living matter where their history is unknown. But at the same time, their principal minerals such as monacytes, xenotymes, and orthites, which appeared in magmas or pegmatite veins under conditions of high temperature and high pressure, are very stable in the thermodynamic
