classification” is always strictly an empirical generalization, based without exception on scientifically proven facts. When the Periodic Table of chemical elements was being created, geochemical facts were not taken into consideration. That is why geochemical classification cannot be replaced by chemical classification. Geochemical classification should be based on the most general phenomena of the history of chemical elements in the Earth’s crust – all particularities should be ignored.
The most general phenomena can be reduced to the following three characteristic features:
1 Presence or absence of chemical or radiochemical processes in the history of the given chemical element in the Earth’s crust.
2 The character of these processes; their reversibility or irreversibility.
3 Presence or absence in the history of the chemical elements in the Earth’s crust of their chemical compounds, or molecules consisting of several atoms.
As in all natural classifications, the limits between the groups may happen to be indistinct. Sometimes, for instance, one and the same chemical element can be placed into different groups. In this case, the history of the main part of the mass of the atoms or the most striking feature of their geochemical history will be crucial.
So, in the history of very radioactive elements (for example in the history of radium) we notice reversible chemical processes for its compounds, and irreversible radiochemical processes for its atoms. Radium will find its place in a group of elements for which the reversibility of the processes will be the most striking feature. I think that the general difficulties we shall come across here do not exceed those inherent in any natural classification, for classification inevitably leads to simplifying parts of nature that are indivisible and inseparable in essence.
At present it is impossible to classify only three elements from the viewpoint described above: the newly-discovered #43, and also the more familiar #85 and #87, whose masses have not been determined as yet. From this point of view, chemical elements can be subdivided into the following six geochemical groups (table 4). The percentages are related to the 92 elements of the Periodic System. The figures in subscript correspond to the atomic masses.
In all these groups, the difference between even and uneven numbers is evident. For groups 1, 4, and 5 it can be expressed quantitatively with sufficient precision (table 5), and for groups 1 and 5 this correctness is without doubt. For group 3, embracing the majority of the elements, it becomes noticeable only concerning widespread elements; that is, elements that make up a large portion of the total mass of matter.
table 4 chemical elements in geochemical groups
table 5
For the other three groups the data are less precise. But F. Clarke’s table, which was presented long before the appearance of our ideas about atomic numbers and the positive charges of nuclei (and quite independently from them) shows that the elements of these groups, which are comparatively widespread, correspond to the even atomic numbers (table 6). So the prevalence of the mass of chemical elements with even atomic numbers is quite evident in five groups of natural classification; only group 4 does not include elements with even numbers.
table 6
The first group – that of rare gases – includes elements that take no part in the main terrestrial chemical processes, and that make up compounds with other elements only in exceptional cases. These atoms are preserved practically unchanged throughout geological time. A closer study of their history makes us discard the early ideas of C. Moureux, who suggested that they are absolutely inert in geological history, and that in them we observe the remains of the cosmic history of our planet. The quantitative intensity of their chemical manifestations in the thermodynamic field of our planet is so different from other compounds, so relatively small, that their actual difference from other terrestrial elements cannot arouse any doubt. However, their geochemical significance is enormous, and their role in the worlds beyond the Solar System must be great, too.
One of them – helium – is very widespread in the substance of celestial bodies, and apparently it plays a significant role there that has not yet been discovered. Its quantity in the Earth’s crust is changeable and seems to be increasing, as it is continuously appearing there due to decomposition of the nuclei of uranium, ionium, radium, radon, RaA, RaC, RaCl, polonium, thorium, radiothorium ThX, thorone, ThA, ThC, protactinium, radioactinium, AcX, actinone, AcA, AcC, AcCl, samarium, and possibly beryllium. It is expected that the process does not stop here, and that there are other elements that secrete alpha particles (just as helium atoms carrying two charges while decomposing, eventually lose their charges and transform to ordinary helium gas).
But there are cases in which the rare gases, called this way by chemists because of the difficulty of creating their chemical compounds under the conditions of our laboratories, do give compounds. These compounds, in the form of water solutions and hydrates as was recently shown by V. G. Chlopin, must play an important role in the structure of the biosphere. Finally, they may include also radon, a rare gas from group five, which is a carrier of great active energy in its different isotopes. In general, the role of the rare gases in the structure of our planet is much greater than their relatively small quantity; and this role is just beginning to reveal itself to us.
The second group – that of inert elements of noble metals in the 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
