Geochemistry and the Biosphere. Vladimir I. Vernadsky
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Institute in Göttingen is at present the largest center for this kind of scientific work.
Geochemistry is developing rapidly now; its influence and significance in purely scientific issues is constantly growing and increasing. The preparatory period is over. Separate branches are beginning to spring up thanks to the close connection of the large complex of its problems with fields that are in fact separated from geological disciplines, one of which is geochemistry. In this way, biogeochemistry has begun to separate. In 1927, the center for research in this field was established in Russia at the Academy of Sciences: its Biogeochemical Laboratory, which is not very powerful as yet.16
table 1
Table I Periodic Table of the Elements. [This is a modern version, included for reference. It is a far cry from the original version by Mendeleyev, Vernadsky’s professor. Elements in italics have been created in laboratories of nuclear physics; all are highly radioactive. The names of these are interesting: After honoring Greek gods, places, and scientists, the namers apparently gave up with #104 and used a numbering system. Ed.]
chemical elements in the earth’s crust; their forms of existence and classification
1 geochemistry classification of chemical elements
The first question that appeared in geochemistry was that of the number of bodies subject to its study: i.e., the number of different chemical elements and atoms that exist or possibly exist in the Earth’s crust. At present we can only investigate this problem as far as it concerns the surface layer of the Earth. As for this area, the question can be answered definitely enough. In general, taking into account only the isotopes we know, and not the possibly existing ones, we can state more than 200 different compositions of atoms, corresponding to the 92 atomic numbers – N. Moselli’s numbers – in the Periodic Table of D. I. Mendeleyev (table 1).
Within the limits of Mendeleyev’s table, all the representatives of its 92 atomic numbers are apparently known; they are either isolated, or their existence on our planet is confirmed by exact data. But it is possible that in the world – on our planet – there are several elements that are not covered by this table elements lighter than hydrogen, for which Moselli’s number is one, or heavier than uranium, which has atomic number 92. We do not understand as yet why the periodic system includes the number of elements we observe now (92), but it is inevitable for scientific thought to try to expand these limits, both by theoretical speculations and by experiment.17
Up till now these efforts were unsuccessful. The attempt of V. Harkins to regard the neutron as a chemical element is sure to be proven unsuccessful too. But as long as it is possible to approach the solution of these problems experimentally, they should not be dropped. This concerns the possible existence of transuranic elements (93 and higher). We should assume that the number of elements (92) and the number of the known isotopes (219) is not final but only temporary, as has been stated empirically.
Geochemistry can study elements only in the thin surface layer of the Earth, which does not exceed 16 to 20 km and which comprises the upper part of the Earth’s crust. I shall dwell upon this crust later, and we shall see that its total thickness reaches 60 to 100 km. The atmosphere is situated above it, but only its lower part, the troposphere, is chemically related to the Earth’s crust: its thickness is 10–15 km. The height of the whole atmosphere (i.e., of the gases following the movement of our planet’s body) is much more considerable, and it undoubtedly exceeds 700 km. The chemical elements of the Earth’s crust and the troposphere are distributed in quite different ways. The difference in the quantity of the various elements contained in it is enormous. The quantity of oxygen – the most widespread element – exceeds the quantity of radium by hundreds of billions of atoms, but radium is not the rarest elementary substance of the Earth’s crust.
Here I give a table (table 2) of the quantities or masses of the chemical elements contained in the Earth’s crust (including the atmosphere), expressed both in weight percentage of the Earth’s crust and in tons. The distribution of chemical elements in the Earth’s crust is given in weight percentage and is categorized by orders of ten: The mass of the Earth’s crust at a maximum thickness of 20 km is 3.25 × 1019 tons.
table 2 the abundance of chemical elements in the earth’s crust as a percentage by weight
(the weight of the earth’s crust at a maximum thickness of 20 km is 3.25 × 1019 t)
Comment: less than 10-11 % are Kr, Xe, Ne, Po, Pa, Ac and Rn.
This table was presented in its general features by the American scientist F. Clarke, who had studied these problems for more than forty years. I have introduced some corrections and changes and given it a different form. It is created on the basis of a huge number of exactly stated facts and many thousands of chemical analyses. The latest calculations of F. Clarke and W. Washington are based on 5508 complete chemical analyses of rocks that were done during the last 30 years. More than 100 years ago, in 1815, the English mineralogist W. Phillips was the first to make such calculations for 10 chemical elements. He returned to this task several times but his calculations, supported by D. Phillips and H. de La Beche, did not become part of science. Still, a small number of scientists, including Elie de Beaumont and A. Daubret, did not drop the task. Much later, in 1889, F. Clarke returned to this problem by systematically studying the principal chemical elements, and at the end of the nineteenth century, I. Focht tried to cover all the chemical elements in this way. Forty years is a sufficient amount of time to judge the correctness of this empirical generalization, and we must say that no significant changes have been made in F. Clarke’s table since then.
Studying the table, we see that there is a correlation between the abundance of chemical elements in the Earth’s crust and the composition of corresponding atoms. This correlation is very complicated and is not quite known to us. Prof. G. Oddo from Pavia had noticed long ago that chemical elements possessing even atomic numbers and containing nuclei of helium; that is, elements whose atomic mass is divisible by four, strongly prevail in the Earth’s crust; they comprise 86.5% of its total mass. Later, similar investigations were made and deepened by Prof. W. Harkins in Chicago. Harkins proved that the same fact could be observed in meteorites, where the prevalence of elements with even atomic numbers is still more considerable. It reaches 92.22% for metallic meteorites, and 97.69 % for stony meteorites.
Meteorites are celestial bodies independent of the Earth, and maybe of the Solar System as well. Their chemical processes have a very indefinite and distant analogy with the processes of the Earth’s crust. But the same regularity is observed in them – the same prevalence (even more pronounced) of elements with even atomic numbers, of atoms with even electric charges of nuclei.18 Nevertheless, this very simple observation raises very important problems. It proves that the chemical composition of the thin surface film of our planet, which as far as we know does not at all correspond to the composition of the whole planet, is not accidental.
The chemical composition of the Earth’s crust is connected with the definite structure of its atoms. Long ago, before Oddo’s time, D. I. Mendeleyev had pointed out that the entire principal mass of the substance of the Earth’s crust consists of light elements (not heavier than iron, #28). Apparently, if the even ordinal elements prevail, the even columns of the Mendeleyev table prevail too. The importance of these observations is evident, for they show that the chemical composition of the Earth’s crust cannot be explained by geological reasons. But no important further conclusions have been drawn yet, and the main point is that the field of empirical observation was not expanded after all, in spite of numerous attempts. Hypotheses and extrapolations dominate here. Very often scientists point to the lesser stability of the nuclei of uneven atoms, but this too is a hypothesis.
This phenomenon may be connected with