Geochemical Classification of the Elements
Geochemical Classification of the Elements
a classification that subdivides the chemical elements according to the criterion of their similarity—that is, their joint concentration in particular natural systems.
The best-known geochemical classifications of the elements were put forth by the Norwegian geochemist V. M. Goldschmidt (1924) and the Russian geologists V. I. Vernadskii (1927), A. E. Fersman (1932), and A. N. Zavaritskii (1950). In Goldschmidt’s classification the chemical elements are divided into four groups—lithophile, chalcophile, siderophile, and atmophile—taking into account the positions of the elements in the periodic system, the types of electronic structures of atoms and ions, the specifics of the appearance of an affinity for a particular anion, and the position of a particular element on the atomic volume curve.
Lithophile elements are the elements of rock. As in the atoms of inert gases, there are eight electrons in each of the outer shells of their ions (in the lithium series there are two). They are difficult to reduce to the elementary state; compounds with oxygen are most characteristic of them. The overwhelming majority of these elements are components of silicates. The lithophile elements also occur naturally as oxides, halides, phosphates, sulfates, and carbonates. For the most part they are paramagnetic; they lie on the descending parts of the atomic volume curve. They include 54 elements: the alkali and alkaline earth metals; boron, aluminum, and scandium; the lanthanides and actinides (actinium, thorium, protactinium, and uranium); carbon, silicon, titanium, zirconium, hafnium, phosphorus, vanadium, niobium, tantalum, oxygen, chromium, and tungsten; and the halogens and manganese (possibly technetium and astatine).
The chalcophile elements (according to Goldschmidt), or the thiophile elements (according to J. R. Hillebrand; 1954), are the elements of the sulfide ores: copper, silver, gold, zinc, cadmium, mercury, gallium, indium, thallium, germanium, tin, lead, arsenic, antimony, bismuth, sulfur, selenium, and tellurium. Their cations have 18 electrons in the outer shell (S2-, Se2-, and Te2- have eight electrons each). In nature they are found as sulfides, selenides, tellurides, and thio salts (an exception is tin, which occurs as cassiterite, Sn02). Silver, gold, copper, arsenic, sulfur, bismuth, and some others occur naturally in the elemental forms. They are for the most part diamagnetic and lie on the ascending parts of the atomic volume curve.
The siderophile elements are elements with an electron shell that is being filled up. They include all the elements of Group VIII of the periodic system, as well as molybdenum and rhenium—11 elements in all. They lie at the minimums on the atomic volume curve, and they are ferromagnetic and paramagnetic. They exhibit a specific chemical affinity for arsenic—sperrylite, PtAs2; loellingite, FeAs2; chloanthite, NiAs2; and cobaltite, CoAsS—and a somewhat lesser affinity for sulfur—pentlandite, (Fe,Ni)9S8; molybdenite, MoS2; and others—as well as to phosphorus, carbon, and nitrogen. In nature the platinum metals occur mainly in the elemental state, and iron occurs both as oxides or silicates and as sulfides, less frequently as arsenides and in the native state.
Atmophile elements are the atmospheric elements. This group includes all the inert gases (from helium to radon), nitrogen, and hydrogen—eight elements in all. The gaseous form is characteristic of them in nature. Most of the atmophile elements have atoms with closed outer electron shells and are situated in the upper parts of the atomic volume curve; they are mainly diamagnetic. Occurrence in the natural state is characteristic of most of them (except for hydrogen, which closely resembles the lithophile elements).
According to the classification given here, all the elements are distributed according to the most important natural genetic and paragenetic associations. The concepts of “bio-phile” (elements of living organisms) and “thalassophile” (seawater elements) are outside this classification.
REFERENCESFersman, A. E. Geokhimiia, vol. 1. Leningrad, 1933.
Shcherbina, V. V. Geokhimiia. Moscow-Leningrad, 1939.
Goldschmidt, V. M. Geochemische Verteilungsgesetze der Elemente, vols. 1-8. Christiana (Oslo), 1923-27.
Voitkevich, G. V. [et al.]. Kratkii spravochnik po geokhimii. Moscow, 1970.
V. V. SHCHERBINA