THE JOURNAL OF
PHYSICAL CHEMISTRY (Registered in U. 8. Patent Office)
(0Copyright, 1956, by the American Chemical Society)
JUNE 19, 1956
VOLUME60 ~~
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NUMBER 6 ~~
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PHYSICAL CHEMISTRY IN GEOCHEMISTRY BY FARRINGTON DANIELS Contributionfrom the Deparlment of Chemistry, University of Wisconsin, Madison, Wisconsin Received December 87, 1856
I n the paragraphs which follow some of the subjects which were discussed a t the symposium will be referred to. Four of them are published in detail in this issue of THISJOURNAL. The phenomena mentioned above occur both in the igneous or hard rocks and in the sedimentary formations produced by ernsion. The primary rocks are formed by the cooling of molten material and efforts have been made to determine what type of material gives an over-all sample of the composition of the earth taken as a whole. Meteorites and the sun apparently are not satisfactory. Geological material may be transported in several different ways as solids, liquids, solutions, gases and colloidal materials. Rivers carry along both dissolved salts and colloidal particles. At high velocities they move large particles and rocks. The glaciers carry large rocks. Volcanic lava and volcanic dust are also important in the movement of material. Air is a very important geochemical agent and it penetrates to considerable depth below the surface. The oxidation state of an element can tell much (1) The following 4 papera are based on a Symposium on Geoconcerning the geochemical history of the mineral chemistry held on Sept. 1 2 , 1955, a t a meeting of the American Cbemiin which it is found. Combining a knowledge of cal Society in Minneapolis. I n addition to the papers published here, I.Urey, “Abundances of the symposium included the following: €C. pH and oxidation-reduction potentials, it is possithe Elements and the Composition of the Earth”; R. M. Garrels. ble to determine the conditions under which a given “Environments of Mineral Formation”; H. G. Thode, “The lsomineral such as iron oxide or lead sulfide will pretopes of Sulfur in Geochemistry”; G. W. Morey, “Experimental cipitate. Conversely, when a given mineral is found Geology”; E. J. Zeller and J. L. Wray, “Factors Influencing the Precipitation of Calcium Carbonate”; A. M. Pommer, “The Reduction the conditions of its environment can be accurately of Vanadium(V) Solutions by Wood or Lignite”; G . C. Kennedy, ascertained. ”Equilibrium Relations in the System SiOP-AhOa-HzO and AbOr The ratio of the sulfur isotopes Sa4t o Sa2offers a Hq0“; J. J. Katr and H. R. Hoekstra, “Chemistry of Uranium and valuable means for studying the geological history. the Genesis of Uranium Minerals”; G. Phair, “Some Aspects of the Geological Cycle of Uranium”; J. R. Arnold, “Age Determination Thode has found that Ss4 is enriched in a fractionand Other Applications of Cosmic-Ray-Produced Radioactivities”; ating process under oxidizing conditions which give J. A. 8.Adams, “The Log-Normal Distribution of Uranium and Alpha sulfates and that it is depleted under reducing conActivity in Obsidians, A Metamorphic Sequence and Wisconsin Water”; P. K. Kuroda, “Some Aspecte of the Geochemistry of ditions which give sulfides. Native sulfur is in a Radium”; D. R. Carr and J. L. Kulp, “Development and Application reduced state and it is depleted in Sa4. The oxidaof the Potassium-Argon Method of Age Determination”; W. R. tion history of a mineral deposit or a petroleum deEckelmann and J. L. Kulp, “Some Aspects of Age Determination by posit may be studied from this isotopic ratio and, the Uranium-Lead Ratio.” 705 The purpose of this symposium is to call attention to the many interesting problems of geochemistry which can be attacked through physical chemistry, and to point out also unique experiments started long ago in the geological past, which are now available for physical chemical studies. Geochemistry is of interest alike to those who want to find new mineral deposits and to those who merely seek an understanding of the geological processes by which nature has laid down our minerals in their present locations. Many branches of physical chemistry are involved in geochemistry, including thermodynamic equilibria of solids, liquids and gases, phase diagrams, oxidation and reduction, pH, solubility, precipitation, crystallization, electrolytic reactions, adsorption, sedimentation, kinetics of rate processes, heat transfer, radioactivity, isotopic tracers, nuclear reactions, radiation damage and thermoluminescence. Only a few of these can be illustrated in this symposium but the general principles are applicable to many geological phenomena.
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in fact, it may be possible to determine when in the geological past the atmosphere acquired its oxygen. Water is another geochemical agent of great importance and it must be remembered that the geological temperatures and pressures have frequently been very high. I n fact, some of our valuable mineral deposits are best explained on the basis of transportation in highly superheated water. The study of the crystallization of molten material to give igneous rocks requires an understanding of the phase relations of at least ten metallic oxides. I n the crystallization from the molten magma water is a n important factor not only in phase relations but in the generation of high steam pressures and the origin of volcanoes. It is important but difllcult to simulate in the laboratory the conditions of very high pressures and temperatures which existed in the geological formation of igneous rocks. Aluminum silicates and aluminum oxides are among the materials which have been studied in water under extreme conditions. Coprecipitated impurities are often important in setting the crystal pattern as illustrated in the classical and confused interpretation of calcite and aragonite deposits. Impurities such as iron, manganese and strontium affect the crystal form of the CaC03, and they in turn are determined by their solubility products and affected by the p H . The pH in turn is influenced by the temperature and the carbon dioxide content of the solution. Quantitative high temperature chemistry being developed now on a laboratory scale above lOOO", is finding new implications for geochemistry, specifically in the reactions of volatile hydroxides and nit rides. The second half of the symposium was concerned with the geochemistry of uranium and nuclear geology, including examples of geological dating. It is well recognized that the Atomic Energy Commission's support of research on isotopes has led to important advances in chemistry, biology and physics. I n a similar manner, its support of projects connected with uranium has advanced considerably the development of geology and geochemistry. The perfection of electronic and chemical methods for measuring minute traces of uranium has given this element new importance in geological studies. Uranium has in fact become a tracer for use in geochemical processes. The use of the Geiger counter and scintillometer has been invaluable in rapid field studies. The chemistry of uranium in its various oxidation states and complex ions is much better established because of recent laboratory studies. From these it is possible to predict the geochemical behavior of uranium. I n the crystallization of igneous rocks such as granites the uranium is expected t o concentrate in the mother liquor because it does not have an ionic radius suitable for fitting into the minerals which crystallize out first. There are still many unsolved problems in explaining the occurrence of the uranium which is found in some minerals. The deposition in veins from hot aqueous solutions is important.
Vol. 60
I n igneous rocks tetravalent uranium and thorium occur together because they -have about the same ionic radii. However, they part company in sedimentary rocks because the uranium is oxidized to a higher valence with a different ionic radius. The hexavalent uranium in solution is precipitated by other ions or adsorbed on the upper layers of lignite beds or concentrated in various other ways to give deposits of, minable value. Even the low concentrations of uranium obtained from the rainwater leaching of volcanic ash are sufficient to account for the formation of some secondary uranium deposits. Studies of the uranium content of various materials are significant because the analytical methods are so accurate even for very low concentrations. The obsidians are interesting because the uranium and daughter elements are sealed in glass so that the effect of weathering is slight. The uranium content of Wisconsin rivers is interesting because it fluctuates seasonally with the rainwater dilution of uranium dissolved from granites, and with the leaching of uranium-rich phosphate fertilizers used on the agricultural land. The lead-uranium ratio, the helium-uranium ratio and the argon-potassium ratio are important in age tests, but continuing research is necessary to improve their accuracy. It is necessary for example to be sure that all the gases are removed from the sample and analyzed, and it is necessary to know that none of the elements involved in the ratio have escaped as gas nor leached away by weathering. Certain elements can be created by natural radioactivity and neutron flux in the earth's crust. If mass spectrometric measurements could be made with greater precision than is now possible gadolinium would be an interesting element for dating purposes. An isotope of gadolinium is formed by neutron bombardment and since both isotopes have the same chemical properties and the same chances of loss, a determination of the isotope ratio and the neutron flux and neutron absorptions would give the age without errors due to weathering. One of the newest and most powerful methods of dating the younger rocks is based on nuclear transitions brought about by cosmic rays in the upper atmosphere. Tritium for decades, carbon14for thousands of years and beryllium1° for hundreds of thousands of years are finding important uses in geological dating. Radiation damage to radioactive materials has been examined as an interesting source of stored heat, but evidence has not been obtained to show that it is geologically significant. Metamict minerals of complex lattice are known which contain up to 130 calories of stored energy per gram and which are so badly damaged as to show no X-ray diffraction pattern until the normal lattice is restored by heating. Thermoluminescence can be produced in the laboratory by heating rocks which have been subject to a-ray bombardment over long periods of time. The a-ray activity comes from traces, about 1 part per million, of uranium or other radioactive elements which are present as an impurity in the rock, The intensity of thermoluminescent light in limestones,
June, 1956
OCCURRENCE OF TECHNETIUM ON THE EARTH’S CRUST
fluorites and certain other rocks can sometimes be used to estimate the age of the rock when the alpha
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ray activity and the sensitivity of the rock to radiation are also known.
REPORT ON THE OCCURRENCE OF TECHNETIUM ON THE EARTH’S CRUST1 BY G . E. BOYDAND Q. V. LARSON Contribution from the Oak Ridge National Laboratory, Oak Ridge, Tennessee Received December 67,1966
A search for primordial technetium has failed to reveal any traces of this element in a variety of terrestrial substances, nor are recent reports of the occurrence of technetium in molybdenite and in yttrotantalite confirmed. Post-war nuclear chemical researches have shown that technetium possesses no beta stable nuclear species, although three long-lived isotopes have been established. Both TcQ7and TcQQ possess half-lives of less than one million years and hence, if initially present, would have disappeared in the approximately 4.5 X loQyears since the formation of the Earth. The possibility for discovering primordial technetium has therefore appeared to depend on whether or not the half-life of long-lived TcQsexceeds 108 years. Results from chemical studies with gram quantities of fission product TcQQ suggest that technetium should exhibit a strong geochemical coherence with rhenium, which is known to be siderophile and chalcophile. Extremely sensitive methods for the detection of sub-microgram quantities were developed. The most reliable method is that of isotopic dilution using Tc9Qand mass spectrometric analysis. The neutron activation method based on the formation of 6.0 h Tc99“ from T c ~ * although more sensitive, is subject to interference, particularly by the Tcgg(n,n’) 6.0 h TcQQn reaction, The negative result obtained in this search appears to be supported by a recent study which indicates a fairly positive disproof of the existence of technetium in the Sun, and by the preliminary finding in this Laboratory that the half-life of TcQsis approximately 106 years.
The problem of the possible occurrence of element 43, eka-manganese, masurium,2aor technetium2bon the crust of the Earth has remained one of long standing in the chemistry and geochemistry of the elements. Quite recently, three notice^^-^ of the finding of technetium in terrestrial materials have appeared. The possible occurrence of technetium in the surface of the Sun has been proposed6 for which support appears to have been afforded recently by quantitative exploration^.^ Easily the most dramatic and possibly the most enduring of all these reports has been that by P. W. Merrill which has established the presence of technetium in Type S stars.8 Numerous previously unidentified lines in R Andromedae can now be assignedg to T c I, including four of the five strongest lines at 4297.06, 4262.26, 4238.19 and 4031.63 B., res$ectively, first accurately measured by W. F. Meggers’O using synthetic technetium. This discovery has stimulated renewed speculation11-16by astrophysicists concerning stellar processes. Technetium has been described by one author’l as “the touch-stone of cosmological theories.” (1) Presented before the Symposium on Geochemistry, 128th National Meeting, American Chemical Society, Minneapolis, Minnesota, September 11-16, 1955. (2) (a) W. Noddack and I. Tacke, Sit& Preuee. Akad. Wiaaenschaften. 400 (1925); (b) C. Perrier and E. SegrB, Nature, 159, 24 (1947). (3) W. Herr, 2. Naturforschg., 9A,907 (1954). (4) W. Noddack and I. Noddack. Angen. Chem., 66, 752 (1954). (5) E. Alperovitch and J. M . Miller, Nature, 176, 299 (1955). (6) C. E. Moore, Science, 114, 59 (1951); aee also, ibid.. 119, 449 (1954).
(7) H. Hubenet, C. de Jager and C. Zwaan, Mem. S O C . roy. mi., Liege, 14, 471 (1954). ( 8 ) P. W. Merrill, Astrophys. J . , 116, 2 1 (1952); Science, 116, 484 (1952). (9) P. W. Merrill, J . Roy. Astron. SOC.Canada, 46, 335 (1952). (10) W. F. Meggers and B. F. Scribner, J . Res. Natl. Bur. Standards, 46, 476 (1950). (11) P. Jordan, Naturwiss., 40, 407 (1953). (12) T. Gold, Mem. aoc. rou. sci., Liege. 14, 68 (1054). (13) St. Temesvary, ibid., 14, 122 (1954). (14) J. L. Greenstein, ibid., 14, 307 (1954). (15) A. G. W. Cameron, Astrophys. J., 191, 144 (1955).
This report will be concerned with the nuclear chemistry and with the geochemistry of technetium as they pertain to whether or not, and possibly where, this element might be expected on the Earth’s crust. Radioactive technetium almost certainly exists in certain terrestrial substances in extremely minute quantities: in uranium ores by virtue of the spontaneous fission of U238and by neutron capture fission of U2aK; in molybdenum-containing minerals as a result of the capture of cosmic ray neutrons; and possibly in other substances, as the end product of extremely high energy reactions caused by other components of cosmic rays. Here, however, the question is asked as to whether or not primordial technetium remains on the Earth today. Does a stable or an extremely long-lived technetium exist? What is the geochemistry of technetium? The answer to the first question would appear to hinge on nuclear stability considerations. Nuclear Stability of Technetium Isotopes.-A summary of available information, including unpublished results from this Laboratory, on the isotopes of technetium is afforded by Fig. 1. The majority of the data listed have been obtained since 1945 and, in addition to the results from Oak Ridge, have been contributed mainly by Prof. P. Sherrer and associates in Zurich and by Prof. M. L. Pool and associates a t Ohio State University. It is readily seen that the nuclear chemistry of technetium is complex: some seventeen activities are known; numerous isomers occur in both the even and odd mass numbers. The information given appears to be consistent with, and explicable in terms of, the single particle model of nuclei.l6 The observed decay energies are consonant with recent nuclear stability systematics. 17s18 I n agreement (16) M. Goeppert-Mayer and J. H. Jensen, “Elementary Theory of Nuclear Shell Structure,” John Wiley and Sons, New York, N. Y..
1955. (17) K. Way and M. Wood, Phya. Rea., 94, 119 (1951). (18) C. D. Coryell, Ann. Rev. Nuc. Sci., 2, 305 (1953).
