Uranium in rocks

Geochemical Nature and Types of Deposits. The cmst of the earth contains approximately 2—3 ppm uranium. AlkaHc igneous rock tends to be more uraniferous than basic and ferromagnesian igneous rocks (10). Elemental uranium oxidizes readily. The solubiHty and distribution of uranium in rocks and ore deposits depend primarily on valence state. The hexavalent uranium ion is highly soluble, the tetravalent ion relatively insoluble. Uraninite, the most common mineral in uranium deposits, contains the tetravalent ion (II). 

SOURCE C. Lewis, The Dating Game (Cambridge Cambridge University Press, 2000) A. Holmes, The Association of Lead with Uranium in Rock-Minerals, and Its Application to the Measurement of Geological Time," Proc. Royal Soc. Lond. A 1911, 85, 248. 

What does the proportion of lead and uranium in rock tell us about the age of the rock ... 

Holmes A. (1911) The association of lead with uranium in rock-minerals, and its application to the measurement of geological time. Proc. Roy. Soc. London A85, 248—256. 

The geochemistry of uranium and thorium has excited considerable interest on accoimt of their strategic importance. Smales determined uranium in rocks by neutron activation followed by isolation of fission product Ba (81). Interference from the fast fission of any thorium present in the sample and from beta-emitting barium isotopes formed by (n,y) reaction is discussed and methods of overcoming the diflSculties are described. The uranium content of two iron meteorites was determined by... 

However, you may be surprised to learn that you are constantly being bombarded with low levels of radiation. This radiation comes from many sources and is referred to as backgrormd radiation. Some of it is in the form of cosmic rays, which are particles that reach Earth from outer space. Small amormts of radioactive elements are found almost everywhere on Earth, as well—in wood and bricks used to make buildings, in the fabrics used in clothing, in the foods you eat, and even inside your body. Traces of uranium in rock layers beneath houses may also produce radioactive radon gas that enters the houses and can present health risks. Various sources of radiation are illustrated in Figure 21.20. 

Holmes A (1911) The association of lead with uranium in rock-minerals and its application to the measurement of geological time. Proc Roy Soc London A 85 248-256 Holmes A (1954) The oldest dated minerals of the Rhodesian Shield. Nature 173 612 Holmes A (1955) Dating the Precambrian of peninsular India and Ceylon. Proc Geol Soc Canada 7 81-106 Holmes A, Cahen L (1955) African geochronology. Col Geol Min Res 5 3-38... 

Radon-222 is a product of the nuclear disintegration series of uranium-238 (Figure 21.3) and is continuously generated as uranium in rocks and soil decays. As Figure 21.25 indicates, radon exposure is estimated to account for more than half the 360-mrem average annual exposure to ionizing radiation. 

Radon is a radioactive gas formed by the natural decay of uranium in rock, soil, and water. 

Singhal, R.K., Sharma, P.K., Bassan, M.K.T. et al. (2011). Comparative determination of uranium in rock phosphates and columbite by ICP-OES, alpha and gamma spectrometry, J. Radioanal. Nucl. Chem. 288, 149-156. 

Another group of radionuclides is generated by cosmic rays in the light elements in the atmosphere. For example, nitrogen forms radioactive carbon C. The natural radioactivity in nature is mainly due to radionuclides of light elements. Approximately 90% of the radioactivity is attributable to K (it represents 0.012% of natural potassium and has a half-life of 1.3x 10 years) and almost all the rest to carbon (half life of 5730 years). Increased radioactivity in certain areas can be caused by abnormally high incidence of radioactive elements (especially uranium) in rocks. 

Isotopes are also used to determine properties of the environment. Just as carbon-14 is used to date organic materials, geologists can determine the age of very old substances such as rocks by measuring the abundance in rocks of radioisotopes with longer half-lives. Uranium-238 (t1/2 = 4.5 Ga, 1 Ga = 10y years) and potassium-40 (t,/2 = 1.26 Ga) are used to date very old rocks. For example, potassium-40 decays by electron capture to form argon-40. The rock is placed under vacuum and crushed, and a mass spectrometer is used to measure the amount of argon gas that escapes. This technique was used to determine the age of rocks collected on the surface of the Moon they were found to be 3.5-4.0 billion years old, about the same age as the Earth. 

Radon is a naturally occurring, chemically inert, radioactive gas. It is colorless, odorless, and tasteless. It is part of the uranium-238 decay series, the direct decay product of radium-226. Radon moves to the earth s surface through tiny openings and cracks in soil and rocks. High concentrations of radon can be found in soils derived from uranium-bearing rocks, such as pitchblende and some... 

The four isotopes, as those of any element, have the same chemical properties. The four are not, however, uniformly distributed in the earth s crust the occurrence of three of them, in minerals and rocks, is associated with the radioactive decay of isotopes of thorium and uranium. In most minerals and rocks the relative amounts (or the isotopic ratios) of the isotopes of lead (often expressed relative to the amount of stable lead-204) are generally within well-known ranges, which are independent of the composition of the mineral or rock they are, however, directly related to the amounts of radioactive thorium and uranium isotope impurities in them. 

The best sealed-in minerals are zircons, zirconium silicate minerals which are formed when melted lava on the flanks of volcanoes solidifies. When the zircons crystallize out, they incorporate radioactive uranium (in particular 238U), which decays in several steps, leading Anally to the lead isotope 208Pb. The rate of decay is very low, as the half-life of uranium-238 is 4.5 x 109 years. Thus, the U-Pb-zircon method for age determination of Precambrian rock is very important. The fossils studied by Schopf were sandwiched between two lava layers (Schopf, 1999). The volcanic layers were dated to 3.458 0.0019 x 109 years and 3.471 0.005 x 109 years the age of the fossil layer (Apex chert) was thus determined to be about 3.465xlO9 years. 

Gale, N.H. and Mussett, A.E. (1973). Episodic uranium-lead models and the interpretation of variations in the isotopic composition of lead in rocks. Reviews of Geophysics and Space Physics 11 37-86. 

Fig. 8.4 Ideal cascade for uranium enrichment. The height of each section is roughly proportional to the number of stages in that section and the width at any stage to the amount of material being processed in that stage (Modified from Spindel in Rock, P. A., ACS Symposium Series 11. Isotopes and Chemical Principles 1975)...

The discovery of the calcrete hosted surfical uranium deposits of Namibia demonstrated the presence of widespread uranium in calcrete filled palaeochannels (Hambleton-Jones 1984) and similar mineral deposits have been observed elsewhere in Southern Africa, USA and Australia (Carlisle 1978 Hambleton-Jones 1978 Mann Deutscher 1978). The host rocks are typically lenticular bodies of alluvium, soil or detritus material cemented by calcite, gypsum, palygorskite, and other mineral phases. Uranium mineralogy is dominated by the mineral Carnotite [K2(U02)2(V04)2.3(H20)] as the main mineral in these channels. However other phases such as andersonite (Na2K3U03(C03)3(H20)6), liebigite (Ca2U02(C03)3(H20)io ... 

Rimsaite, J. 1982. Mineralogical and petrochemical properties of heterogeneous granitoid rocks from radioactive occurrences in the Grenville structural province, Ontario and Quebec. In Maurice, Y.T. (ed) Uranium in Granite. Geological Survey of Canada, SI-23, 19-30. 

At the time of the discovery of radio-activity, about seventy-five substances were called elements in other words, about seventy-five different substances were known to chemists, none of which had been separated into unlike parts, none of which had been made by the coalescence of unlike substances. Compounds of only two of these substances, uranium and thorium, are radio-active. Radio-activity is a very remarkable phenomenon. So far as we know at present, radio-activity is not a property of the substances which form almost the whole of the rocks, the waters, and the atmosphere of the earth it is not a property of the materials which constitute living organisms. It is a property of some thirty substances—of course, the number may be increased—a few of which are found widely distributed in rocks and waters, but none of which is found anywhere except in extraordinarily minute quantity. Radium is the most abundant of these substances but only a very few grains of radium chloride can be obtained from a couple of tons of pitchblende. 

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