URANIUM-GEOCHEMISTRY

or, The Geochemistry of Uranium Deposits

My master's thesis at the University of Utah covered the topic on GEOCHEMISTRY OF URANIUM ORE DEPOSITS and their relationship to paleostructures. My field study was conducted in the uranium-rich area of southeastern Utah, which is part of the Colorado Plateau. It is also one of the largest dinosaur fossils hunting grounds of Jurassic age (about 150 to 180 million years ago).

URANIUM is a naturally occurring element in nature, and is very radioactive. It is commonly detected on the ground using a scintillometer or geiger counter, which measures the amount of radiation emitted. Below you will read about its chemical aspects and how useful it is in our atomic age.

The general geochemistry of uranium in near-surface environments is a very fascinating topic. This element is commonly found chiefly in oxidized state as dioxide, UO2, which occurs in the well-crystallized variety uraninite and the microcrystalline form pitchblende in vein deposits.

Uranium, like copper or iron, is an element showing changes from one oxidation state to another in geologic environments. Details of its chemistry provides an explanation for much of its behavior. Uranium has many valence states (+2, +3, +4, +5, +6), but only the +4 and +6 states are of geologic interest. In its two lowest valences uranium is such a powerful reducing agent that it can liberate hydrogen from water, and the +5 valence in the presence of water is unstable with respect to +4 and +6. Primary uranium minerals are oxidized to uranyl ion, which is somewhat mobile in weakly acid solutions, and also in neutral and alkaline solutions if CO3= is present. Surface waters in contact with uranium minerals should contain a few parts per million of uranium. From such solutions uranium may be precipitated in the sexi-valent state by a variety of anions, forming the familiar oxidized minerals; or it may be reduced by any one of a number agents such as organic matter, forming UO2 or one of its hydrates. These processes take place in the famous deposits of the Colorado Plateau and central Wyoming, where oxidized minerals are disseminated in sandstones near the surface, and at lower levels black unoxidized ore is found in sediments containing much organic material. This type is conspicuous in concentrated minerals amounts around buried logs, fragments of bone, and isolated lenses of dark shale.

Incipient oxidation and loss of uranium by radioactive decay may increase the oxygen-uranium ratio, so that uraninite and pitchblende seldom show precisely the composition often approaching a composition symbolized by U3O8.

PITCHBLENDE - U3O8 - a massive mineral, brown to black in color with a distinctive luster, contains radium, and is the chief ore-mineral source of uranium.

URANINITE - UO2 - an ore-source of uranium, black octahedral or cubic oxide, contains thorium, the cerium and yttrium metals and lead, gives off helium gas when heated

In the zone of weathering (near-surfaces) these two chief sources of uranium are converted to one or more of the bright-colored uranium minerals, such as

CARNOTITE - [K2(UO2)2(VO4)2.3H20]--bright yellow in color

TYUYAMUNITE -- [Ca(UO2)2(VO4)2.nH2O] -- yellow-green color

AUTONITE - [Ca(UO2)2(PO4)2.nH2O] --

RUTHERFORDINE - UO2CO3

These minerals are slightly soluble, so that their uranium can be carried by surface water or groundwater into reducing enviroments (a bed of lignite or black shale, for example) and precipited as pitchblende or coffinite (USiO4.nH2O). The above ore minerals are the result of different degrees of oxidation below the surface. In a nutshell, this is the chemical process that gave origin to the ore deposits in Utah some 160 million years ago when there was still dinosaurs roaming the land in Jurassic time.

Uranium is used mainly in development of NUCLEAR ENERGY and generation of ELECTRICITY in power plants.

During World War II scientists dealt with a rather unexpected application in connection with a very complicated chemical problem. It had been found that the isotope of uranium having a mass of 235, has a nucleus unstable to collisions with neutrons. Such collisions result in a splitting of the uranium nucleus into lighter fragments (fission) and the liberation of large amounts of ENERGY in the form of heat and gamma-rays. It became necessary to separate 235-Uranium from the much more plentiful (but not fissionable) isotope 238-Uranium. These two are well-known isotopes and both contain the same amount of protons, but differ in the number of neutrons, 143 and 146, respectively.

"light isotope": mass number = 92+143 = 235 (uranium 235)

"heavy isotope": mass number = 92+146 = 238 (uranium 238)

Because of the great chemical similarity of the isotopes of an element, the chemical resolution of uranium into its isotopes was not feasible, and some physical method was sought. Since the rate of diffusion of a gas varies with its molecular weight, the composition of a gas mixture coming through an orifice will not be quite the same as that in the original sample, and the resolution of a gas mixture by sucessive diffusions is possible, at least in principle. Preliminary diffusion experiments with uranium-hexafluoride, UF6, a volatile uranium compound, indicated that 235-UF6 could indeed be separated from 238-UF6 by diffusion. Thus an enourmous plant was built for the purpose in Oak Ridge, Tennessee.

So the volatile compound UF6 of uranium and fluorine was used in a major process to isolate 235-uranium. In the process, UF6 diffuses many thousands of times through porous barriers, with lighter fractions moving on to the next stage and the heavier fractions being recycled through earlier stages. If UF6 were not volatile we might never have developed nuclear energy.

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