Geochemistry of Uranium and Thorium
2-1 Introduction:-
Atomic number of Th is "90", of U is "92", both U and Th are belong to Actinide series. In the earth's crust they are classified as lithophile elements that are concentrated in silicate phase.
U has 3-natural isotopes (238, 235, and 234) the most abundant one is U238 (99.277 %) which has a half life time (4.5*10-9 year). In chemistry U has valences (3, 4, 5, and 6), in nature is tetravalent and hexavalent. U and Th nearly have the same behavior in igneous and metamorphic rocks, while in sedimentary environments we find that U is more mobile than Th. U is mobile under oxidizing conditions, especially in acidic or carbonate rich waters, and immobile under reducing conditions, water rich in organic matter and Fe-oxides where they absorb it.
2-2 Geochemistry and Occurrence of uranium and Thorium:-
The geochemistry of Th, U, Ce, and Zr as these elements appear in igneous rocks is very similar and is governed largely by the low concentration and high valence of their ions. The activation energy of migration of their ions (E-value) is relatively high, which would tend to freeze the ions in the main stage of crystallization of the magma, but their concentrate is ingeneral too low to permit the appearance of phases in which these elements are essential constituents. In broad terms they either concentrate in residual solutions or are included in solid solution in the minerals that form the bulk of the rock.
Uranium and Thorium owing to their high charge and large size of their quadra valent ions do not permit entrance into the normal rock minerals, and as the crystallization proceeds the residual solutions become enriched in these elements.
Zirconium much commoner than U & Th in part crystallizes in the magmatic stage as accessory zircon and may then house a certain amount of Th & U.
Both Monazite and Xenotime appear to house Th & U more readily than Zircon. Zr and rare earth, (Ce IV) together with more polyvalent elements such as Nb and Ta also tend to concentrate in the residual solutions.
Th, Ce, and U have a marked affinity for alkalic rather than granitic or intermediate rocks in the broad coarse of magmatic differentiation.
The main types of deposits that carry concentration of Th minerals are pegmatites, hydrothermal veins and detrital deposits. Pegmatites associated with alkalic igneous rocks, particularly nepheline syenites and their variants, are notably rich in Thorium. These pegmatites are also relatively high in rare earths, Ce, Y, Zr, Nb, Ca, P, and F while Ta & U are minor constituents.
These pegmatites are feldspathic but generally lack quartz and contain nepheline, together with pyroxenes and a variety of complex silicates containing Zr. Apatite is a characteristic accessory mineral in the pegmatites and it sometimes occurs separately as very large deposits associated with alkalic igneous rocks.
Pegmatites derived from granitic rocks tend to contrast with those from alkalic rocks in containing on the whole, a smaller amount of Th and are relatively enriched in Y over Ce, Ta over Nb and U over Th.
The granite pegmatites generally are quartz rich, with Zr present chiefly as Zircon.
Thorium may possible be obtained as a by product from magnetite apatite analogous to the production of uranium from sedimentary phosphate rocks.
Hydrothermal vein deposits containing thorium have become know only during the past few years. They differ from the base-metal sulphide type of veins, in which thorium is lacking in significant amounts, and they show resemblance to the Ce, and Th rich pegmatites associated with alkaline igneous rocks.
Thorium also occurs as a very minor constituent in deposits other than veins and pegmatites, such as with Nb in carbonatites and in a few contact metamorphic deposits. In the sedimentary cycle, thorium unlike uranium is not a significant constituent of carbonaceous marine black shales and apparently does not play an important biochemical role. Zirconium is like thorium in this respect and is deposited chiefly as detrital zircon in near shore clastic sediments. In sea water uranium is enriched relative to thorium.
Alluvial deposits contain the largest known reserves of Th. The thorium-containing mineral, monazite, occurs widely distributed as an accessory mineral in igneous and metamorphic rocks and in pegmatites. The high specific gravity, hardness and general stability of monazite cause it, when freed by weathering to become mechanically concentrated in alluvial deposits. The known deposits of monazite sands are large. The monazite is associated with ilmenite, zircon, magnetite and garnet chiefly, and is separated and concentrated by magnetic methods.
Deposits may contain concentration of uranium more than 10 times the average in the earth's crust (> 0.002% U), are formed by igneous and sedimentary rock forming processes, by ore forming processes.
Uranium has a large atomic radius, high chemical activity; its hexavalent compounds are relatively soluble in aqueous solutions. All these properties permit it to form compounds with many other elements, to inter the structure of a wide variety of minerals, to take part in many chemical reactions and to be deposited in many rocks and minerals of a diverse origin and compositions. These properties also that leads to the wide geologic distribution, lead to its dispersion, so the concentrations of U are not as great as these of other less active metals "e.g. lead, molybdenum…." The concentration of U in valuable deposits seams to be formed by its large radius and high valence, which prevent it from concentration in ordinary rock forming minerals, and also by the relative insolubility in aqueous solutions of its common tetravalent compounds, which lead to the precipitation of U in a wide variety of environments where reducing conditions prevail.
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