General Properties and Geochemical Behaviour of Uranium & Thorium

 URANIUM

-         Uranium is lustrous white, radioactive, metallic compound.  It is the last of the naturally occurring elements in the periodic table, with the greatest atomic weight 238 (At.  No. 92).  Sp. Gr. 19 (gold = 19.32, iron = 7.85).  M.P. = 1850o C.

-         Uranium consists of three natural isotopes - U238 (99.2739%, half life 4.5 X 109 years), U235 (0.7025%, half life 7.3 X 108 years) and U234 (0.0057%, half life 2.48 X 105 years).

-         Unlike gold, it is never found in free metallic state in nature.

-         It is the only element having a readily fissionable form -the isotope U235 has the greatest amount of energy stored in its atomic structure.

-         Uranium is ductile, malleable and capable of taking a high polish. In powder form, as obtained by reduction, it is pyrophoric i.e. it takes fire spontaneously on exposure to air.  Iron alloys containing more than 20% U are also pyrophoric, producing sparks when scratched or struck.

-         In course of atomic disintegration, U produces a series of elements including helium, radium, actinium and lead:

U238 --> 8 He4 + Pb206, U235 --> 7 He4 + Pb207

-         Salts of U absorb energy from light which may be given off in the form of fluorescence.

 Geochemical Behaviour:

-         Uranium is widely distributed throughout the earth's crust with minute amounts present in every rock type, natural waters (including sea-water).

-         Estimated concentration in the crust is 4 ppm, in sea-water 1 gm/1000 tons.  These figures compare with those for other important metals.

-         The metal is more abundant than gold, platinum, silver, bismuth, mercury, cadmium and antimony.

-         It is equal in abundance to tin, arsenic and molybdenum.

-         It is less in abundance than cobalt, lead, zinc, copper and tungsten.

-         In nature U may be tetravalent or hexavalent.  It is essential that tetravalent compounds are poorly soluble and precipitate, while hexavalent forms are sufficiently soluble facilitating uranium migration.

-         At the early stages of granitic magma crystallization, in reducing and alkaline environments, tetravalent U compounds enter rock forming minerals as isomorphous admixtures and form relatively high U concentrations in granites.

-         The highest U contents (upto 50% of the total quantity) are observed in such minerals as sphene, orthite, monazite, zircon, apatite, ilmenite and others.

-         The principal rock forming minerals, particularly the dark ones, contain 5-15% of the total amount of U; the remaining being present as microinclusions and in intergranular seams.

-         At the late stage of granitic magma, U is removed by hydrothermal solutions in the form of carbonate complexes to be later precipitated to form hydrothermal ore deposits which constitute significant endogenous deposits.

-         In exogenous environments, tetravalent U compounds become unstable and change to hexavalent ones.  It is therefore leached out of the near surface parts endogenous ore bodies and redeposited in the zone of secondary oxidized and reduced ores.

-         U may be adsorbed out of solutions by organic substances like peat, humus, decaying animal and plant remains, and may be precipitated with phosphates, glauconite, clay and iron hydroxides to form sedimentary U deposits.

 Abundance in Rock Forming Minerals:

-         Uranium occurs in a wide variety of minerals, but is characteristically concentrated in a few species of minor abundance.

-         Where present as trace quantities in such minerals as quartz and feldspars, its mode of occurrence is uncertain.  The following are some possibilities:

a)      Isomorphous substitution in the lattice.

b)      Concentration in lattice defects.

c)      Adsorption along crystal imperfections and grain boundaries.

d)      Inclusions as microcrystals of uranium.

 Abundance in Common Igneous Rocks:

-         Concentration in dunites and other ultrabasic rocks is extremely low (0.014 ppm).

-         There is a general tendency for uranium to increase towards the more silicic varieties of igneous rocks.

 Behaviour During Magmatic Processes:

-         Commonly occurrs in veins as uraninite or pitchblende.

-         Found in pegmatites along with rare earths (samarskite and euxenite).

 THORIUM

-         Atomic No. 90, Atomic Wt. 232, Sp. Gr. 11.7

-         Thorium is a heavy, gray, hard to fuse metal belonging to the titanium group.

-         Unlike uranium, thorium does not have a natural fissionable isotope, but when bombarded by nutrons, it transforms to U233, which is a fissionable material.

-         It is thus a "fertile" nuclear material since it can be made fissionable.

 Geochemical behaviour:

-         Like uranium, it is distributed widely in nature but in relatively small amounts.

-         Estimated thorium content in the crust is 0.001% (2.5 times U).

-         It is more abundant than tin, arsenic, molybdenum and precious metals.

-         It is equal in abundance to beryllium and cobalt.

-         It is less in abundance as compared to lead, zinc and copper.

-         A perceptible increase in the average thorium content from basaltic to granitic rocks is noted. (5 X 10-7% in ultrabasic rocks, 5 X 10-4% in basic and intermediate rocks and 1.8 X 10-3% in acidic rocks).

-         The Th content increases markedly in alkaline rocks of both basaltic and granitic series reaching a maximum of 6.5 X 10-3%.

-         Consequently the geochemical behaviour of Th in endogenous processes is manifested in its accumulation in granites and especially in alkaline magmas, dispersion in their accessory minerals and concentration in post-magmatic products viz.,

-         pegmatites, albitites and hydrothermal veins related to them.

-         Th is associated with minerals of rare elements eg tantalum and niobium, rare earths, cerium, yttrium, and uranium.

-         Common Th compounds are deposited prior to Uranium ones and are post magmatic products of a higher temperature.

-         Most of the Th bearing minerals are resistant to oxidation and under exogenous conditions are accumulated as placers.

Classification of Uranium and Thorium Minerals

-         Most of the Uranium- and thorium bearing minerals are oxides or silicates.

-         About 100 uranium and uranium bearing minerals are known in nature.  Of greatest practical value is uraninite (nasturan, pitchblende) UO2 (92%) and its amorphous variety (up to 60%).  Various types of mineable ores include:

 

Brannerite   (U,Ca,Fe,Y,Th)3Ti5O16?   28-44%

Davidite   (Fe,U)TiO3            20%

Uranothorite   (Th,U)SiO4         upto 17%

Uranophane   CaH2[UO2(SiO4)2]5h2O   67%

Coffinite   U[SiO4]1-x[OH]4x         68%

Autunite   Ca[UO2]2[PO4]2.8H2O      60%

Torbernite   Cu[UO2]2[PO4]2.12H2O   61%

Zeunerite   Cu[UO2AsO4]2.10H2O   56%

Carnotite   K2[(UO2)2V2O8].3H2O      64%

 

-         About 30 thorium and thorium bearing minerals are known, the most important of which are:

 

Thorianite   ThO2            88%

Broggerite   (U,Th)O2         6-15%

Thorite      ThSiO4         81.4%

Uranothorite   (Th,U)SiO4         50-70%

Ferrithorite   (Th,Fe)SiO4         45-65%

Thorogummite   (Th,U)[(SiO4)(OH)]4      45-65%

Aeschynite   (Cl,Th)(Nb,Ti)2O6      20%

Priorite      (Y,Th)(Nb,Ti)2O6      8%

Th-bearing Monazite  (Ce,Th)[(P,Si)O4   3.5-40%

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