Properties and Geochemical Behaviour of Uranium & Thorium
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
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).
gold, it is never found in free metallic state in nature.
is the only element having a readily fissionable form -the isotope U235
has the greatest amount of energy stored in its atomic structure.
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
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
is widely distributed throughout the earth's crust with minute amounts present
in every rock type, natural waters (including sea-water).
concentration in the crust is 4 ppm, in sea-water 1 gm/1000 tons.
These figures compare with those for other important metals.
metal is more abundant than gold, platinum, silver, bismuth, mercury, cadmium
is equal in abundance to tin, arsenic and molybdenum.
is less in abundance than cobalt, lead, zinc, copper and tungsten.
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
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.
highest U contents (upto 50% of the total quantity) are observed in such
minerals as sphene, orthite, monazite, zircon, apatite, ilmenite and others.
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
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.
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.
in Rock Forming Minerals:
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:
Isomorphous substitution in the lattice.
Concentration in lattice defects.
Adsorption along crystal imperfections and grain boundaries.
as microcrystals of uranium.
in Common Igneous Rocks:
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.
During Magmatic Processes:
occurrs in veins as uraninite or pitchblende.
Found in pegmatites along with rare earths (samarskite and euxenite).
No. 90, Atomic Wt. 232, Sp. Gr. 11.7
is a heavy, gray, hard to fuse metal belonging to the titanium group.
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
uranium, it is distributed widely in nature but in relatively small amounts.
thorium content in the crust is 0.001% (2.5 times U).
is more abundant than tin, arsenic, molybdenum and precious metals.
is equal in abundance to beryllium and cobalt.
is less in abundance as compared to lead, zinc and copper.
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).
Th content increases markedly in alkaline rocks of both basaltic and granitic
series reaching a maximum of 6.5 X 10-3%.
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.,
albitites and hydrothermal veins related to them.
is associated with minerals of rare elements eg tantalum and niobium, rare
earths, cerium, yttrium, and uranium.
Th compounds are deposited prior to Uranium ones and are post magmatic products
of a higher temperature.
of the Th bearing minerals are resistant to oxidation and under exogenous
conditions are accumulated as placers.
of Uranium and Thorium Minerals
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:
About 30 thorium and thorium bearing minerals are known, the most
important of which are:
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Department of Geology
Aligarh Muslim University, Aligarh - 202 002 (India)