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CHAPTER - II
GEOCHEMISTRY OF URANIUM
Uranium is one of the most abundant element found in the Earth's crust. It can be
found almost everywhere in rock and soil, in rivers and oceans. Traces of uranium are
even found in food and human tissue. However, concentrated uranium ores are found
in just a few places, usually in hard rock or sandstone (Abhas Singh, 2010). Naturally
occurring uranium consists of three different isotopes: U238, U235 and U234. No natural
isotopic fraction of U238 and U235 has been observed, and totally all natural materials
have a ratio of U238/U235. The third isotope of uranium U234 is an intermediate in
decay series of U238 and is formed from U238 by emission of an alpha followed by
two beta particles. The formation of uranium valence states between 2+ and 6+. In
minerals, only the valences 4+, 5+ and 6+ are known (Wedepohl, 1969). The present
chapter deals the geochemistry of Uranium in rocks, soils, plants and water.
2.1. Lithogeochemistry of Uranium:
Uranium occurs in a variety of minerals but characteristically is concentrated in a few
of minor abundance. Uranium generally formed in Di – valent (UO), Tri – valent
(UF3), Tetra – valent (UCl4), Penta – valent (UF5), Hexa – valent (UF6). Uranium is
present as trace amount in such major minerals as quartz and feldspars, its mode of
occurrence is uncertain, though following possibilities are 1. Isomorphous substitution
in the lattice, 2. Concentration in lattice defects, 3. Adsorption along crystal
imperfections and grain borders, 4. Inclusion as microcrystals of uranium minerals
(Ali, 2001). The fact that the uranium and thorium concentration of igneous rocks are
both closely related to the compositions of those rocks makes it difficult to subdivide
igneous rocks into meaningful radiometric groups (Assaf et al., 1997). In the igneous
environment, uranium is mobile element and tends to accumulate in the late
differentiates of igneous melts, primarily because the uranous (U4+) ion has a high
charge, which prevents it from substituting for any element its own size. The
abundance of uranium 4 ppm in granite (Table 2.1), 0.10 ppm in Oceanic tholeiitic
basalt, 0.01 ppm in Dunite and peridotite, 0.53 ppm in Plateau basalt, 0.45 ppm in
sand stones, 0.20 ppm in metamorphic rocks (Wedepohl, 1969).
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Table 2.1 abundance of uranium in various sources (Levinson, 1980)
Granite
4.8 ppm
Sedimentary rock
2 ppm
Average in earth continental crust
2.8 ppm
Soil
1 ppm
Vegetation Ash
0.5 ppm
Sea water
1- 4 X 10 -3 ppb
Drinking water
30 ppb
Uranium (IAEA, 1996) is ubiquitous. Its concentration, however, is conditioned by its
specific properties such as its high affinity to oxygen, its stability in tetravalent form
under conditions of high temperature and pressure and its stable because of large ionic
radii and absence of crystallo - chemical analogue (Jayaram, 1978). The low polarizing
properties of uranium and the consequent high solubility result in its enrichment as
are forming solutions in the late stage of magmatic crystallization in the pegmatoid
and hydrothermal phases. Although uranium occurs under diverse conditions it is
primarily associated with magmatic rocks. At crustal depths, however, it decreases
because of the increase in the content of mono and bivalent elements.
The concentration of uranium varies according to the substances it is mixed with and
the places where it is found. For example, when uranium is mixed with granite that
covers 60% of the Earth's crust, there are approximately four parts of uranium per
million, i.e. 999,996 parts of granite and four parts of uranium.
General abundance ratio (content in percent) of uranium in the Earth crust according
to A.P. Vinogradov is 2.5 x 10-4%. It forms part of all rock forming the Earth crust
and is also present in natural waters and living organisms. The lowest concentration
of uranium is present in ultrabasic rocks, the highest in sedimentary rocks (phosphate
rock and carbon shale).
Due to radioactive decay the amount of uranium in the Earth crust in the geological history
is reducing. When plutonic rock crystallizes uranium is squeezed out in the residual liquor
and concentrates in accessory minerals (Abdel Warith, 2000). Proximity of ionic radius
of U4+ to ionic radius of Th, Ca, determines a broad spectrum of isomorphous substitutions
of uranium in thorium and rare-earth minerals.
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During the hydrothermal process uranium actively migrates in the form of uraniumcarbonate complexes. Decrease of oxidation-reduction potential of solutions, decrease
of partial pressure of carbon dioxide, change of pH of solutions lead to dissipation
of such complexes and precipitation of uranium in the form of mineral phases.
Temperature intervals where hydrothermal uranium minerals are formed are estimated
at 100 - 220оС, sometimes at 300 - 350оС. Metamorphic processes from amphibolitic
to granulite
facies
are
commonly
accompanied
by carry
over
of uranium.
Ultrametamorphism sometimes is accompanied by some addition.
2.2. Pedogeochemistry of Uranium:
Uranium (U) is predominantly found in the +VI and +IV oxidation states in the
environment. U (VI) is generally more soluble and consequently more mobile than U
(IV) in soil. Depending on the redox conditions and the pH of the environment,
uranium can exist in different predominant forms; under oxic conditions uranium are
primarily present as the uranyl ion UO2, 2+ and its associated hydroxyl complexes for
low pH values. For sufficiently high total uranium concentrations at near-neutral pH,
U (VI) can precipitate as schoepite [UO3· 2H2O]. In the presence of high phosphate
concentrations, U (VI) can form uranyl phosphate solids that are less soluble than
other U (VI) solids. For under saturated conditions, U (VI) can be solubilized by
forming dissolved uranyl- phosphate complexes (Abhas Singh, 2010). U (VI) can
readily adsorb to iron (III) oxyhydroxides such as ferrihydrite, goethite, and hematite
and clay minerals. Adsorption to iron-bearing minerals is favored by the high binding
affinity of uranyl ion to geomedia. Iron (III) oxyhydroxides like goethite are common
minerals in soil and groundwater. They act as strong adsorbents for heavy-metals
because of their reactive surfaces and high specific surface areas [10]. U (VI)
adsorption to iron oxides typically increases from low to near neutral pH conditions.
However, at higher pH values, in the presence of inorganic carbon, U (VI) forms
stable dissolved complexes with carbonates that can limit U (VI) adsorption to iron
oxyhydroxides and increase the solubility of U (VI) precipitates. At reducing
conditions, uranium exists primarily as the mineral uraninite, UO2, which may be
oxidized to more mobile U (VI) species when exposed to oxidizing conditions (El
Mansi, 2000).
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In-situ immobilization is also likely to promote the most stable solid associated forms
of uranium. Based on its geochemistry there are different potential approaches to
immobilizing U (VI). It can be reduced to the relatively immobile U (IV) under
reducing conditions by microbial activity and by chemical reductants.
When
sustaining reducing conditions is not feasible, reduced U (IV) may remobilize by
oxidizing to U (VI) species, other in-situ remediation approaches are needed.
Phosphate addition is a potential strategy for in-situ uranium immobilization for
oxidizing conditions. The injection of phosphate-containing compounds may facilitate
formation of uranyl phosphate solids; these solids have relatively low solubilities and
are expected to form stable precipitates. This strategy could be particularly useful for
sites at low pH when carbonate effects are not dominant and uranyl phosphates may
precipitate out readily. Because phosphate is not abundant in most soils and aquatic
systems, a source of orthophosphate must be added to the subsurface. Orthophosphate
can be obtained from minerals or from organic compounds. A recent study reported
uranyl phosphate precipitation as a result of bacterial phosphates activity; bacteria use
an organophosphate compound for their metabolism and in-turn produce 4
orthophosphate that can combine with uranium. Uranyl phosphate precipitates were
also observed in an oxidizing bedrock aquifer resulting from interactions with iron
oxyhydroxide, the abundance of uranium 1 ppm in soil.
2.3. Hydrogeochemistry of Uranium:
Organic material in the pore spaces of rocks creates a reducing environment in the
water. The oxidizing, uranium-bearing water is passing through the rock precipitate
uranium in the rock where the reducing environment exists. Sometimes whole logs
(organic matter) buried in the rocks become rich in uranium deposited through this
process. Uranium is a widespread and ubiquitous element. It has a crustal abundance
of 2.8 parts per million, slightly more than tin. Primary deposits of uranium tend to
concentrate in granitic or alkalic volcanic rocks, hydrothermal veins, marine black
shales, and Precambrian age placers (Lidman et al., 2012). Secondary (or epigenetic)
deposits of uranium are formed later than the surrounding rocks that host the mineral
deposit. Uranium is soluble in oxidizing aqueous solutions, especially the U6+ valence
state, and can be redistributed from primary source rocks into porous sedimentary
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rocks and structures by groundwater and form secondary (epigenetic) uranium
mineral deposits.
Uranium concentrations are constant in sea water and are in the range 1- 4 x 10 -3 ppb
(1 – 4 ppm). These values may be compared with an average of 10 -5 to 10 -6 ppm for
thorium. Concentrations of uranium in fresh water are considerably more variable
than in the oceans. The concentrations are clearly controlled by the variety of factors,
such rock type leached by the water, rate of flow and rate of evaporation (Wedepohl,
1969).
Natural waters carrying the uranium carbonate and phosphate will also carry many
other anions, such as the sulphate, fluoride and chloride which may also form
complexes with uranium. Uranium complexes of some of these anions are important
under specific environmental conditions:
(a) Uranous fluoride in reducing water below pH 3-4.
(b) Uranyl fluoride, uranyl ion (UO2 2+), in oxidized water at Eh values of + 0.2 to 0.1 volt, and pH values 1-7.
(c) Hydroxide complexes in neutral to alkaline environments.
It is now clear that uranium in natural waters is normally complexed. The nature of
the complexing inorganic ions, such as carbonate, phosphate, hydroxide and fluoride,
will depend on the relative activities of these anions and the specific conditions of
Eh-pH. They are also affected by other variables such as the total dissolved solids
content and the presence of other ions.
Therefore, there is a trend toward analyzing for many of the variables found in water
samples collected for uranium surveys in an attempt to assistant in interpretation of
the results. These includes: U, PO4, HCO3, SO4, Cl, F, Ca, Mg, K, total dissolved
solids, conductivity, pH, Eh, and pathfinder elements (Levinson, 1980).
2.4. Biogeochemistry of Uranium
Uranium is low in the list of element abundance, comprising only 0.0027% of the
earth's crust. Apart from ore deposits, uranium is most abundant in high silica igneous
rocks and in shales, especially black shales. Black shales are commonly enriched in
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uranium with respect to other sedimentary rocks, sometimes to the extent of causing
these rocks, rich in organic matter, to be considered as possible commercial sources of
uranium. With atomic number 92 and atomic weight 238.03, uranium is the heaviest
naturally occurring element. There are eleven known isotopes, of which three—with
atomic weights 234, 235, and 238—occur in nature. The uranium content of coals is
highly variable and its occurrence sporadic; where higher than average, the uranium
tends to be concentrated in the stratigraphically highest coals or in the vicinity of an
unconformity. It is thus most probable that virtually all the uranium in coals has been
extracted from solution long after the deposition of peat and that almost no uranium
was associated with the original vegetation (Levinson, 1980). The plants ash contains
0.5 ppm of uranium general limit.
No plants have been recognized that are direct indicators of U mineralization,
although in Alaska it has been noted that lupine tends to favour U-rich soil
(Kovalevsky, 1972).
Several indirect indicators are known (notably Astragalus)
which have an affinity for selenium, and therefore are actually selenium indicators. U
is associated with Se in the sandstone roll-front type deposits of the mid-western
USA, where Se indicator plants (Anderson et al., 1954) have been used successfully
in the discovery of U mineralization in plants. Another geobotanical indicator of U
mineralization is the development of abnormal growth patterns, which have been
observed in the field and in laboratory experiments (Bakhit, 1989).
Such
morphological changes are not, however, a reliable guide to U mineralization, since
many other physical and chemical parameters may effect similar changes. Three
studies have noted changes from normal flower colours near U mineralization (P.W.
Swarzenskia et al., 1999). Shacklette noted a paling in the pink hue of fireweed;
Kovalsky and Vorotnitskaya noted albinism and varicoloured flowers of Caragana;
and Brooks indicated that U anthocyanins may give a bluish tint to flowers which are
typically red or pink (Anderson et al., 1954).
The fixation of U by plants and other organic matter in general. Plants grown in soils
spiked with U accumulated more U in their roots than in their aerial parts, although
the amount of U in the branch tips was found to bear a definite relationship to that
available in the soil. Roots with a high cation exchange capacity can absorb the most
U. Another study suggested that U compounds in plants probably form as a result of
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ion-exchange reactions between metal-bearing solutions and plant tissues. Organic
compounds can bind and transfer U. Studies have shown that U occurs as oxonium
complexes in cellulose and lignin, and as chelates. Humic and fulvic acids are the
dominant concentrators and transporters of U and they are believed to play a
significant role in the formation of secondary U deposits.
Low levels of soil phosphate greatly increase the ability of plants to accumulate U,
whereas high levels of carbonate have a similar effect. U is effectively absorbed by
plants that take up very little potassium (e.g. rose and pine families). Where formation
waters have a high salt content U tends to stay in solution, and in the presence of salts
and calcium, U can migrate considerable distances. Important observations for
biogeochemical prospecting are: 1) U is absorbed best by plants which have a fairly
acid cell sap and a high cation exchange capacity in the root. This acidity is estimated
to be pH 5, pH 4-5 and less than pH 5.2. One Russian study found the highest U
uptake to take place in peat at pH 6; and 2) plants with high transpiration rates
transport most ions to their upper parts. Kovalevsky indicated that plants in the boreal
forests of Siberia appeared to have a physiological barrier to U uptake, whereas Ra
migrated readily from the soils to the plants and no barrier seemed present. He noted
that physiological barriers are much more significant in the aerial portions of plants
than their roots, and that the barriers are absent in the roots of high-order plants and of
low significance in the lower plants. A later study defined U as a "low barrier"
element (i.e. only small quantities can be taken up by the high-order plants), but noted
that element absorption by plants is, on average, 3000 times more vigorous from
aqueous solutions than from the solid phase of soils. Thus where U is dissolved in
groundwater, much higher U concentrations in plants may result than where U is
present in the soils. This study concluded that the "barrier" exists not in the roots but
in the twigs, where U dissolved in fluids passing up the xylem tissues precipitates due
to a change in pH that occurs during photosynthesis U is in low concentrations in the
soils, but available in the formation waters that the roots are tapping (IAEA, 1996).
Thus there is a wide spectrum of results from a good to poor correlation, perhaps
dependant upon the porosity, permeability and hydrology regime of the substrate.
Each study area must be treated on its own merits. If a better geochemical profile is
obtained from soils than plants, then the soils should be collected. If a similar
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response is found in both sample media, then whichever is more practical should be
collected. If the U in the bedrock is incorporated in the crystal lattices of resistate
minerals (Levinson, 1972), there is likely to be no more than a subtle biogeochemical
response to the mineralization, despite the highly corrosive micro-environment
established around rootlets. However, if the U is labile (e.g. as pitchblende or other U
oxides in fractures and at crystal boundaries) then it can be readily assimilated by
some plants and a strong biogeochemical response may result.
2.5. Uranium mineralogy
Uranium is amphoteric (Frondel, 1958; IAEA, 1974). On account of this, in acid and
neutral media its forms cations, (bivalent uranyl ions UO2++) and in alkaline media it
forms urinate and polyuranate anions.
The large ionic radii, variable valency of uranium and its amphoteric property explain
the complex chemical behavior of uranium and the consequent large number of
mineralogical, varieties. There are on record over hundred identified uranium
minerals. Uranium is most stable in hexavalent state. In oxygen rich systems the
hexavalent uranium forms (U6O2++) ions. Minerals having (U6O2++) are stable but
relatively more soluble in leachants. They form fluorescent yellowish – green
coloured compounds, characteristic of uranyl ions. Tetravalent uranium minerals, on
the other hand, require suitable redox conditions before them can be solubilised. The
compounds of pentavalent uranium are transitory and quickly disproportionate to U4+,
and U6+ ions. Thus, no stable minerals of pentavalent uranium are formed in nature.
Uranium is associated with Th, Ra, Zr in simple and multiple oxides because U4+ is
diadochic with them. Nb, Ta and Ti are found multiple oxides, although uranium is
not isomorphous with them because of the valency state and ionic radii. Thus it also
occurs in monazite, zircon, columbite – tantalite, samarskite etc.
2.6. Uranium ore minerals
An ore mineral, is a “mineral which may be used for the extraction of one or more
metals”. A uranium ore mineral is therefore a mineral possessing such physical and
chemical properties and occurring in a deposit in such concentrations that it may be
used for the profitable extraction of uranium, either alone or together with one or
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more other metals. There are only a few of the many uranium minerals that meet these
qualifications and still fewer in which uranium is the major constituent.
Pitchblende and uraninite contains theoretically up to 85 per cent uranium but actually
between 50 and 80 percent; Carotids, torbernite, tyuyamunite, autunite, uranophane,
and brannerite are 45 to 60 percent.
Davidite, samarskite, and euxenite, for example contains only 1 to 18 per cent. The
majorities of uranium-bearing minerals however contain uranium in small or trace
amounts.
2.7. Primary Uranium ore minerals
Uraninite (combined with UO2 and UO3; 50-85 percent U3O8).
Uraninite is a naturally occurring uranium oxide with cubic or octahedral crystal
form. It has a specific gravity of 8-10.5 (iron = 7.85), a grayish-black color sometimes
with a greenish cast and a, it is also an important constituent of nearly all important
primary deposits, occurring closely associated with its massive variety, pitchblende
Hardness of 5-6, about the same as steel. Its streak is black. Its most widespread
occurrence is in pegmatites, in which it is found in small amounts, throughout the
world.
Pitchblende (combined with UO2 and UO3; 50-80 percent U3O8).
Pitchblende is the massive variety of uraninite, without apparent crystal form, that
occurs most abundantly in the rich primary vein deposits of uranium. It is the chief
constituent of nearly all high-grade uranium ores and has provided the largest part of
all uranium produced throughout the world.
Davidite (rare earth-iron-titanium oxide; 7-10 percent U3O8).
Davidite is a dark brown to black mineral with a glassy to sub metallic luster. It has
about the same hardness as pitchblende (5-6) and is somewhat lighter in weight
(specific gravity, 4.5). It occurs most commonly in angular, irregular masses,
sometimes with crystal outlines, but never in round, botryoidal shapes like
pitchblende.
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Coffinite
Coffinite is a uranium-bearing silicate mineral with formula: U (SiO4)1-x (OH)4x. It
occurs as black incrustations, dark to pale-brown in thin section. It has a grayish black
streak. It has a brittle to conchoidal fracture. The hardness of coffinite is between 5
and 6.It was first described in 1954 for an occurrence at the La Sal No. 2 Mine,
Beaver Mesa, America and named for American geologist Reuben Clare Coffin (1886
-1972). It has widespread global occurrence in Colorado Plateau-type uranium ore
deposits of uranium and vanadium. It replaces organic matter in sandstone and in
hydrothermal vein type deposits. It occurs in association with uraninite, thorite, pyrite,
marcasite, roscoelite, clay minerals and amorphous organic matter.
Coffinite's chemical formula is U (SiO4)1-x (OH)4 ray powder patterns from samples
of coffinite allowed geologists to classify it as a new mineral in 1955. A comparison
to the x-ray powder pattern of zircon (Zr SiO4) and thorite (Th SiO4) was the basis for
this classification. Preliminary chemical analysis indicated that the uranous silicate
exhibited hydroxyl substitution. The results of Sherwood’s preliminary chemical
analysis were based on samples from three locations. Hydroxyl bonds and siliconoxygen bonds also proved to exist after infrared absorption spectral analyses were
performed The hydroxyl substitution occurs as (OH)44- for (SiO4)2-. The hydroxyl
constituent in coffinite later proved to be nonessential in the formation of a stable
synthetic mineral. Recent electron microprobe analysis of the submicroscopic crystals
uncovered an abundance of calcium, yttrium, phosphorus, and minimal lead
substitutions along with traces of other rare earth elements. Coffinite was first
discovered in sedimentary uranium deposits in the Colorado Plateau region, but has
also been discovered in sedimentary uranium deposits and hydrothermal veins in
many other locations. Samples of coffinite from the Colorado Plateau were found
with black fine-grained low-valence vanadium minerals, uraninite and finely
dispersed black organic material.
Other materials associated with later finds from the same region were clay and quartz.
In vein deposits of the Copper King Mine in Colorado, coffinite was also found to
occur with uraninite and pitchblende. Formation of coffinite requires a uranium
source, and may happen in reducing conditions, as evidenced by the associated
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presence of low-valence vanadium minerals. Silica-rich solution provides such a
reducing condition in cases where coffinite results as an alteration product of
uraninite. This finding is consistent with the coffinite samples of the Colorado
Plateau, which included fossilized wood. In China, coffinite can be found in granite in
addition to sandstone. Concluded that coarse-grained coffinite most likely forms in
high temperature environments. Coffinite and uraninite precipitate inside brecciated
and fractured regions of altered granite at pressures between 500 to 800 bars and
temperatures at 126 to 178 °C.
2.8. Secondary Uranium ore minerals
Carnotite (K2O 2UO3 V2O5 nH2O; 50-55 percent U3O8). Carnotite, a potassium
uranium vanadate, is the most important of the secondary uranium ore minerals,
having provided possibly 90 percent of the uranium production from secondary
deposits. It is a lemon-yellow mineral with an earthy luster, a yellow streak, and a
specific gravity of about 4.
Tyuyamunite (CaO 2UO3 V2O5 nH2O; 48-55 percent U3O8).Tyuyamunite is closely
related to carnotite as indicated by the chemical formula, which is the same except
that calcium substitutes for the potassium of carnotite. The physical properties of
tyuyamunite are the same except for a slightly more greenish color than carnotite and,
in some cases, a very weak yellow-green fluorescence not found in carnotite.
Torbernite and Meta-torbernite (CuO U2O3 P2O5 nH2O; 60 percent with U3O8).
Torbernite and meta-torbernite are hydrous copper uranium phosphates, the only
difference between the two being the number of water molecules present; their
physical properties are identical.
Autunite and Meta-Autunite (CaO2UO3 P2O5 nH2O; 60 percent U3O8. Reference to
the chemical formula will show that these two minerals have the same composition as
torbernite, with calcium substituting for copper.
Uranophane (CaO2 UO3 2SiO2 6H2O; 65 percent U3O8) Uranophane is a hydrated
calcium uranium silicate containing silica in place of the phosphate of autunite.
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Schroeckingerite (NaCa3 (UO2) (CO3)3(SO4) F 10 H2O; 30 percent with U3O8).
Schroeckingerite is a complex hydrated sulfate, carbonate, and fluoride of calcium,
sodium, and uranium.
2.9. Ore genesis of Uranium
There are several themes of uranium ore deposit formation, which are caused by
geological and geochemical features of rocks and the element uranium. The basic
themes of uranium ore genesis are host mineralogy, reduction-oxidation potential, and
porosity.
Uranium is a highly soluble, as well as a radioactive, heavy metal. It can be easily
dissolved, transported and precipitated within ground waters by soluble changes in
oxidation conditions. Uranium also does not usually form very insoluble mineral
species, which is a further factor in the wide variety of geological conditions and
places in which uranium mineralization may accumulate.
Uranium is an incompatible element within magmas, and as such it tends to become
accumulated within highly fractionated and evolved granite melts, particularly alkaline
examples. (Rogers et al., 1969) These melts tend to become highly enriched in uranium,
thorium and potassium, and may in turn create internal pegmatite or hydrothermal systems
into which uranium may dissolve.
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