Mineralogy and Occurrence of
Uranium & Thorium
The chemistry of uranium is such that it can form both a cation and oxyanion in natural waters (ions that are positively- and acutely-negatively charged) and can combine with different chemical groups to be soluble under both acid and alkaline conditions. Such versatility can be extremely useful in mineral processing. The mineralogy of uranium is therefore quite complex – the element can form oxides, silicates, hydroxides, vanadanates, arsenates, phosphates, sulfates, carbonates, molybdates and even urananates. Consequently there are hundreds of minerals in which uranium is a major constituent –almost a third of all uranium minerals can occur in economic quantities. As the susceptibility of these minerals to acid or alkaline solutions, temperature, organic matter and oxidants varies widely, it is necessary to characterize the mineralogy of the ore in order to select the most efficient method of processing.
A uranium or thorium ore
mineral is one possessing such physical and chemical characteristics and
occurring in a deposit in such concentrations that it may be used for the profitable
extraction of uranium or thorium alone or with one or more other metals.
There are only a few
of the many uranium minerals that meet these qualifications and still fewer
in which U occurs as a major constituent.
The U-content of some important ore minerals is: pitchblende and uraninite 50-80% U3O8 (theoretically upto
85%); carnotite, torbernite,
tyuyamunite, autunite, uranophane and brannerite
45-60%; davidite, samarskite
and euxinite 1-18%.
The majority of U-bearing
minerals contain U in small or trace amounts or as an accessory to other
major constituents. Most of the
Uranium minerals in pegmatites and placers are
refractory i.e., they are extremely difficult to breakdown chemically to
A. PRIMARY URANIUM
These are found in veins
Pitchblende and uraninite in sedimentary
rocks are important. Refractory
minerals are found in placers. The
primary minerals are generally black to dark brown, heavy, with a shiny or
At or near the surface they
are altered to form bright coloured secondary
minerals. There a number of important
primary ore minerals. Uraninite and pitchblende are varieties of the same
(combined UO2 and UO3; 50-80% U3O8).
Naturally occurring oxide, cubic or octahedral crystal form, color greyish black with a greenish cast, streak black, D. = 8-10.5, H. = 5-6.
has a widespread occurrence in nature.
It occurs in small amounts in pegmatites and
is closely associated with the massive variety pitchblende. Also occurs as finely disseminated crystals
(combined UO2 and UO3; 50-80% U3O8).
Massive variety of uraninite occurring as
irregular masses with a rounded, layered or botryoidal
structure. Lighter than uraninite, D. = 6-9.
Occurrence: Pitchblende is the chief constituent
of all high grade uranium ores. (It
has provided the largest part of all U produced in the world). The principal occurrences are vein deposits
of the mesothermal type, in igneous, metamorphic
and sedimentary rocks. It is
associated with one of the primary ore minerals of Fe, Cu, Co, Pb, Ag and Bi. In
fact, the presence of these minerals in a deposit is an indication of
conditions favourable for pitchblende.
The common gangue minerals are quartz and carbonates
with fluorite, barite and hydrocarbons.
In vein deposits, it occurs as cavity fillings rather than
replacements and likewise, the richest deposits have formed where large open
fractures were present. Deposition of
pitchblende gives rise to strong alteration of the wallrock. Presence of hematite extending from the
pitchblende vein a few inches to a few feet into the wallrock
is the most characteristic feature.
Other alteration products are kaolin, chlorite, sericite
and silica minerals.
Pitcblende also occurs as
flat-lying deposits in sedimentary rocks where it is deposited between and
around the grains and available openings Eg
"copper-uranium" deposits in southern
Davidite: (Rare earth iron-titanium oxide;
7-10% U3O8). Angular, irregular
masses, sometimes crystalline, never in rounded botryoidal
shapes. Color dark brown to black, lustre glassy to sub metallic, a thin yellow-green
coating of carnotite or tyuyamunite
forms on surface on exposure to air, H. = 5-6, D. = 4.5.
occurs in hydrothermal veins of high temperature and pressure having
characteristics of pegmatites. Such veins occur in gneisses and schists, sometimes in gabbro
and anorthosite also. It is associated with ilmenite,
hematite, biotite, mica, quartz, calcite and pink
feldspar. It is never found as a pure
mineral, rather as complex intergrowths with ilmenite
which has similar physical properties and chemical composition.
[U(SiO4)n (OH)n]; 70% U3O8). Massive
with an irregular fracture, color black, lustre
adamantine, H. = 5-6, D. = 5.
Occurrence: Coffinite is found associated with uraninite
and intimately mixed with fine grained carbonaceous material and black
vanadium minerals. It is also found in
pitchblende veins. Because of its
close association and apparent physical similarity with uraninite,
its identification is of no practical concern to the prospector.
B. SECONDARY URANIUM
These are spectacular in
appearance, being variously coloured -- bright
yellow, orange, green. They occur as earthy or powdery materials or
as fine, delicate, needle-like, platy or flaky crystals. They fluoresce under ultra-violet
light. The secondary uranium ore
minerals have two major modes of occurrence: a) they occur in the weathered
or oxidized zones of primary deposits where they are formed by the
decomposition of primary minerals, and b) as irregular flat-lying deposits in
sedimentary rocks, primarily sandstones, also in conglomerates, shales and limestones formed by
precipitation from solutions.
(K2O.2UO3.V2O5.nH2O; 50-55% U3O8).
Soft powdery aggregates of finely crystalline material in thin films
or stains. Color lemon-yellow, lustre earthy, streak yellow, D. = 4. Carnotite is
Occurrence: As flat-lying, irregular, partially
bedded orebodies in sandstones. Associated with other secondary U-minerals
viz., tyuyamunite, torbernite,
and uranophane, rare oxides, carbonates, arsenates,
vanadates, phosphates and silicates. Carnotite is also
associated with vanadium minerals -- corvusite, hewettite, roscoelite; with silicified or carbonized wood, and a variety of coal-like
and asphaltic material.
(CaO.2UO3.V2O5.nH2O; 48-55% U3O8). The
physical properties are the same as those of carnotite except for a slightly more greenish color and
in some cases a very weak yellow-green fluorescence.
Occurrence: Always associated with carnotite, more abundant where there is appreciable
amount of calcium, usually as calcite or limestone.
iii) Torbernite & Meta-torbernite: (CuO.2UO3.P2O5.nH2O;60% U3O8) These are hydrous copper-uranium
phosphates. The difference between the
two being the number of water molecules present. Their physical properties are also
identical. They occur as flat, square,
translucent crystals that fluoresce with a faint green color. Color bright emerald green, lustre pearly, H. = 2-2.5, D. = 3.5.
Occurrence: These minerals occur with other
secondary U-minerals where copper has been present in depositing solutions or
surrounding rocks. Associated with
primary deposits where oxidation has occurred (except pegmatites
where there is no copper). Other
associated minerals are clays, limonite, quartz, pyrite, copper sulfides and
iv) Autunite & Meta-autunite: (CaO.2UO3.P2O5.nH2O; 60% U3O8). These minerals have the same composition as
torbernite with Ca substituting for Cu. They are generally associated with these
minerals and where copper is lacking, only these
minerals are found. Physical
properties of autunite are similar to torbernite except for color which is predominantly lemon
or sulfur yellow (sometimes apple green), and yellow to greenish yellow
fluorescence. Streak colourless to pale yellow or green,
occur as small square, rectangular or octagonal flat crystals or as thin
coatings or stains. h> = 2-2.5, D.
Occurrence: These minerals are an oxidation
product of pitchblende and uraninite and other
primary minerals. Likewise, they are
important constituents of primary oxidized ore deposits. They occur in all deposits of secondary
uranium minerals and are common secondary minerals in most pegmatites.
Uranophane: (CaO.2UO3.2SiO2.6H2O; 65%
U3O8). Hydrated uranium-calcium
silicate containing silica in place of phosphate in autunite. Occurs as stains or coatings without
apparent crystal form or as finely fibrous or radiating crystal
aggregates. Color lighter than autunite, D. = 3.85.
Occurrence: Origin and occurrence are very
similar to autunite and torbernite. Found abundantly where Cu and P are
absent. Though as widespread as autunite and torbernite, it is
usually present in smaller quantities.
Nevertheless it is an important constituent of secondary uranium
deposits in limestones. It is the most common secondary mineral in
non-commercial deposits in granites and pegmatites.
The number of thorium
minerals is small compared to uranium because thorium does not form secondary
minerals. For this reason the thorium
minerals are hard to recognize in the field.
They occur principally in granites and pegmatites,
or in veins or placers derived from these.
All thorium minerals contain some uranium, just as all primary uranium
minerals contain some thorium.
i) Monazite: [(Ce,La,Th)PO4; 1-15% ThO2]. Prismatic crystals or angular fragments or
as rounded small glassy grains in placers, color pink to brown; yellowish
brown; honey yellow; green, streak colourless to
pale brown or yellowish, lustre resinous, H. =
5-5.5, D. = 4.6-5.3.
Occurrence: Sparsely scattered in granites,
gneisses and pegmatites. Also found in hydrothermal vein
deposits. Concentrated by weathering
and erosion in sands and gravels along rivers and beaches (placer deposits),
and in conglomerates. Commonly
associated with ilmenite, gold, rutile,
zircon, magnetite, cassiterite, garnet, and primary
ii) Thorianite: (ThO2; upto
90% ThO2 and 33% U3O8). Small cubes
which become worn on edges when subjected to erosion, color black to brownish
or greyish, lustre
sub-metallic to greasy, streak black; grey or greenish, H. = 5-7, D. = 9 or
Occurrence: It is the richest of all thorium
minerals and occurs scattered in granites, gneisses
It occurs in sand and gravel deposits throughout the world, the
principal occurrence being in the gold and tin placers.
Thorite: (ThSiO4; upto
80% ThO2 and 25% U3O8). Small, square,
prismatic crystals with pyramid like points similar to zircon, colour black; greenish black or brown, lustre glassy or greasy, streak brown to orange, H. =
4.5-5, D. = 4-6.
Occurrence: Occurs in small amounts in granites,
granitoid rocks, gneisses and pegmatites,
and in sands and gravels derived from these.
It occurs also in beach sands where it is associated with magnetite,
garnet, ilmenite, and gold. A large number of varieties found in
different parts of the world are given different names viz., auerlite, thorogummite, calciothorite, ferrothorite, hyblite and enalite etc.
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