Mineralogy and Occurrence of Uranium & Thorium Ore Minerals
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.
THE URANIUM ORE MINERALS.
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 recover U.
A. PRIMARY URANIUM ORE MINERALS:
These are found in veins and pegmatites. 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 pitch-like lustre.
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 mineral.
i) Uraninite: (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.
Occurrence: Uraninite 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 in conglomerates.
ii) Pitchblende: (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 Utah and northern Arizona. It also occurs in limestones.
iii) 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.
Occurrence: Davidite 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.
iv) Coffinite: [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 ORE MINERALS:
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.
i) Carnotite: (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 not fluorescent.
Occurrence: As flat-lying, irregular, partially bedded orebodies in sandstones. Associated with other secondary U-minerals viz., tyuyamunite, torbernite, autunite, schroeckingerite 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.
ii) Tyuyamunite: (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 carbonates.
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. = 3.1.
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.
v) 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 THORIUM ORE MINERALS
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 uranium minerals.
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 above.
Occurrence: It is the richest of all thorium minerals and occurs scattered in granites, gneisses and pegmatites. It occurs in sand and gravel deposits throughout the world, the principal occurrence being in the gold and tin placers.
iii) 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.
NOTE: For an excellent review of Uranium, its uses, production, deposit types and mineralogy see this article ‘Uranium Geology’ by Andrew Jackson.
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