Uranium in the earth's crust was introduced hydrothermally or magmatically. The Source of uranium present at specific deposits has been postulated to be alkalic granites, arkoses or volcanic ash-tuffs. In many areas, the alkalic granites have an unusually high uranium content. Uranium is leached from the granites, syngenetically disseminated in the sediments and epigenetically concentrated by ground-water. Once the uranium concentrations are emplaced, they are remobilized only for relatively short distances within the same environment.
In nature, uranium exists in different valence states: U+4 (tetravalent), U+6 (hexavalent and U(OH)+3 (trivalent). Mineralogy and geochemistry of the valence states are dissimilar. Tetravalent uranium is stable only under reducing conditions. It oxidizes readily and releases energy into the +6 state. Hexavalent uranium forms a complex uranyl ion (UO2)+2 which combines with other elements, as oxides, silicates, sulfates, vanadates, arsenates, carbonates, phosphates and molybdates.
Uranium minerals may be termed PRIMARY or SECONDARY, depending upon their degree of oxidation and origin. The most important primary ore minerals are uraninite, an oxide and coffinite, a silicate. Pitchblende, also a primary mineral, is a colloform variety of uranitite. Through progressive oxidation, the secondary minerals may be formed from elements and ions derived from the progenitor (primary) minerals, from intrinsic host constituents, or from migrating ground-water, under varying pH conditions. Unlike the primary minerals, they usually occur in groups rather than as one mineral.
The transformation from primary to secondary minerals is gradual and complex. Oxidation is progressive and influenced by migrating ground-waters, ore exposure to atmosphere, topographic and climatic conditions.
Ideally, uraninite is UO2; however, due to decay of the uranium and oxidation, it occurs mostly as black amorphous ore, UO3. These ores are primary and occur in water-saturated rocks below the water table and the zone of oxidation.
Primary minerals other than uraninite and coffinite are a group containing niobium, columbium, tantalum, and titanium which are known as multiple oxides of uranium. Most of these minerals occur in placers and pegmatites, and in relatively small quantities. The group includes the uranium minerals: davidite, brannerite, betafite, euxenite, fergusonite, pyrochlore and samarskite. All of the multiple uranium oxides are refractory, and are not generally amenable to chemical treatment. Therefore, the extraction of uranium from minerals in this group is very costly.
In the beginning stages of oxidation, uranium in vanadiferous ores forms hydrous oxides or black colloidal materials. These changes may be hardly recognizable and include an increase to 6-valent uranium and redistribution of ore. Redistributed ores are frequently brownish or grayish and are the intermediate step between uraninite-coffinite and the yellow secondary uranyl minerals. Trace elements in the ores during further oxidation combine with the uranyl ion to form particular assemblages of secondary minerals. It is postulated that the oxides form first, followed by hydrates of lead and uranium, then vanadates or silicates or phosphates. Carbonates and sulfates are the last to form.
The oxidation of nonvanadiferous ores in the absence of arsenic, silicates and phosphates generally form uranium sulfates and carbonates. Since the sulfates and carbonates are soluble, they do not occur in large deposits and are mostly found as evaporation products, efflorescences, or halos around the cores of unoxidized ores. Zippeite and johannite are the most common uranyl sulfates, whereas the most abundant uranyl carbonate is schroeckingerite.
Where oxidized, the vanadiferous uranium ores generally form the relatively insoluble minerals: carnotite, tyuyamunite and metatyuyamunite. All are yellow uranium hydrated vanadates. They are ore minerals initially mined in the Colorado Plateau region, and are derived from the rauvite, a poorly defined uranyl vanadate that is the intermediary mineral and earlier oxidation product of uraninite. In contrast to the dull gray, black or brownish-black primary minerals, the secondary minerals have bright colours with shades of yellow, yellowish-green, green and orange. Once crystallized, carnotite is remarkably stable and fixes uranium. Commonly, the secondary minerals are found as crusts, powdery aggregates, platy, acicular, fan-like coatings and films, in contrast to the heavy and massive primary minerals.
Hydrated uranium oxides are not abundant because sulfates, carbonates or other anions are usually present to combine with the (UO2)+2 ion. Small quantities of becquerelite, schoepite and fourmarierite are closely associated with oxidizing uraninite. They develop next to the cores of unoxidized uraninite and are removed from the outside of the rim until the source is gone.
Some uranium minerals are formed by the evaporation of uranium-bearing ground-water. These caliche or calcrete deposits may also be termed primary in that they formed first rather than being oxidation products.
The most common and widespread secondary minerals are carnotite, tyuyamunite, tetatyuyamunite, torbernite, metatorbernite, autunite, metaautunite, uranophane, schroeckingerite and zeunerite. Minerals prefixed by meta, are chemically and physically the same, but are hydrated and have a lower water content. Limits of the fully hydrated minerals are between 10 and 12 H2O and 4 to 8 for the meta-hydrates.
Copper is associated with uranium at many deposits and in the oxidized zone, torbernite and metatorbernite may be found. Both are hydrous copper-uranium phosphates. In the same phosphates group are autunite and metaautunite; calcium uranyl hydrated phosphates. Both frequently occur with torbernite, and are similar except that calcium substitutes for copper. Uranophane is also similar to torbernite and autunite, except that silicate is contained in place of phosphate. Generally, it is found when calcium and silicate predominate. Similarly, autunite and torbernite are found when calcium/phospate; copper/phosphate predominate, respectively. Since autunite and uranophane contain calcium, they are relatively insoluble, and may form as caliche deposits or oxidation products.
Uranium minerals can be categorized according to their predominant chemical composition, as oxides, silicates, vanadates, phosphates, sulfates, carbonates, arsenates, molybdates, selenites, tellurites, multiple oxides of niobates-tantalates-titantates and urano-organic complexes. Minerals in the 12 groups and their characteristics are summarized below.
Uraninite, the most important uranium ore mineral, ideally is (UO2)+2. By oxidation and during radioactive decay, the uranium atoms disintegrate through a series of daughter products leaving an excess of oxygen atoms, resulting in (UO2)+2 becoming transformed into PbO and UO3. When UO3 is reacted with H2O, hydrated oxides, UO2(OH)2H2O are formed and are the ions comprising the secondary minerals.
As the only PRIMARY MINERAL in the uranium oxide group, uraninite has a high radioactivity and is the heaviest mineral in its pure state. It occurs in pegmatites, vein and stratiform deposits. Generally, it is least oxidized in the pegmatities, more oxidized in veins and relatively highoyt oxidized in sandstone stratiform deposits. Texture of uraninite varies from granular to dense and massive. Depending upon its combination with other elements, its colouration may be dull gray, black, brownish or olive-green.
Pitchblende, the colloform variety of uraninite, has a pitch-like luster, massive and frequently botryoidal. When redistributed, the pitchblende may be found as sooty material. Broggerite and clevite are varieties of uraninite containing rare earths.
Other minerals in the oxide group are uncommon to very rare. They may occur as hydrated oxides such as becquerelite and schoepite, or as hydrous oxides combined with the elements Ba, Bi, Pb, Na, K and Cu. These are all secondary minerals being the oxidation products of uraninite and other primary uranium minerals.
Although not widespread, the most prevalent of the secondary oxides found are schoepite, becquerelite and fourmarierite. These minerals usually occur at the rim, as halos, encrusting uraninite and in very small quantities. They are yellow to golden-orange in colour, and occur as microcrystalline to dense aggregates.
Primary minerals of the silicate group are coffinite and uranothorite. Coffinite is an important uranium ore mineral and is frequently found together with uraninite. It is fine-grained, blackish, friable and usually indistinguishable from uraninite. Positive identification can only be made by x-ray diffraction tests. It has a low solubility in reducing environment.
Uranothorite is a variety of thorium containing up to 12% uranium. It occurs in placers and fractured gneisses. In colour it is black, grayish to brownish, heavy with a specific gravity slightly less than uraninite.
Uranophane is the most common secondary uranium silicate mineral. It is found in a wide variety of environments, but most commonly in pegmatites and granites, as non-ore deposits. As a hydrous calcium uranium silicate, it has a straw-yellow colour and found mostly as fibrous radiating aggregates, or felted coatings. Frequently, it occurs as a pseudomorph after uraninite. Beta-uranophane is much rare than uranophane and has the same chemical composition. It has different optical properties and may convert to uranophane when crushed to powder.
Although not often recognized, boltwoodite is a fairly common secondary silicate mineral that is found in sandstone deposits. It is hydrated, contains potassium, is yellow and may be found as groups of slender needles, spherical pellets or felted masses. Kasolite is the only uranium silicate containing lead. It is fairly common, yellow to yellowish-brown and derived in the oxidized zones of lead-bearing uranium deposits. It may occur as rosettes of lathlike crystals, granular masses or dense gumlike crusts. Distinctive properties include its very high index of refraction and stout prismatic crystals. Soddyite is the only uranyl silicate that does not contain some other cation in addition to uranium. It resembles kasolite but lacks lead and is not common.
Other uranyl silicate minerals include hydrates of magnesium and uranium such as sklodowskite, or copper and uranium as cuprosklodowskite, both rare.
Carnotite, tyuyamunite and metatyuyamunite are important uranium ore minerals. Prior to the discovery of blackish unoxized uranium ores in the mid-1950's, these three minerals were the source of uranium mined in the Colorado Plateau region. All three minerals are relatively insoluble, and once formed, the uranium remains in a fixed state.
Usually, carnotite is found disseminated as canary-yellow earthy masses and coatings or replacements of fossilized logs and bones. It occurs where potassium is available in the interstitial hydrous micas of vanadiferous sandstone. Its bright canary-yellow colour may be dulled or concealed by iron-oxide staining, or organic matter. Distinguishing properties include its low hardness of 2 to 3, non-flourescence and x-ray pattern.
Tyuyamunite is the calcium equivalent of carnotite, and is similar in appearance, although it may be somewhat more greener in colour. It forms where calcium is available from limy host rocks or ground-waters. Although it contains more water than carnotite, it dehydrates easily to metatyuyamunite. Water content in metatyuyamunite varies from 3 to 5 H2O, compared to 8 to 10 H2O for tyuyamunite. Tyuyamunite has lower indices of refraction than metatyuyamunite and carnotite. Positive identification can only be made by x-ray powder-diffraction tests.
Rauvite is a hydrous calcium uranium vanadate, purplish to brownish-black in colour, and found as fine-grained masses, coatings and crusts. It probably has been derived by the alteration and oxidation of uraninite, and may be an intermediate mineral formed prior to the formation of carnotite or tyuyamunite. Since it is found only in small quantities, it has not been important as an ore mineral. Unvanite is similar to rauvite and its status is uncertain.
All of the uranium vanadates are secondary minerals. Francevillite, sengierite and vanuralite contain Ba and Pb; Cu and A1, respectively, and are all rare.
Uranium phosphates are all secondary minerals and commonly have a sheet-like structure and constitute the largest group of uranium minerals. Autunite and torbernite and their "meta" forms are widespread, found mostly as fracture fillings, joint coating and crystals.
Autunite and torbernite are commonly found together, and where crystallized, occur as thin to thick tabular plates. Most autunites are apple green to yellowish green in fan-like platy crystals or earthy masses. As a calcium uranium phosphate, its occurrence is related to calcium-rich ground waters or host rocks. It has a brilliant yellowish-green fluorescence which distinguishes it from non-flourescent torbernite
torbernite is apple to emerald green in colour, contains copper and is found in the oxidized zones of copper-ruanium deposits. It has distinctive small square crystals, and is not fluorescent. Often the crystals occur in groups, forming books. Minerals similar in appearance are zeunerite and autunite. Its x-ray pattern is distinctive.
Phospuranylite has a similar chemical composition to autunite, but is not fluorescent, and has a higher indice of refraction. It has a distinctive golden-yellow colour, high uranium content, and occurs as scaly to earthy coatings for aggregates. Although it is widespread, its occurrences have been of small quantity and not of economic value.
Hydrated phosphates containing lead are dewindtite and dumontite. Both are yellow, translucent, fluoresces green and are rare secondary uranium minerals.
All uranyl arsenates are secondary minerals, derived from alteration of arsenides and uraninites. They occur in small quantities as crusts and crystals. Probably, the most prevalent are zeunerite, metazeunerite and uranospinite.
Zeunerite hydrates easily, and most zeunerite is probably metazeunerite. Both closely resemble torbernite, contain copper, have a vitreous luster and are green in colour. They can be distinguished from torbernite by its higher indice of refraction and x-ray spacing pattern. It does not fluoresce.
Uranospinite, and its meta-form also are hydrated copper uranium phosphates, and closely resemble autunite and novecekite. They are yellowish-green in colour, and fluoresce bright lemon-yellow. Occurrences are rare.
All minerals in this group are of secondary origin, and are hydrated. The uranyl ion forms soluble complex minerals with carbonate ions. During oxidation, where pyrite is present, calcite is dissolved to give a solution high in HCO3. Acid or alkaline conditions tend to break up the carbonate complex and reduce the solubility of uranium in solution. As the evaporation of ground water continues, the solid uranyl carbonates are precipitated. Since the uranium carbonates are very soluble, they are not widespread and occur mostly as crusts, films and efflorescences.
Schroekingerite and liebigite are the two most common minerals in this group. Found as caliche deposits and in the oxidized portions of primary deposits, schroeckingerite has a greenish-yellow colour, and bright fluorescence of the same colour. It may occur as crusts, pellets, clusters and aggregates. In composition, it is a hydrated fluoride-carbonate-sulfate of sodium, calcium and uranium, and is the only mineral in this group to contain sulfate.
Liebigit3 is a hydrated carbonate of calcium and hexavalent uranium, formed from alkaline carbonate solutions. It has a green to yellow-green colour, occurring as short prismatic crystals, scaly aggregates or crusts. It is strongly fluoresces light green.
These secondary minerals are formed when uraninite and coffinite oxidize in the presence of iron or copper sulfides, in the absence of vanadium, phosphate or arsenic. Similar to the uranytl carbonates, they are soluble and are formed by the evaporation of ground-water. Commonly, they are seen as bright efflorescents on mine walls.
Zippeite, a latae secondary mineral, is a hydrated potassium uranyl sulfate. Mostly it occurs as thin coatings or warty aggregates, with a bright golden-yellow colour, and earthy appearance. Resembling zippeite is uranopilite and its meta-form. It also occurs as crusts, films and felted aggregates with a straw-yellow colour, and bright lemon-yellow fluorescence. Uranopilite is insoluble, and as a hydrated basic uranyl sulfate does not contain potassium or copper. Johannite is a hydrated copper uranyl sulfate with a light- to dark-green colour. It is found as scaly and fibrous coatings or small aggregates and is not fluorescent.
All uranium Molybdates are secondary minerals and are rare. Umohoite, the most known of the group, is soft, opaque and has a bluish-black colour.
Moctezumite is a bright orange rare secondary uranyl tellurite, containing both lead and tellurium.
Demesmaekerite Derriksite Guilleminite
All three uranyl selenties are rare.
Thucholite is black, similar in appearance to low-rank coal, and is a complex uraniferous hydrocarbon. It is organic and occurs in sandstones saturated with asphaltite, as nodules or pellets. Uranium content may vary from a trace to a few percent.
Uranium minerals in this group are multiple oxides and are considered to be Primary Materials. They occur mostly in pegmatities and placers in relatively small quantities. Content of uranium varies considerably from less than one percent to over 40 percent in some materials.
The most important minerals of the group are betafite, brannerite, davidite and euxenite. Davidite has been mined in Australia and Mozambique for its uranium content. During the 1950's the Bear Valley placer deposit in Idaho were mined. Euxenite was recovered for its uranium value as a by-product of columbium and tantalum production.
Euxenite is a rare earth titanium-tantalate containing 10 to 13 percent uranium oxide. When fresh, it is jet-black in colour, and brownish or reddish when altered. It may be disseminated in pegmatites, or found in placers near euxenite-rich pegmatites.
Davidite is a rare earth iron-titanium oxide, containing 7 to 10 percent uranium. It is a dark-brown to black mineral with a glassy to submetallic luster. Most occurrences are as angular irregular masses, sometimes with crystal outlines. When oxidized, thin yellowish-green secondary minerals may form on its surface.
A uranium-titanium oxide, brannerite is found in placers and pegmatites. It is identical to euxenite in appearance and contains 27 to 44 percent uranium. Upon oxidation, it alters to a brownish-green colour.
Minerals in this group offer potential for uranium recovery as by-products in mining for other metals.