Uranium minerals coffinite

Uraninite crystals retain most of the actinides produced by the fission reactions and most of the fission products that have ionic radii close to that of uranium. When uraninite becomes hydrothermally altered or transformed during supergene weathering, that is, in the weathered zone of the Bangombe reactor, the reduced conditions in the close vicinity of the U ore allows its precipitation in newly formed Si-P-REE-uranium minerals (coffinite). 

The concentration of uranium in the sediments of roll-front deposits is typically in the 1000 to 2000 ppm range (8). The uranium occurs as coatings on grains and as interstitial material the predominant uranium mineral is uraninite Coffinite is often... 

Uranium existing in coal as silicate mineral coffinite and uraninite (UOj) poses a potential environmental hazard. Following combustion of coal, the refractory coffinite remains in the bottom ash and slag while the uraninite is vaporized and is later condensed on the fly-ash particles as the flue gases cool (Chadwick et aL, 1987). Comparative radiation exposure assessment studies on coal and nuclear-based electricity generation reveal that emissions from both are very low, but dose levels from coal-fired plants are equal to or slightly lower than from a nuclear power plant (UNGA, 1980 Chadwick et al., 1987). 

Most important mineral Pitchblende and uraninite, essentially UOj, but usually partly oxidized to UjOg (both in Figure M76). The uranium minerals camotite and coffinite are also important... 

Uranium occurs as concentrations of pitchblende with some coffinite in veinlets along faults, in brecciated zones and in satellite structures associated with the major structure. These minerals may occur also as fine disseminations in selected horizons in the host rocks. Secondary uranium minerals are locally abundant, especially in near-surface portions of deposits. Small amounts of quartz and carbonates are the gangue minerals. Associated minerals vary in type and abundance and determine whether a deposit is classified as monometallic or polymetallic. The latter may contain a wide variety of associated minerals, including sulphides. Hematitization and chloritization are the most common forms of alteration... 

Vein-like deposits in metamorphic rocks also occur in Pre-cambrian Shield areas, but they differ from unconformity-related uranium deposits in that they are not associated with major regional unconformities, the geometries of orebodies are different and they extend to greater depths. Vein-like deposits are closely associated with steeply dipping, brecciated major fault systems. Uranium minerals (pitchblende with some coffinite and brannerite) occur as open fracture fillings and as fine disseminations adjacent to the fractures in Proterozoic meta-igneous andmetasedimentary rocks. Common associated minerals are chlorite, hematite and pyrite. 

Dominant ore minerals in the reduced zone are pitchblende and coffinite and, in some deposits, associated primary vanadium oxides—for example, montroseite. In oxidized zones the important uranium minerals are the uranyl vanadates (carnotite, tyuyamunite or francevillite). Accessory elements include Mo, Se and Cu. Average uranium content ranges from 0.01 to 0.40 7o U3O8. 

The Waldel deposit contains two mineralized structures one is a north-south shear zone and the other is related to a northwest quartz vein. Uranium mineralization is contained in pitchblende and coffinite, in close association with pyrite. Evidence of major reworking within the reduced zone was recorded. In the upper parts numerous hexavalent uranium minerals were identified. It seems that the Dylen deposit is also bound to a quartz vein. 

The sedimentary cover reaches a thickness of some 2000 m in the centre of the basin and 125 m above the deposit. Uranium mineralization occurs mainly in the Stephanian, especially in carbonaceous siltites and argillaceous sandstone lenses with organic matter, at the contact between both layers. Mineralization occurs conformably or as scattered clusters, uranium being contained in pitchblende, coffinite, autunite, torbernite, heinrichite and uranophane. 

U-bearing minerals and adsorption processes (Salah et al. 2000 Perez del Villar et al. 2000). The vertical and lateral flow of groundwater is responsible for the oxidation and dissolution of primary sulphides, leading to acidic solutions that facilitated the oxidation and dissolution of uraninite. The resulting uranyl cations migrated and precipitated as uranyl minerals, mainly phosphates, silicates, silico-phosphates. In certain local conditions, reduction of these uranyl cations allowed precipitation of coffinite with a high content of P and LREE. Adsorption of uranium, together with P, mainly occurs on Fe-oxyhydroxides, but this kind of uranium retention seems less efficient than the precipitation, at least in the close vicinity to the... 

Fig. 4. Relation between clays and coffinite at Cigar Lake (a, b, c, e and f) and at Oklo (d) (a) to (d) uranium progressively invades the clay phase until complete replacement in (d) (e) coffinite replacing a chlorite mineral (precipitation of coffinite around a chlorite flake) (f) map of the distribution of uranium corresponding to the rectangle in (e). (C = clay mineral Co = coffinite.)...

High amounts of Ca (and potentially Ra), Sr, and LREE were co-precipitated by Al-phosphates from the crandallite group, which crystallized in the clay halo. Clay minerals (mainly chlorite and to a lower extent illite) and Ti-oxide were found to have sorbed significant amounts of uranium. Sorption onto mineral surfaces was followed by the formation of coffinite, USi04-nH20, rimming clay, and rutile particles. 

Unconformity-related deposits are found near principal unconformities. Examples include the ore bodies at Quff Lake, Key Lake, and Rabbit Lake in northern Saskatchewan, Canada, and in the Alligator Rivers area in northern Australia. Sandstone deposits are contained in rocks that were deposited under fluvial or marginal marine conditions. The host rocks nearly always contain pyrite and organic plant matter. The sediments are commonly associated with tuffs. Unoxidized deposits of this type consist of pitchblende and coffinite in arkasoic and quartzitic sandstones. Upon weathering, secondary minerals such as camotite, tuyamunite, and uranophane are formed. More information on these and other uranium deposit types is available (1). 

Uranium is a moderately rare element. Its abundance is estimated to be about 1 to 2 parts per million, making it about as abundant as bromine or tin. The most common ore of uranium is pitchblende, although it also occurs in other minerals, such as uraninite, carnotite, uranophane, and coffinite. 

Coffinite an important mineral in some uranium deposits, U(Si04)i— ... 

Uranous ion (U ) and its aqueous complexes predominate in groundwaters of low Eh. U(IV) is the major oxidation state in the most common uranium ore minerals uraninite [U02(c)]—pitchblende is roughly U02(am)—and coffinite (USi04). The U(IV) concentrations in groundwater at low... 

Uranium U(VI) minerals are most often products of the oxidation and weathering of nearby primary U(IV) ore minerals such as uraninite [U02(c)I and coffinite [USi04(c)l (cf. Pearcy et al. 1994). They also form by evaporative concentration of dissolved U(VI), particulary under arid conditions. Schoepite (/J-UOj 2H2O) is fairly soluble and, therefore, is a rare mineral, whereas carnotite K2(U02)2(V04)2j and tyuyamunite (Ca(U02)2(V04)2j, which have lower solubilities (particularly above pH 5) are the chief oxidized ore minerals of uranium. The plots in Figs. 13.5 and 13.6 indicate that uranyl minerals are least soluble in I0W-CO2 waters, and, therefore, are most likely to precipitate from such waters. This is con.sistent with the occurrence of carnotite and tyuyamunite in oxidized arid environments with poor. soil development (Chap. 7), such as in the calcrete deposits in Western Australia (cf. Mann 1974 Dall Aglio et al. 1974), and in the sandstone-hosted uranium deposits of the arid southwestern United States (cf. Hostetler and Carrels 1962 Nash et al. 1981). The... 

Coffinite U(SiO, )j (OH),j is a black mineral, important as a uranium ore. It occurs in sandstone deposits and hydrothermal veins. 

Coffinite was first described as a new mineral by Stieff and co-workers from several localities in the sandstones of the Colorado Plateau deposits often intimately associated with asphaltic material. It was also found in vein-type deposits in Spain by Arribas and has since been found in almost all types of deposits. The composition has been reported as U(SiOi4)i AOrganic matter was always present and organometallic complexes of uranium may have accounted for the excess uranium rather than requiring excess (OH) to account for the U Si ratio deviating from unity. USi04 has been prepared by Fuchs and Gebert with no evidence of OH substitution. 

At Montmassacrot (Fig. 5) the mineralized formations trend WSW-ENE and dip 60-80° to the north. They consist of very brecciated seams of disseminated pitchblende and coffinite (average grade, 0.8%o). Radon clearly indicates the mineralization with anomalous values close to the ore deposit. There is a good correlation between these results and those of the dry residual uranium content. 

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