Coffinite

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Bound in minerals such as uraninite, pitchblende, coffinite, etc. fixed as a replacement ion for Y, Ce, Zr, Th, Ca, and Ba in other, particularly accessory minerals and adsorbed as ion on clay minerals, hydrous iron oxides, etc. 

Zhao Ewing (2000) examined altered uraninite from the Colorado Plateau with quantitative electron microprobe analysis in order to determine the fate of trace elements, including Pb, Ca, Si, Th, Zr, and REE, during corrosion under oxidizing conditions. The alteration phases identified included schoepite, calciouranoite, uranophane, fourmarierite, a Fe-rich U phase, and coffinite. The primary uraninites and alteration phases generally had low trace element contents. The electron microprobe analyses indicated that the trace elements preferentially entered the secondary U phases. Alteration caused the loss of U, Pb, and Zr, and incorporation of Si, Ti, Ca, and P into U phases. 

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... 

After uraninite, coffinite (USi04-nH20) is the most important U-bearing mineral at Cigar Lake. It often occurs as a secondary alteration product of uraninite along microfractures, cracks, and boundaries of uraninite grains. Along with uraninite dissolution, coffinitization of uraninite is another process responsible for the release of U. 

In clay samples Zr-Th-rich coffinite was found around remnants of zircon. It is likely that it is the result of solid solution with zircon, ZrSi04 and thorite, ThSi04, which are isostruc-tural with coffinite (Finch Murakami 1999 Jensen Ewing 2001). The presence of phosphorus and sulphur in coffinite suggests that both elements substituted for Si in the coffinite structure. A previous study at the Bangombe site in Gabon has clearly shown that coffinites are most important secondary minerals for the retention of fissiogenic lanthanides and actinides (Stille et al. 2003). 

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. 

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). 

Jensen, K. A., Janeczek, J., Ewing, R. C., Stille, P., Gauthier-Lafaye, F. Salah, S. 2000. Cran-dallites and coffinite retardation of nuclear fission products at the Bangombe nuclear fission reactor. Material Research Society Symposium Proceedings, 608, 525-532. 

Under Reflected Light. Radioactive minerals (zircon, uraninite, coffinite) enclosed in organic material are surrounded by a clear halo of more than 20 microns width (Figure 1). This halo corresponds to a zone of increased anisotropy, reflectivity, and hardness as first observed by Stach (24). 

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... 

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). 

Fig. 66 shows the undersaturation of some mineral phases of interest. If the saturation index is attained, the respective mineral is precipitated by the model and acts as a limiting phase (kinetics are not considered). The possible limitation by coffinite, uraninite, and pyrite from 500 days onwards (not distinguishable in the figure coffinite is not a limiting mineral phase any more from 2000 days on furthermore it is questionable that coeffinite forms under this conditions) is remarkable. Kaolinite is supersaturated after 2000, calcite after 7000, and Al(OH)3... 

For a closer look on the reactions at the beginning of the degradation, the modeling was redone with unchanged boundary conditions in 100 time steps for a period of 600 days. Fig. 68 shows better than Fig. 65 and Fig. 66 the step wise decrease of the pE value. The first drop is connected to the occurrence of Fe2+, the second to the elimination of Fe3+ and the reduction of As51 to As3f. Shortly after that the reduction of U6- to U4+ occurs. In the model uraninite and coffinite... 

Canada and Australia are the world s largest producers of uranium. All Canadian production is from rich deposits in the Athabasca basin of northern Saskatchewan among those is the McArthur River mine, which has the world s largest high-grade deposit, estimated at 1.52 X 10 t of uranium from ore grading 15-18% uranium. These unconformity -type Saskatchewan deposits, which are also the principal deposit-type for Australian uranium production, contain mainly uraninite [UO2] with associated coffinite [U(Si04)i jc(0H)4j and brannerite [(U,Ca,Y,Ce)(Ti,Fe)206] (Plant et al, 1999). The chief uses of uranium are in nuclear power plants and weaponry. 

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. 

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