Uraninite weathering

Weathering of natural uraninite (Finch Ewing 1992 Finch Murakami 1999) and synthetic U02 (Wronkiewicz et al. 1992, 1996) under oxidizing conditions can result in the formation of a series of U6+ secondary phases, such as U6"1" oxide hydrates (e.g., dehydrated-schoepite, UO3-0.8H2O), alkali U6+ oxide hydrates (e.g., compreignacite, K2[(U02)604(0H)6j 8H20), U + silicates (e.g., uranophane, Ca(U02)2(Si03)2... 

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). 

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... 

Gummite [12326-21-5] UO,.nH,0 Ms 304.043 Amorphous Isotropic n.a. n.a. Generic name for a mixture of uranium and lead oxides of intense yellow color resulting from the weathering of uraninites. 

An example of veins with complex mineralogy is the mined-out uranium deposit at Shinkolobwe, Zaire, at the northwest end of the African Copperbelt. It occurs in a faulted transported fold in metamorphosed dolomitic shales of the middle Proterozoic Mine Series of the Roan Group.Uraninite and uranophane mineralization (about 620m.y. ago) was followed by several later mineralizations in which pyrite, molybdenite, monazite, selenium, Co-Ni sulphides and selenides and copper minerals were formed. It is believed that this orebody was formed by redistribution of metals originally deposited in marine sedimentary rocks." The uranium, originally weathered from granites and deposited in the marine sediments, was in low... 

In the Phanerozoic uranium oxide mineralization in granites tends to occur as a lower-temperature phase, such as low thorium pitchblende in veins, which is less resistant to weathering than euhedral thorian uraninite crystals and, hence. 

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