No abstract available

[en] The results are critically reviewed of the thermal decomposition of johannite, uranopilite and Zn-zippeite. The study is part of the scientific inventory of secondary uranium minerals in the collections of the National Museum in Prague. (B.S.)

[en] Mineralogical and isotopic studies of vein-type uranium deposits revealed the presence of several generations of radioactive mineral assemblages that formed and survived under specific environmental conditions. Aqueous solutions, percolating along the rejuvenated fractures, dissolved radioactive and radiogenic minerals in one part of the deposit and redeposited them in the other part of the deposit in a different crystalchemical form during a time span of several hundred million years. The following secondary mineral assemblages and their spatial and temporal evolution have been studied: (1) Secondary pitchblende rims formed on sulphides, arsenides, carbonates and quartz between 870 and 200 million years ago; (2) Uraniferous mixed-layer phyllosilicates along fractures in altered rocks where uranium was trapped in the interlayer of montmorillonite component about 400 million years ago; and (3) Reactions between mobilized uranium and other ions, such as silica, sulphur, carbon and water to form coffinite, uranyl-bearing silicates, carbonates, sulphates and hydrocarbons continue at the present time. It is concluded that, by changing chemical-mineralogical compositions and adapting suitable crystal structures, uranium can survive under diverse environmental conditions. (author)

[en] During the oxidation of naturally occurring uraninite, UO₂, uranyl ion is released and may persist in aqueous solution, if the pH remains low enough or anions which form very insoluble solids with UO₂²⁺ are absent. If sulphides are oxidized in the vicinity of uraninite a complex suite of basic sulphates of intermediate solubility may form and these can control the levels of uranium in groundwaters. Zippeites were among the first of these to be recognised. Attempts to understand the chemical processes involved during the deposition of secondary uranium minerals prompted an examination of the stability of monovalent cation zippeites, M₄(UO₂)₆(SO₄)₃(OH)₁₀.4H₂O (M = Na, K, NH₄). The compounds were synthesised using the methods of Frondel et al. and analysed thermogravimetrically. X-ray powder diffraction patterns corresponded closely with reported d-spacings. Calculations of stability constants and ΔG⁰sub(f) values were carried out at 298.2 K using techniques reported elsewhere. The results are reported and discussed. (author)