Uranium minerals uranyl silicates

Hydrated Silicates of uranium occur in nature as gummite and allied minerals (see p. 273) uranophane or uranotil is a complex calcium uranyl silicate. 

Uranyl silicates are an important group of minerals in the altered zones of many uranium deposits, and uranophane, the most common member of this group, is... 

The order of presentation of the uranium minerals will follow chemical groups. The U minerals are discussed first, followed by the niobates, tantalates and titanates. These two groups include the primary reduced minerals. The uranyl minerals are considered in the order hydrated oxides, silicates, phosphates and arsenates, vanadates, molybdates, sulphates, carbonates, and selenates and tellurates. Each section includes an evaluation of the known crystal chemistry and its effect on chemical variability and occurrence of mineral species. 

Chemically, the uranyl silicates form three groups depending on the uranium/silicon ratio. The most populated group, the 1 1 group, is one of the best studied. Stohl and Smith and Sidorenko and co-workers reviewed the crystal chemistry of these minerals. They showed that all 1 1 minerals have essentially the same basic structural unit [(U02)Si04]J" , an infinite chain of edge-shared uranyl pentagonal dipyramids and silicate... 

The author would like to thank the several students and colleagues who have contributed to this study of uranium minerals. Frances V. Stohl, Christine A. Anderson and Barry E. Scheetz have contributed greatly to the study of the uranyl silicates. Michael E. Zolensky is still engaged in the studies of the uranyl phosphates. He has also helped review this manuscript. Steve A. Markgraf assisted with the literature search and compiling of data used in the tables. 

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... 

Uranium in its highest valence state forms a large number of colourful minerals that may deposit in the oxidized zone associated with the primary deposit or the uranium may go into solution and be transported a considerable distance from its source area before reprecipitation. Minerals that form at the source may mimic the original phases by direct replacement, but more often they form a nondescript mass that destroys any original structure. These minerals are usually hydrated uranyl oxides, silicates or phosphates. Further from the source the minerals usually form as one or more of the many hydrated uranyl oxysalts. 

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