Uranyl silicates

Uranyl Silicate is formed as a crystalline preeijutate when a solution of sodium silicate is added to a 10 per cent, solution of a uranyl salt. ... 

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... 

Figure 19. The uranophane anion-topology (a) and uranyl silicate sheets based upon this anion topology (b-d) which differ only in the orientations of the sihcate tetrahedra.

S5uithesized [67], and the two compounds have identical uranyl silicate fiameworks. There are sheets of vertex-sharing silicate tetrahedra that contain both four and eight-membered rings (Fig. 22). Adjacent silicate sheets are... 

Figure 23. The uranyl silicate framework structures in Na2[(U02XSi04)] (a) and RbNa[(U02)(Si206)](H20) (b).

Nguyi n, S. N., R. J. Silva, H. C. Weed, and J. E. Andrews, Jr. 1991, Standard Gibbs free energies of formation at the temperature 303.15 K of four uranyl silicates soddyite, uranophane, sodium boltwoodite and sodium weeksite. J. Chem. Thermo. 24 359-76. 

One of the major outputs of these model simulations is a prediction of a sequence of secondary phases. These phases arc important because they can act as sinks for radionuclides and other toxic components of the waste, limiting their migration away from the point of emplacement. Even phases not containing such components affect the overall course of reaction via effects on pH, redox conditions, etc. Figure 1 shows the results of one simulation for reaction of spent fuel and J-13 well water at 25°C. Haiweeite (a calcium uranyl silicate), soddyitc (a uranyl silicate), and schoepite (a uranyl hydroxide) are successive (and overlapping) sinks for uranium, the dominant constituent of spent fuel. 

The uranyl silicate minerals occur in all types of deposits. They may be found close to the uraninite or other primary minerals as one of the first-formed oxidation products, or they may be found in isolated occurrences as trace coatings far from any obvious source. By far the most common silicate is uranophane, which may actually be the most common of all the... 

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 1 3 uranyl silicates comprise haiweeite and weeksite. These minerals have the same occurrences as the 1 1 minerals, but they are not as common. These minerals also show acicular to prismatic habits and yellow to white colours. The mineral ranquilite, which was described by de Abeledo and co-workers is evidently identical with haiweeite. Ursilite may be a valid magnesium haiweeite, but more work is needed to validate its existence. A sodium analogue of weeksite occurs in hydro-thermal experiments at 300°C with synthetic nuclear reactor waste. 

Fig. 11 Structure of soddyite, (U02>2Si04-2H20 (uranium 2-5 polyhedra ruled and Si04 tetrahedra stippled chains, as found in other uranyl silicates, lie 90° to one another and share Si04 tetrahedra)...

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. 

Uranous complexes tend to be insoluble at low temperatures and at pH 4.5-7. At temperatures above 150°C uranous transport may become dominant. Depending on ligand concentrations, uranous fluoride, phosphate, sulphate and especially hydroxide compounds are important species under these conditions, but uranous carbonate complexes are not. Uranyl species are soluble over a wide range of conditions. In normal groundwater, at temperatures of 25°C, uranyl fluoride complexes are dominant at pH <4, uranyl phosphates at pH 4-7.5 and uranyl di- and tricarbonate complexes at pH >7.5. Uranyl silicate complexes are probably insignificant, and at temperatures near 100°C uranyl hydroxides predominate, whereas uranyl carbonate complexes dissociate. ... 

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