Uranyl oxide hydrates

Fig. 8. Transmission electron micrograph of corroded SNF exposed to moist-air conditions for 3 years. A thin layer of Pu-rich precipitate was observed on the weathered fuel surface along with uranyl oxide hydrate and Cs-Mo uranyl oxide hydrate alteration phases (adapted from Buck et al. 2004).

Buck, E. C., Wronkiewicz, D. J., Finn, P. A. Bates, J. K. 1997. A new uranyl oxide hydrate phase derived from spent fuel alteration. Journal of Nuclear Materials, 249, 70-76. 

The uranyl oxide hydrates are important corrosion products of uraninite and UO2 in spent nuclear fuel under oxidizing conditions. However, the systematics of the structures had not been well described until the studies reported by Miller et al. With the exception of the synthetic U02(0H)2 polymorphs, all hydrate crystal structures are based on sheets of edge-sharing uranyl pentagonal or square bipyramids. Only four structural unit chains are required to construct the uranyl oxide hydrate sheets (as well as the structurally similar UgOg sheets). One chain is made up... 

Schindler and Hawthorne [29] presented novel insights into the chemical composition and occurrence of uranyl oxide hydrates using a combined binary representation and bond-valence approach. In essence, this approach looks for conditions of overlap of the bonding requirements emanating from the stmctural unit and interstitial components. The outcome is a series of predicted compositions and mineral stabilities. 

The autunite anion-topology is a simple array of squares located such that their comers correspond (Fig. 5). Two uranyl oxide hydrates, y-[(U02)(0H)2] [33] and P-[(U02)(0H)2] [34], contain sheets with this anion topology. The topologically identical sheets in these two stmctures contain only uranyl square bipyramids that are located in half of the squares of the underlying anion topology. Each square bipyramid shares all four of its equatorial vertices with four different uranyl square bipyramids, thus each is four-connected and all of the connections within the sheet involve only the sharing of vertices. The sheets in these stmctures are electroneutral, there are no interlayer constituents, and the sheets are linked directly by H bonds. 

Sheets in the structure of the rare Pb uranyl oxide hydrate mineral sayrite [48] are based upon the anion topology shown in Figure 11. The topology contains the P, R, U and D chains with the sequence RUPURDPDRUPU... The... 

The structure of a-[(U02)(0H)2] [54] contains sheets of uranyl hexagonal bipyramids (Fig. 17), and the sheet anion topology consists only of hexagons. This is the only uranyl oxide hydrate that contains hexagonal bipyramids, although this coordination type is common in uranyl carbonates, uranyl nitrates, and uranyl phosphates of the phosphuranylite group. 

Table 5 Uranyl oxide hydrates (gummite minerals) ... 

Alkali and alkaline-earth uranyl oxide hydrates... 

Table 7 lists 19 known minerals that may be classified as alkali or alkaline-earth uranyl oxide hydrates. They have often been referred to as uranates as well as uranyl oxides, but as details of their crystal structures become known it is apparent that they are closely related to the uranyl oxides described above. In fact, Sobry and Noe-Spirlet and Sobry" have shown that substitutional series exist between schoepite and all the minerals on the list. It was proposed that the minerals can be explained by the general formula... 

This group of uranyl minerals occurs almost exclusively in alteration haloes on uraninite in association with the uranyl oxide hydrates. The Pb minerals are common because of the available radiogenic Pb especially in geologically older deposits. The phases are usually very fine-grained and intimately intergrown with other minerals or with one another. Only rarely do they form as recognizable small crystals. Colour can be a guide to specific mineral identifications, but X-ray diffrac-... 

Christ C. L. and Clark J. R. Crystal chemical studies of some uranyl oxide hydrates. Am. Miner., 45, 1960, 1026-61. 

Although the list of 231 X-ray patterns has been essentially restricted to accepted uranium mineral species, several poorly described unnamed minerals have been included along with the best characterized uranyl oxide hydrates. These latter synthetic phases have been listed because of the possible existence in the gummite alteration rinds often associated with uraninite. 

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