Meta-autunite

Figure 1. Catalog page of meta-autunite, including 3-D perspective excitation-emission plot, maximum excitation and emission spectra, and rhodamine dye equivalencies.

There are many examples of U contamination in the environment, from U-mining sites to U production facilities (Bertsch et al. 1994 Abdelouas etal. 1999 Buck etal. 1996) and disposed U ordnance (Salbu et al. 2003). The major U-bearing phases found in soils at the former U-processing site at Femald in Ohio, USA, were Ca-meta-autunite, U-Ca-oxide, uraninite, and uranium (IV) meta-phosphate (U3PO4) (Buck et al. 1996). Bertsch et al. (1994) and Allen et al. (1994) used XAS to determine the oxidation state of U in bulk soil samples. The position of the U-Lm edge indicated that 80% of the U was hexavalent. 

Autunite Ca (002)2 (P04)2 10 020 Can lose water to form meta-autunite dissolves in acids... 

Uranium(VI) readily precipitates in the presence of phosphate to form a number of sparingly soluble U-phosphate phases (U phases, such as saleeite, meta-autunite, and autunite) and also is removed by sorption and co-precipitation in apatite. Several studies have shown that hydroxyapatite is extremely effective at removing heavy metals, uranium, and other radionuclides from solution (Gauglitz et al., 1992 Arey and Seaman, 1999). 

Autunite is well-known to dehydrate rapidly to form meta-autunite, for whieh no satisfactory structure model has yet been presented. A method of direet synthesis of meta-autunite has not been reported, and its formation by dehydration of autunite likely induces so much mosaic spread that single crystal structure determinations have not been successful. Autunite may be eonsidered to exhibit pseudosymmetry, as the transformation matrix [- /2OO/OOI/OIO] yields a metrically tetragonal cell. 

Makarov YS, Ivanov VI (1960) The crystal stracture of meta-autunite, Ca(U02)2(P04)2 6H20. Dokl Akad Nauk SSSR 132 601-603... 

J. Beintema, On the Composition and the Crystallography of Autunite and the Meta-Autunites, Rec. Trav. Chim. Pays. Bos 57 (1938) 155-75. 

The uranyl phosphates and arsenates comprise the largest group of uranium minerals and, except for uranophane, the most abundant mineral group. Table 10 lists the known minerals in this group. The autunite and meta-autunite families... 

Several new autunite-like and meta-autunite-like minerals include arsenuranospathite, heinrichite and metaheinrich-... 

The meta-autunite family is the largest family, probably because many of the minerals are found in sandstone deposits in arid or semi-arid climates. Actually, practically all museum specimens of the autunite minerals prove to be meta-autunites when rechecked. This situation suggests that many specimens were initially misidentified or it may be that they have dehydrated in the museum atmosphere during storage. Once an autunite has dehydrated, which appears to happen readily, rehydration does not occur. The occurrence of an autunite thus implies formation in a cool humid climate and minerals from such localities should be carefully collected and maintained in a humid state for study. 

Crystal structure studies have only been made on members of the meta-autunite family. Abernathyite and metatorbernite have been studied by Ross and Evans, and Ross and coworkers,meta-uranocircite by Zolensky and Khosrawan-Sazedj, threadgoldite by Piret and co-workers, and hydrogen-meta-autunite by Morosin." These studies verify the general meta-autunite structure proposed by Beintema, but show that there are subtle structural differences among the species. The basic unit is an infinite sheet of 2-4 UOe polyhedra and (P,As)04 tetrahedra, which share corners. Ideally, this sheet has tetragonal symmetry. It is shown in Fig. 13, which also shows how the sheets are proposed to... 

Fig. 13 Ideal uranyl phosphate sheet structure as found in autunite and meta-autunite families, (a) (top), structure of [(U02)(T04)] sheet (b) (centre), stacking of sheets in autunite family (c) (bottom), stacking of sheets in meta-autunite family. After Beintema °...

Specimens of trogerite and meta-uranocircite are known to transform from tetragonal to lower symmetry at temperatures near room temperature. For trogerite the transformation is around 25°C for meta-uranocircite it is at 108°C. A slight distortion in the structure results and the crystals show an extensive cross-grid twinning. Similar transformations occur in other minerals also—for example, Ca and Pb meta-autunites, and may explain the reported meta-autunite II, which is orthorhombic. 

Beintema J. On tbe composition and the crystallography of autunite and the meta-autunites. Reel Trav. chim. Pays-Bas Belg., SI, 1938, 155-75. 

Vochten R. and Deliens M. Transformation of curite into meta-autunite paragenesis and electrokinetic properties. Phys. Chem. Minerals, 6, 1980, 129-43. 

Zolensky M. E. The crystal structure and twinning of barium uranyl phosphate hexahydrate (meta-uranocircite), and comparison to the other members of meta-autunite group. Ph.D. thesis, Pennsylvania State University, 1983. 

Tetravalent uranium replaces calcium in the apatite structure. Hexavalent minerals (autunite, meta-autunite and torbernite) are also present. Reserves are estimated at 10 000 ton uranium. The origin of this occurrence is uncertain, so an exploration model for this type deposit has not been constructed. 

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