Autunite

LIF is also used witii liquid and solid samples. For example, LIF is used to detect lJO ions in minerals the uranyl ion is responsible for the bright green fluorescence given off by minerals such as autunite and opal upon exposure to UV light [23],... 

Uranium, not as rare as once thought, is now considered to be more plentiful than mercury, antimony, silver, or cadmium, and is about as abundant as molybdenum or arsenic. It occurs in numerous minerals such as pitchblende, uraninite, carnotite, autunite, uranophane, and tobernite. It is also found in phosphate rock, lignite, monazite sands, and can be recovered commercially from these sources. 

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

Uranium occurs in nature in many rocks, minerals and sediments. The principal uranium minerals are pitchblende, carnotite, uranitite, tobernite, uranophane, autunite, and davidite. Uranium also is found in very small quantities in monazite sand, phosphate rock, and lignite. Although uranium is present in very smaU quantites, these sources also are used for commercial recovery of the metal. 

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. 

Secondary U and REE minerals include autunite, Ce-phosphate, and Ld-Nd phosphates. The geochemical behaviour can be explained through pyrite oxidation that increases acidicity and releases sulphate and Fe(III), that would allow oxidative dissolution of the U ore, possibly precipitating uranopilite. When the pH increased at sites more distant from pyrite dissolution, U(VI) was hydrolysed and eventually co-precipitated with Fe3+-oxyhydroxides. 

AUTUNITE. This mineral is a hydrous phosphate of calcium and uranium, crystallizing in the tetragonal system, usually in thin tabular crystals. Good basal cleavage hardness. 2-2.5 specific gravity, 3.1 luster, subadamantine to pearly on the base color, lemon yellow streak, yellow transparent to translucent strongly fluorescent. 

Originally from near Autun in France, whence the name, it is a secondary mineral associated commonly with uraninite. In the United States, it occurs sparsely in the pegmatites of Connecticut, New Hampshire and North Carolina. Autunite also is known as calco-uranite. See also Uraninite. 

TORBERN1TE. An ore of uranium with the composition, CudJO h (PO4)2 8 — I2H2O, green, radioactive, tetragonal, and lsomoiplious with autunite. Occurring in tabular crystals or in foliated form, the mineral is commonly a secondary mineral. 

Appendix C contains the chemical formulae for the minerals used in this book. There are very few minerals that have the ideal crystalline structures discussed above. There are sufficient substitutional impurities, crystal defects, and distortions that make the CBPC structure significantly different from the models discussed above. Several well-established minerals exhibit these features, as are many of those listed in Appendix C. For example, Ca(UO2)2(PO4)2T0H2O is formed by the substitution of Ca in autunite by uranyl (UO2) ions, making the autunite a mineral of radioactive uranium. Similarly, (Ce,Th)P04 is formed by the substitution of the Ce in monazite by Th. Numerous minerals can be formed by substitutions and provide a researcher sufficient degree of freedom to synthesize very complex minerals to produce useful CBPCs. 

As was mentioned before, Arey et al. [19] conducted batch equilibration experiments to evaluate the ability of hydroxyapatite to remove uranium from contaminated sediments at the Savannah River Site of DOE and showed that removal of U was due to secondary phosphate minerals that had solubility even lower than autunite (Ca(U02)2(P04)2- IOH2O). The authors suggest formation of Al/Fe secondary phosphate. A similar conclusion was reached by Fuller et al. [20], who showed that uranyl ions can be removed by using hydroxyapatite. 

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