Uranyl minerals

At the third level, the most detailed partition of luminescence minerals is carried out on the basis of metals in the mineral formulae, hi rare cases we have minerals with host luminescence, such as uranyl minerals, Mn minerals, scheelite, powellite, cassiterite and chlorargyrite. Much more often luminescent elements are present as impurities substituting intrinsic cations if their radii and charges are close enough. Thus, for example, Mn + substitutes for Ca and Mg in many calcium and magnesium minerals, REE + and REE substitutes for Ca, Cr substitutes for AP+ in oxygen octahedra, Ee substitutes for Si in tetrahedra and so on. Luminescence centers presently known in solid-state spectroscopy are summarized in Table 4.2 and their potential substitutions in positions of intrinsic cations in minerals in Table 4.3. 

Fig. 4.68. Laser-induced time-resolved luminescence spectra of uranyl minerals...

The spectra of the green laser-induced luminescence represented in Fig. 4.4a, together with their decay time, also allows its association with These luminescence spectra strongly differ from the spectral parameters of all known uranyl minerals. For this reason it is not possible to connect this type of green luminescence with finely dissipated uranyl phases. On the other hand, this luminescence is very similar in such different host minerals as sedimentary apatites, opalites, chalcedony, chert, quartz and barites. Luminescence independence from the minerals structure evidences that it may be connected with uranyl adsorption on the minerals surface, supposedly in the form of (UO2 X nH20)2+. 

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... 

Uranyl carbonate minerals are common in the altered zones of uranium deposits, and tend to be amongst the most soluble of the uranyl minerals. Currently, there are 22 known stmctures of uranyl carbonates, of which 12 correspond to minerals (Table 3). In slightly alkaline to alkaline groundwater the uranyl tricarbonate cluster, with formula [(U02)(C03)3]" is the dominant species, and uranyl minerals that form from such solutions contain the uranyl tricarbonate group as an isolated cluster. 

The crystal chemistry of phosphate minerals has recently been reviewed [9, 10]. These references present a stmctural hierarchy based on the pol5mierization of polyhedra of higher bond-valence, especially tetrahedra and octahedra. In a similar fashion, an extensive stmctural hierarchy of uranyl minerals and inorganic compounds has been developed over the last decade [11, 12]. This chapter follows the concepts and principles of both of these stmctural hierarchies, but places the primary emphasis on actinide coordination. As the coordination environments of the actinides differ with valence state [13, 14], it has been found convenient to discuss the compounds of the lower valence-state actinides separately from those of the higher valence-states. 

Uranium U(VI) minerals are most often products of the oxidation and weathering of nearby primary U(IV) ore minerals such as uraninite [U02(c)I and coffinite [USi04(c)l (cf. Pearcy et al. 1994). They also form by evaporative concentration of dissolved U(VI), particulary under arid conditions. Schoepite (/J-UOj 2H2O) is fairly soluble and, therefore, is a rare mineral, whereas carnotite K2(U02)2(V04)2j and tyuyamunite (Ca(U02)2(V04)2j, which have lower solubilities (particularly above pH 5) are the chief oxidized ore minerals of uranium. The plots in Figs. 13.5 and 13.6 indicate that uranyl minerals are least soluble in I0W-CO2 waters, and, therefore, are most likely to precipitate from such waters. This is con.sistent with the occurrence of carnotite and tyuyamunite in oxidized arid environments with poor. soil development (Chap. 7), such as in the calcrete deposits in Western Australia (cf. Mann 1974 Dall Aglio et al. 1974), and in the sandstone-hosted uranium deposits of the arid southwestern United States (cf. Hostetler and Carrels 1962 Nash et al. 1981). The... 

The pH range of minimum solubility of the uranyl minerals is also the pH range of maximal U(VI) sorption on most important natural sorbents, including organic matter (van der Weijden and Van Leeuwen 1985), Fe(III) oxyhydroxides, Mn and Ti oxyhydroxides, zeolites and clays (cf. Langmuir 1978 Turner 1995). In terms of approximate values, where /f (ml/g) = (wt adsorbed/wt sor-... 

The structures of several uranyl minerals have been investigated using X-ray techniques. Camotite was found to have a similar stmcture to Cs2[(U02)2(V20g)], which contained infinite... 

Many other natural materials, including several metal hydroxides (Fe, Al, Mn) as well as clays, are capable of adsorbing uranium. Sorption proceeds to a variable extent. It usually depends on the pH of the solution and the pH range for the greatest sorption of uranyl overlaps the pH range of minimal solubility of uranyl minerals. 

Most uranium minerals occur in all of the several types of ore deposits. A given deposit usually has no more than two reduced minerals. The oxidized minerals that occur in the deposit depend on the Eh-pH conditions and the availability of reactive anions. In the absence of reactive anions, hydrated oxides and uranates form. The uranyl ion is, however, fairly soluble and groundwater can effectively disperse it a considerable distance from the reduced source. The uranyl minerals that are then deposited are complex compounds that employ available oxyanions. The rate of formation of these secondary minerals can be very rapid, as is evidenced by mineral formation on the walls of mine drifts in a matter of months after the drifts have been opened. In all deposits there is usually a zonation of mineralogy in which a reduced mineral... 

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. 

The uranyl minerals are considered in groups, depending on their associated anion. This approach is useful because each of these groups has many characteristics in common, including those of occurrence and crystal chemistry. Within each group sub-classification by UO2 XO ratios leads to interesting comparisons and some very specific mineral families. 

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-... 

The two common oxidation states of uranium are U (uranyl) and U (uranous). Uranyl minerals occur in oxidized environments, tend to be bright yellow, red, orange or green in colour and are common in oxidized portions of uranium ore-bodies. Some common uranyl minerals include autunite, t)niyamunite, torbernite and uranophane. Uranous minerals occur in reduced environments, tend to be black or brown in colour and are variably metamict owing to natural radiation... 

Fig. 4.170 (a-i) Laser-induced time-resolved luminesc ce spectra of uranyl minerals (zippeite, autunite, zeunerite, agate, orangite, thorite, monazite, xenotime)... 

The spectral-kinetic parameters of the green laser-induced luminescence of the sedimentary apatites allow its association with 1102 " emission. The spectra presented in Fig. 4.5b, c are typical for uranyl minerals and it is possible to suppose that we are dealing with separate uranyl mineral phase on the apatite surface. Comparison with known uranyl minerals laser-induced luminescence shows that the most similar spectral-kinetic parameters have minerals andersonite Na2Ca (U02)(C03)3 X 6H2O and liebigite Na2(U02)(C03)3.10H20, but this identification is ambiguous. 

Keywords Aqueous speciation Bond valence Borate minerals Crystallization Dissolution Morphology Surface structure Uranyl minerals... 

The crystal structures of uranyl minerals are dominated by the structure of the complex uranyl cation (1102) ". The central U is coordinated by two anions... 

Average equatorial U-

bond-valences for [6]-, [7]- and [8]-coordinated are 0.64,0.54, and 0.45 v.u., respectively ([25], calculated with the parameters of [40]). Individual equatorial U-

bond-valences vary over a larger range for example, the U-cp bond-valences in schoepite vary between 0.27 and 0.73 v.u. [36], giving rise to a range of intrinsic acidity constants for one t5q>e of anion termination. The types of anion termination in uranyl minerals are limited by the occurrence of [6]-, [7]- and [8]-coordinated U e.g., [6]- and [8]-coordinated U never occur together, and always occur with [7]-coordinated U . The conformation of an anion termination can be denoted by the symbol where q> is an... 

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