Mineral dominant species diagrams

The species distribution diagrams calculated from these equilibria are shown in Figs. 3.6a-3.6c. For wolframite, both Mn + and Fe are dominant species for pH < 2.8. The mineral... 

The release of uranium and thorium from minerals into natural waters will depend upon the formation of stable soluble complexes. In aqueous media only Th is known but uranium may exist in one of several oxidation states. The standard potential for the oxidation of U in water according to equation (2) has been re-evaluated as E° - 0.273 0.005 V and a potential diagram for uranium in water at pH 8 is given in Scheme 3. This indicates that U" will reduce water, while U is unstable with respect to disproportionation to U and U Since the Earth s atmosphere prior to about 2 x 10 y ago was anoxic, and mildly reducing, would remain the preferred oxidation state in natural waters at this time. A consequence of this was that uranium and thorium would have exhibited similar chemistry in natural waters, and have been subject to broadly similar redistribution processes early in the Earth s history. Both U " and Th are readily hydrolyzed in aqueous solutions of low acidity. A semiquantitative summary of the equilibrium constants for the hydrolysis of actinide ions in dilute solutions of zero ionic strength has been provided by Liljenzin and coworkers. The results obtained for Th ", U and UO are summarized in Table 1. The tetracations of the oxidation state (TV) metals are the dominant species only below pH 1. Above pH 8 the monoanionic M(OH)s complexes are expected to be the dominant species in solution, although the bulk of the metal will have precipitated at this... 

Figure 3 log/o -pH diagram for the solubility of gold as Au(HS)2, at 200 °C and saturated water vapor pressure. Solid lines delineate mineral-stability fields dotted lines delineate the fields of dominance for dissolved sulfur species (total dissolved sulfur = 0.01 m), and dashed lines show gold solubility contours. Drawn for an activity of water equal to unity (after Wood and Samson, 1998). 

From the reactions (1.86) and (1.87) and chemical equilibrium involving Cu-and Fe-chloro-complexes, iso-concentration contours for Cu- and Fe-minerals are drawn on logfo -pH diagram (Fig. 1.32 Crerar and Barnes 1976). Above calcida-tions were carried out, assuming that Cu- and Fe-chloro-complexes are dominant Cu- and Fe-species (Crerar and Barnes 1976). Bisulfide-, thio- and carbonate-complexes are also important in determining solubility of sulfides as well as chloro-complexes (Table 1.5, Brimhall and Crerar (1987)). Above argument clearly indicates that dominant base metal complexes in hydrothermal solution depend on concentration of ligand, pH, foj, temperature and so on. 

Although small amounts of copper do occur in uranium ore in the Uravan district, the dominant elements that are associated with uranium mineralization are ones that either must or can be readily transported in neutral to somewhat alkaline solutions elements that require acid solutions for mobility, such as copper, are lacking in the ore . Harshman and Adams gave an excellent summary of the geochemistry of roll fronts, including an Eh-pH diagram to explain the zonation of a roll front in terms of dissolved and solid species. 

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