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Fields of stability of uranium (Vl) and uranium (IV) hydroxides and oxides in water solution at 25 C and one atmosphere pressure have been calculated as functions of Eh and pH* Equilibrium values of the ++ 4+ actiTity of UOa ion and of U ion also have been calculated and are shown as contours on the stability fields c Thermodynamic relations among the uranium (Yl) hydroxides and hydrated oxides indicate that the free energy differences among" the various species are small,, The data are interpreted to mean that a variety of such uranium (VI) compounds may form and even coexist 0 Similar studies of the uranium (IY) hydroxide indicate that it is unstable relative to the oxide and might well be expected to change to the oxide at a finite rate* Uranium ,(?) compounds probably have a transitory existence because of the instability of the ionj uranium (ill) oxides and hydroxides wold not be expected to occur naturally because the uranium (ill) ion would decompose water* A comparison of the behavior of the vanadium (ill) and (IY) hydroxides with uranium (XY) oxide and uranium (VI) hydroxides indicates that vanadium (III) hydroxide should oxidize to the vanadium (IV) hydroxide at a lower potential than that required for the change from uranium (IT) to uranium (Yl)* A rather highly speculative diagram showing probable fields of stability of many of the major minerals of the Colorado Plateaus is presented and the suggestion made that a consistent picture results if it is assumed that the ores , as viewed today P represent the superimposition of a -weathering environment on a mineral assemblage formed in a primary reducing environment «> It is emphasized that such an interpretation is consistent but not necessarily unique e Prior to about 1950? the yraniumvanadium ores of the Colorado Plateaus presented few problems in the interpretation of the oxidationreduction conditions of their environment of formation* The oxidationreduction "sensitive" elements of the major minerals uniformly exhibited the higher valences consistent with moderate to strongly oxidizing environments* During the last several years this consistency has disappeared! with the discovery of large quantities of -"blue -black" or "black" ores 9 a variety of minerals containing the same elements , but in lower valence states , have become of economic importance* The genetic relation of the "blue -black" ores to the more oxidized^ or "carnotite 11 ores p is not clear, but definite possibility is that the primary ores were an assemblage of minerals deposited under reducing conditions , and hence with the "sensitive" elements in their lower valence states , and that the more oxidized ores have beenp at least in part;, derived from them by ordinary weathering processes. If so? the primary oxidizing agent was oxygen carried by groundwaters , and the temperature and pressure of the reactions were close to 25 C.and I atmosphere » It is possible to calculate the stability relations of many of the minerals involved under such conditions* In a previous paper (Garrels s 1955) relations of some of the vanadium oxides were considered* In this

Some thermodynamic relations among the uranium oxides and their relation to the oxidation states of the uranium ores of the Colorado Plateaus