THERMOCHEMICAL STABILITY OF NINGYOITE
1965 年 4 巻 4 号 p. 245-274
詳細
1965 年 4 巻 4 号 p. 245-274
The solubilities of ningyoite, autunite, and hydrogen-autunite, all synthetic except autunite, were measured mainly at 25° and 100°C by immersing them in water with pH ranging from 0 to 6. Equilibrium constants of the dissolution reactions of the minerals were obtained by estimating activities of the concerning ions with the aid of the Debye-Hückel limiting law from the uranium concentrations and the pH values of the solutions. The standard free energies of formation of the minerals were then calculated from the equilibrium constants. The values are -948 Kcal for ningyoite CaU(PO4)2•2H2O, -1728 Kcal for autunite Ca(UO2)2(PO4)2•10H2O, and -1598 Kcal for hydrogen-autunite H2(UO2)2(PO4)2•10H2O with probable errors of ±2 Kcal. The standard enthalpy of formation of ningyoite was evaluated to be -1035 ±5 Kcal from the equilibrium constants at 25° and 100°C.
Stability fields of some uranium minerals in the forms of oxide, carbonate, sulphate, and uranate in addition to the three minerals were considered in Eh-pH diagrams constructed by using the above free energy values together with the available values of the relating compounds and ions. Stability of ningyoite in comparison with uraninite was estimated by examining chemical equilibria of some reactions including them. Ningyoite stably precipitates at lower temperatures around 25°C under neutral or acidic conditions with phosphorus activity around or above 10-8, which seems to be similar to that in supergenic waters, while at higher temperatures around 100°C, only under weakly or moderately acidic conditions with high phosphorus activity around or above 2×10-7. The conditions of the ore solution in the uranium deposits around the Ningyô-tôgé area were discussed by examining equilibrium relations among the paragenetically occurring minerals including ningyoite. The reason why ningyoite has never been found in other uranium ores with high phosphorus content, e. g. phosphorite, is explained from the limitation on pH and temperature for the formation of the mineral.
