Ningyoite occurs extensively in the unoxidized zones of the Ningyôtôgé and the neighbouring uranium deposits as the principal ore mineral. It is a hydrated calcium uranous phosphate with minor amount of various substituting ions, and its chemical formula can be written as Ca
1.0U
0.8R.E.
0.2 [P(O, OH)
4]
2•1-2H
2O. Ningyoite is presumed to be comparatively stable, in spite of the similarity of its crystal structure with that of rhabdophane, a meta-stable rare mineral of rare earths, as indicated by their thermal behaviours. Ningyoite contains almost all the rare earths and their abundance ratio resembles that of apatite. This, together with other evidences, strongly suggests that ningyoite was formed by the reaction between apatite and uranium-containing solution.
Ningyoite was synthesized in various conditions from mixtures of uranium-calcium-phosphorus-containing compounds and solutions for the purpose of determining the possible environment of the precipitation of ningyoite. The results of the syntheses from apatite and various uranyl complexes under reducing conditions and the additional experiments indicate that ningyoite is formed only when the excess phosphate ion other than that combined with calcium is contained in solution, and also that the carbonate ion is not favourable for its precipitation. The properties of the synthetic ningyoites are a little variable depending upon the pH values of the solution and other factors, though ningyoite could be formed in a wide range of pH from 1.2 to 7.6 under the reducing condition. Natural ningyoite is considered to have been formed in weakly alkaline condition in comparison with the synthetic minerals.
The nature of the solution from which ningyoite was precipitated in the Ningyô-tôgé area has been considered also from the paragenesis of ningyoite in the uranium ores. Seeing from the fact that the close accociation of ningyoite and such reducing agents as carbonaceous matters and pyrite is common, uranium had probably been transported to the precipitation site in the hexavalent state and was subsequently reduced. The intimate association of ningyoite and gypsum formed by hydration of the original anhydrite indicates that the uranium bearing solution contained sulphate ion as the principal anion, and also that the temperature of the formation of the ningyoite, was above 35°C. The temperature was estimated also by the fact that kaolinite was partly transformed to sericite during the uranium mineralization, to be in the range from 40° to 80°C.
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