Yasuo Ukai

The relation between the minute content of Uranium in Zircon and its Mineralogical characters

Zircon is one of the most common and familiar minerals which are distributed widely in various rocks. Therefore the mineralogical characters of zircon have attracted the interest of geologists, petrologists and mineralogists and have been investigated from various stand points of geology. Some of these characters are very intimately related to the content of radioactive elements, and this investigation was made as an attempt to account for these relationships.The uranium contents of representative zircon specimens collected from the weathered granite sands of the Tensi Mine were analyzed by the microfluorimetric method. To eliminate the quenching effect of fluorescence intensity derived from the bluk zirconium, the actual contents were calibrated by multiplying the coefficient 1.31 to the apparent value determined by the direct method.The radioactivity of zircon was measured by the decatron type countmeter, and it was generally proportional to the uranium content. The natural fluorescence intensity activated by the ultraviolet ray was determined by means of the sensitive photomultiplier detector, and the fluorescence intensity of zircon containing a comparatively large uranium was lower than that containing smaller amount.An attempt to find some relation between the colour and the uranium content in zircon was fruitless except for the facts that the colorless zircon was relatively low in its radioactivity but that the greenish brown member was moderately high. The lattice defect of zircon was also investigated with the Geiger counter diffractometer and related to the uranium content.Various minor elements contained in zircon were semiquantitatively determined by spectrographic method.From the above experimental results, the intrusion of granite batholith was classified into three following stages, first the stage in which the first intrusion of granite represents the northern part of this district and the zircon content in granite is rich and the uranium content in zircon is small, and secondly the stage in which the second intrusion of granite represents the southern part and the zircon content is small and the uranium content is slightly richer than that of the granite found in the northern part, and finally the stage in which the third intrusion of granite represents the middle part and the zircon content is of moderate amount but the uranium content ranges from a small to a large amount.