【摘要】 <正> 由于萤石（CaF₂）的钙离子半径（1.06）与钍离子半径（1.10）、特别是与四价铀离子半径（1.05）相似,所以铀和钍常常置换萤石中的钙而进入萤石晶格。此外,铀和钍还能以吸附形式赋存在萤石裂隙中或其表面上,或者成为独立的矿物以固体包体形式存在于萤石中。因此,萤石常含有铀和钍,加之周围岩石中也经常含有放射性元素,从而形成了放射场。萤更多还原

【Abstract】 Fluorites occurring in the Bayan Obo dolomite can be roughly divided into three types according to their colours: violet black, violet and colourless. Fluorite colours are closely related to uranium content and geochemical environment. Violet black fluorite generally contains 10 ppm U and 18 ppm Th and occurs mainly in Nb-RE iron ore deposits of high radioactivity (200—1,000γ); violet one usually has<2 ppm U and<1 ppm Th and is largely distributed in the inner contact zone of the iron ore deposits and the dolomite; and colourless one is poor in U and Th, generally less than 0.1ppm, and is common in the calcitic sperulites of the dolomite.Obvious changes in colour are noticed in naturally occurring fluorites after irradiated in the nuclear reactor. With the colour changing from light to dark, fluorites have become from isotropic to anisotropic. At the mean time, fission tracks and micro-fractures have increased correspondingly.It is evident that the colouration of fluorites is indirectly dependent on the content of U and Th, and directly on irradiation. It is also suggested that irradiation damage is another colouration mechanism of fluorites.更多还原