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Brown, green, yellow, and red CaF2:U crystals were investigated by means of optical spectra and chemical analyses in order to determine the cause of the different colorations. Whereas earlier work claims the green coloration is due to U2+, we have shown it is due to U4+. Whereas earlier work claims the yellow coloration is due to U4+, we have shown it is due to U6+. The brown coloration appears to be due to U4+ with a different charge compensation that the green coloration.
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McLaughlin et al. (1970) studied this question.
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