Abstract
THE absorption spectra of fluorites coloured by natural or artificial irradiation show, in general, principally two regions of strong absorption1: from 350 to 470 mμ and from 530 to 650 mμ. The latter has already been attributed to reduced calcium, either as isolated atoms or as colloidal particles ; the former can now be attributed to the bivalent ions of the rare earth elements, which, as has been shown before2, are responsible for the diffuse fluorescence bands of fluorites. The blue Eu++-band has its maximum of excitation3 in the region of 350–390 mμ, the red band of Sm++ in CaF2 most likely towards 430 mμ. The red Sm++-fluorescence of some natural fluorites is connected with absorption maxima between 420 and 470 mμ. The different colours of fluorites can now be explained by the varying intensities of the different absorption maxima: Eu++ alone gives a light yellow colour (for example, Derbyshire), Sm++ a deeper yellow brown (Wölsendorf and others), the bivalent rare earths together with the Ca-maxima give the green colours (Weardale), predominance of the Ca-maxima the blue and violet colours, violet corresponding to a more disturbed lattice (larger colloidal particles?).
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PRZIBRAM, K. Absorption Bands and Electron Transitions in Coloured Fluorites. Nature 141, 970 (1938). https://doi.org/10.1038/141970b0
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DOI: https://doi.org/10.1038/141970b0
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