Abstract

Spectral luminescence analysis, using a laboratory fluorescence spectrometer, of 85 scheelite-powellite specimens from 50 localities in the western United States show that 46 specimens are within the sensitivity limits of a Fraunhofer line discriminator, an airborne electro-optical device for the detection of materials stimulated to luminesce by the sun. Three-dimensional perspective plots of excitation and emission spectra of the natural specimens are markedly similar in shape to perspective plots of 21 artificial scheelite-powellite specimens, synthesized for discrete mole proportions of tungsten and molybdenum within the end members of the scheelite-powellite solid solution series. Natural specimens show wider range in luminescence intensity than the synthetics, however. Luminescence intensity has been shown by other workers in the fabrication of artificial tungstate phosphors to be controlled by varying the amount of calcium, tungsten, rare earth element and trace element impurities, and temperature; these and other variable factors could readily occur in nature and are the probable causes for the wide range of luminescence intensity noted in the natural specimens.

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