Abstract
It is well known that crystals of topaz from the Eastern Brazilian Pegmatite Province may turn blue by the irradiation with60Co gamma rays followed by heat treatment. Also, it is known that the sensation of color changes with the thickness of these crystals. The dependence of the color, given by 1931 CIE chromaticity coordinates, with the thickness of the crystal was analyzed. The absorbance used in the calculation of these coordinates was given by the sum of Gaussian lines. The parameters of these lines were determined through the decomposition of the optical absorption spectra in the ultraviolet and visible regions. The decomposition revealed several lines, whose assignment was made considering studies in spodumene and beryl crystals and highly accurate quantum mechanical calculations. The transmittance becomes very narrow with increasing thickness, and the CIE chromaticity coordinates converge to the borderline of the CIE Chromaticity Diagram at the wavelength of maximum transmittance. Furthermore, the purity of color increases with increasing thickness, and the dominant wavelength reaches the wavelength of maximum transmittance.
Highlights
Most gemstone crystals have an intrinsic color due to the optical absorption and presence of color centers that are produced by impurities like transition metals and elements of Earth-rare, or defects in, the crystalline lattice [1]
The color shown by a gemstone reflects intrinsic factors like the actual form and presence of color centers as well as external factors like the light produced by different light sources
We began the process of decomposition of the spectra of topaz submitted to heat treatment at 200◦C using as reference for the far ultraviolet region the lines observed in spodumene
Summary
Most gemstone crystals have an intrinsic color due to the optical absorption and presence of color centers that are produced by impurities like transition metals and elements of Earth-rare, or defects in, the crystalline lattice [1]. In 1975, Nassau and Prescott reported a remarkable feature obtained from 86 colorless topaz samples with probable Brazilian origin; 21 samples exhibited a blue color after gamma irradiation followed by moderate heating [17]. This blue color in transmitted light is attributable to an optical absorption band near 620 nm. We report in the present work the effect of thickness of topaz samples on the 1931 CIE xy chromatic coordinates calculated using the absorbance described as the sum of Gaussian lines The parameters of these lines were determined through a spectral decomposition analysis of the optical absorption in the visible and ultraviolet regions. In this paper we report an analysis of the dependence of transmittance, color purity and dominant wavelength with thickness
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