Color Dependence on Thickness in Topaz Crystal from Brazil

Abstract

It is well known that crystals of topaz from the Eastern Brazilian Pegmatite Province may turn blue by the irradiation with 60 Co 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.

Optical absorption and luminescence studies of fast neutron-irradiated complex oxides for jewellery applications

“…1, curve 4). Two lines around 230 and 305 nm belong to a single defect such as silanone (=Si=O) as it was suggested in [28] based on similar lines observed in spodumene. A blue color is related to a broad absorption band in the red part of the spectrum generated by the socalled R-centers (two F-vacancies with two trapped electrons) [29].…”

Optical Properties of Natural and Synthetic Minerals

“…After irradiation in absorption spectra there appears an intense absorption at 230 nm and occur the characteristic maxima at 305, 410, 446, ~590 nm. Two lines around 230 and 305 nm belong to a single defect such as silanone (=Si=O) as it was suggested in [9] basing on similar lines observed in spodumene. Blue color is connected with a broad absorption band in the red part of the spectrum generated by the so-colled R-centers (two F-vacancies with two trapped electrons) [10].…”

Isothermal annealing of a 620nm optical absorption band in Brazilian topaz crystals

“…In the figure the well-defined absorption band is observed at 620 nm under the influence of a strong absorption in the ultraviolet region. A recent study indicates that the strong absorption observed below 400 nm in the present spectrum is due to an optical absorption band around 314 nm and that this band can be tentatively assigned to silanone (QSiQO) [14]. A small and broad shoulder of the 620 nm band appears in the region of 800-1100 nm.…”