Abstract
Quartz with aluminum as impurity absorbs energy from ionizing radiation and modifies its color. Colorless quartz becomes smoky or dark smoky (morion quartz) when exposed to gamma rays. By heat-treatment, smoky quartz may become successively greenish, yellowish, or brownish as the irradiation dose increases. Natural, colorless quartz is routinely colored by irradiation with gamma rays and heat-treatment for jewelry production. The color formation in natural quartz through this procedure is explained based on EPR, UV-VIS, and IR studies of irradiated and irradiated and heat-treated samples. Smoky quartz shows absorption bands in the visible region and a strong EPR signal. After heat-treatment it shows absorption bands in the near UV region with extensions into the visible region and a weak EPR signal. The intensity of the absorption bands is proportional to the irradiation dose. These changes of color are explained by the model of Itoh, Stoneham, and Stoneham. [AlSi O4 /h+]0 centers are produced by irradiation, causing the EPR signal and the absorption bands in the visible region. [AlSi O4]- centers are created from [AlSi O4 /h+]0 centers by heat-treatment. They cannot cause an EPR signal and have absorption bands in the near UV region with extensions into the visible region. The highest concentration of [AlSi O4]- centers occurs when the charge compensators have medium mobility. Lithium should give the best condition for color formation. Sodium (low mobility) and hydrogen (high mobility) should make smoky quartz colorless after heat-treatment.
Highlights
Natural or cultured quartz have usually a small content of aluminum as a substitute for silicon
Colorless quartz is routinely colored by irradiation with gamma rays and heat-treatment for jewelry production
The temperature in the irradiation room was in the range of 290 K to 330 K
Summary
Natural or cultured quartz have usually a small content of aluminum as a substitute for silicon. Colorless quartz is routinely colored by irradiation with gamma rays and heat-treatment for jewelry production. One observes that natural quartz shows absorption bands of light in the visible (VIS) and near ultraviolet (UV) regions after irradiation with gamma rays at temperatures from 290 K to 330 K. The intensities of these bands increase with the dose. Electrons are removed by gamma rays from the oxygen atoms adjacent to AlSi. With an electron hole (h+), the AlSiO4 tetrahedron, denoted as [AlSiO4/h+]0 center, releases the M+ to diffuse away in the quartz crystal lattice (1). It is proposed a role for the charge compensator in color formation
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