The optical absorption of gamma irradiated and heat-treated natural quartz

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. [AlsiO 4/h4+]0 centers are produced by irradiation, causing the EPR signal and the absorption bands in the visible region. [Al siO4]- centers are created from [Al siO4/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 [A1 siO4]- 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.