Potential development of green and purple colors in colorless natural quartz from geodes in rhyodacites, Serra Geral Group, Brazil

Abstract

Colorless quartz can develop a green color when submitted to gamma irradiation and its development is related to the presence of molecular water and hydroxyl groups in the quartz crystalline structure. In the present study, colorless quartz crystals hosted in geodes within rhyodacites of Serra Geral Group, Southern Brazil, were selected to test the potential development of green color. The samples were analyzed by infrared (IR) and thermogravimetric (TGA) techniques, in addition to the Amethyst Factor (f a ) method. All samples showed an absorption bands in the IR, indicating possible development of a green or purple color, and f a analysis indicated the samples would develop a green color. All samples became colored after irradiation, presenting color zoning: from colorless to grayish-green, and from grayish-green to slightly purplish. Some samples showed a greenish colored zone and developed an amethyst phantom crystal in its apical portion. In addition, a high content of water in samples that remained colorless after irradiation was detected. It is here interpreted that water concentration varied during quartz crystallization, enabling the development of the green color, and that the excess of water can inhibit the development of radiation-induced color in colorless natural quartz crystals.