Color Variation in Gemmy Amethyst : The Role of Contaminant Diffusion

Abstract

The role of contaminant diffusion purple quartz, otherwise known as amethyst, is commonly known for its occurrence as clusters of gemmy crystals that grow on the walls of fluid-filled fractures and vugs in hydrothermal metamorphic systems. These crystals often show color intensity variations that are thought to result from variable exposure to radiation in the natural environment. Some amethyst localities, however, present crystals that exhibit progressive color variations along the c-axis of the crystal and suggest that progressive diffusive mobility of impurities within relatively wide crystallographic channels may also have played a role in generating color variation in these crystals. In earlier studies conducted in our laboratory, we have shown that crystallographically-preferred diffusion of hydrous impurities is nearly ubiquitous within Alpine and Brazilian colorless quartz specimens. Here, we use high-resolution Fourier Transform Infrared Spectroscopy (FTIR) to document the variable abundances of mobile LiOH, HOH, and AlOH species within crystals extracted from the classic amethyst locality in Veracruz, Mexico. We show the relationship of violet color intensity with contaminant concentration and provide a test of the hypothesis that diffusion processes are integral to the development of color vibrancy in these crystals.