Optical absorption, electron spin resonance (ESR) and nuclear (NMR) studies of silicate minerals such as quartz and topaz help to reveal the nature of a variety of colors which are derived from defects within the crystal structures involving the presence of impurities, trapped holes, and electrons. The present study was inspired by color changes in cinnabar, which upon exposure to sunlight, turns from vermillion red to black under certain conditions, the solid state physical reasons for which have not yet been described. Smoky and amethyst quartz are also bleached by energy from the Sun; reactions that can be reversed by the process of artificial irradiation and heat treatment. Topaz, the focus of this study, exhibits the imperial yellow variety from Ouro Preto, Brazil, which bleaches upon exposure to high temperatures and gives rise to a pink color if chromium is present as an impurity. For the blue variety of topaz, which arises from the irradiation of colorless topaz to smoky, then heat treating to blue, the crystal chemistry remains undefined. Many color centers found in topaz are believed to have a relationship to the presence of aluminum in tetrahedral sites, also related to trapped hole/electron defects. Although NMR studies have targeted the presence of 27Al with uncertain results, optical absorption and ESR studies show clear connections to the production of electronic defects related to absorbing centers caused by high energy irradiation. ESR studies indicate that significant information can be attained relative to these defects when the magnetic vector is parallel to the c axis of the crystal. This paper begins to shed light on the responsible mechanisms that may define the crystal chemistry in terms of the electronic environment, with particular emphasis on topaz.
Investigations of color center phenomena in Topaz and Quartz through electron spin resonance with reference to optical absorption and nuclear magnetic resonance: Implications for extended mineral applications