Pequenos polarons ligados e centros de cor em topázio

Abstract

In the present work we studied the color beneficiation processes of colorless topaz, an abundant gemstone in Brazil. Blue topaz is the most internationally traded gemstone in the gem and market, reaching about 100 million carats per year. Almost 100% of blue topaz received some treatment to improve the color that, generally, involves some type of irradiation and heat treatment. Although these treatments havebeen done for more than forty years, we still know very little about the center causer of the color and details of physical processes for their creation, thus generating the motivation for this work Topaz has the chemical formula Al2SiO4(F,OH)2 with orthorhombic symmetry and, mostly, it is naturally colorless. For this work we used samples from four regions of Brazil: Tocantins, Rondônia, Marambaia (Minas Gerais) and Hematita (Minas Gerais). We treated these samples with gamma radiation, with fast electrons and neutrons, and thermal treatments. For characterization, we have used Optical Absorption (OA), in the region of light in the visible spectrum, and Electron Paramagnetic Resonance (EPR). By EPR, we identified four paramagnetic centers in topaz (Fe3+, O2 -, X and O-) and analyzed their behavior with the treatments. The centers Fe3+ and O2 - show no relation with the color of topaz, the first being an impurity substituting Al3+ ions and the second created by fast neutrons. The X center is produced by gamma rays and related to the brown color, both presenting low thermal stability. Its concentration strongly depends on the origin of the sample. We showed that the O- center is the cause of blue color and it can be created using gamma radiation, electrons or fast neutrons, the generation with fast neutrons and electrons being more efficient and not depending on the origin of topaz. The production of O- and X centers is independent and their optical absorption bands were explained through the model of small bound polarons, which means charge transfers (holes) between equivalent oxygen sites. The first polaron, O-, causes an optical absorption band at 620 nm and involves six oxygens in octahedral symmetry, while the second, X, causes a band at 460 nm and involves four oxygens in tetrahedral symmetry.