Abstract
We report a comparative study between natural and treated green, blue and black color tourmalines from granitic pegmatites of the Minas Gerais state, Brazil, using Electron Microprobe, Electron Paramagnetic Resonance (EPR) and Mossbauer spectroscopy. Electron Microprobe analysis confirms that all studied samples are schorl species. The Electron Paramagnetic Resonance spectra of green and blue tourmaline are characterized by line widths of about 1000 Gauss. These unusual broad lines are probably due to spin-spin interactions caused by the high Fe concentrations and structural disorder. Mossbauer spectra collected at room temperature confirm that iron is dominantly present as Fe 2+ . It is incorporated at two or three different Y sites for the green and blue tourmalines, respectively. From those features, we conclude that blue tourmalines are characterized by a high degree of disorder, whereas a smaller degree of disorder is observed in green tourmalines. In addition, heat treatments in oxidizing and reducing atmospheres yielded structural readjustments at the Fe sites, de-hydroxylation, oxidation and generation of a second, different phase.
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