Correlation of irradiation-induced yellow color with the O- hole center in tourmaline

Tourmaline with the general formula XY(3)Z(6)(BO3)(3)Si6O18(OH,O)(3)(OH,F) and the trigonal space group R3m (C-3v(5)) is known as a gemstone with great variety of colors. Some color centers are related to transition metal ions, and others to electron or hole traps. In this paper we report on a combined study using electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and the optical detection of EPR (ODEPR) on a yellow color center produced by gamma-irradiation in colorless Li-bearing elbaite tourmaline from Brazil. The color center is an O- hole trap center, which is stabilized within the plane spanned by three Y sites, and is located in the structural channels formed by Si6O18. We suggest that two of the Y sites are substituted by Al-27 and the other by Li-6,Li-7. During the irradiation process atomic hydrogen H-0 is also produced, which shows the same thermal stability as the hole center (250 degreesC). Therefore, we assign H-0 to be the local charge compensator for the hole trap. From the ODEPR measurements we conclude that the yellow color is caused by the O- hole center. The large negative isotropic Al superhyperfine interaction of the O- hole trap center is consistent with a calculation of the transferred hyperfine interactions by exchange polarization supporting the proposed defect model of an O- at the O-1 sites, whereby the O- is relaxed into the plane formed by three Y ions.