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

Tourmaline with the general formula XY ₃ Z ₆(BO ₃) ₃Si ₆O ₁₈(OH,O) ₃(OH,F) and the trigonal space group R3 m ( C _{3 v} 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 ?-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 Si ₆O ₁₈. We suggest that two of the Y sites are substituted by ²⁷Al and the other by ^6,7Li. During the irradiation process atomic hydrogen H ⁰ is also produced, which shows the same thermal stability as the hole center (250 °C). 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.