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

Abstract

Tourmaline with the general formula XY 3 Z 6(BO3)3Si6O18(OH,O)3(OH,F) and the trigonal space group R3m (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 γ-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 27Al and the other by 6,7Li. During the irradiation process atomic hydrogen H0 is also produced, which shows the same thermal stability as the hole center (250 °C). Therefore, we assign H0 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 O1 sites, whereby the O− is relaxed into the plane formed by three Y ions.