Radiation-induced defects in euclase: formation of O- hole and Ti3+ electron centers

Irradiation techniques are often applied to gem minerals for color enhancement purposes. Natural green, blue and colorless specimens of rare gemological quality euclase, BeAlSiO4(OH), from Brazil were irradiated with gamma rays in the dose range from 10 to 500 kGy. Although the colors of the different specimens were not strongly influenced, two different irradiation-induced paramagnetic defect centers were found by electron paramagnetic resonance (EPR). The first one is an O- hole center interacting with one Al neighbor and the second is a Ti3+ electron center. The EPR angular rotation patterns of both irradiation-induced defects were measured and analyzed. The results suggest that O- hole centers are formed by dissociation of the hydroxyl ions, similar as in topaz crystals. In euclase the OH- ions interconnect distorted Al octahedra and Be tetrahedra in O-5 positions. During irradiation, the electrons are captured by titanium ions (Ti4+ + e(-)), leading to the formation of paramagnetic Ti3+ ions. From the EPR rotation patterns it is clear that these ions substitute for Al ions. The spin Hamiltonian parameters of the irradiation-induced defects are analyzed and compared to similar defect centers in other mineral specimens. Thermal annealing experiments show that the O- hole centers and Ti3+ electron centers are directly connected through the radiation process.