Estudo da suscetibilidade ao escurecimento por radiação gama de quartzo róseo-leitoso da província pegmatítica da Borborema

Abstract

This work has investigated the changes induced by γ-radiation on impurity-related point defects in massive rose quartz from one deposit located at The Borborema Pegmatite Province (Northeast Region, in Brazil). Samples extracted from rose and colorless (milky) quartz blocks were irradiated with doses of 60Co, from 0.5 to 96 kGy. Point defects related to Al, Ge, Li and OH were measured by optical, infrared, and electron paramagnetic resonance spectroscopy, prior and after irradiation. The contents of Al, Li, Ge, Fe, Ti and other impurities were measured by inductively-coupled plasma mass spectrometry in quartz fragments exhibiting rose, pale-rose, and milky colorations. It was found that [AlO4]0, [AlO4/H]0 and [GeO4/Li]0 were generated by the dissociation of [AlO4/Li]0 and [Li-OH] centers with doses as lower as 0.5 kGy. Above 8 kGy, the electron paramagnetic resonance signal related to [GeO4/Li]0 decreases due to the intense mobility of Li species throughout the quartz lattice, giving rise to E’1 centers perturbed by Ge. The increase in [AlO4]0 content with γ doses and the consequent rise in the intensity of smoky color were similar for both rose and colorless quartz. Scanning electron microscopy carried out in insoluble residues obtained after chemical dissolution of each type of quartz revealed the presence of nanometric fibers only in rose specimens. These results suggested that the cause of rose color in massive quartz from Borborema Pegmatite Province is probably related to the presence of dumortierite inclusions.