Study of thermal treatments and irradiation on natural amethyst by optical absorption

Thermal treatment effects on the samples of amethyst from Minas Gerais and Rio Grande do Sul, through optical absorption measurements, are studied. The effects of cobalt 60 gama ray radiation on the amethyst from Minas Gerais through optical absorption measurements, are studied. The optical absorption spectra shows a basic line and bands in 10,500 cm-1 (k), in 18,300 cm -1 (θ) and in 28,000 cm-1 (ζ). The correlation between thermal treatment effects and radiation effects shows that the θ and ζ bands belongs to a same center and the k band to another center. The basic line vanishes by thermal treatments at 2700C. The analyses of the isothermal treatment decay and dose-curve reveal a complex kinetics, suggesting that the kinetic mechanisms involve the electron (s) and hole (s) migration in the lattice. The amethyst from Minas Gerais after being discolored at 4700C and irradiated again shows that the optical absorption bands don't recover the original absorbance, suggesting the existence of iron diffusion mechanisms in the lattice. After being isothermally annealed the amethyst from Rio Grande do Sul above 4000C temperature yield a yellow-brown color, probably due to the formation of Fe2O3 precipitate. We suggest the this formation is due to iron diffusion and pre-existent oxygen vacancies. This model is reinforced through the observation the amethyst from Minas Gerais, isothermally treated at 4700C in highly reducing, also gets a yellow-brown color. (author)