Colouration in natural beryls: A spectroscopic investigation

Beryl samples from Badmal mines, Orissa were studied by electron spin reson ance, optical absorption and Mossbauer spectroscopy. It was found that on a rtificial irradiation, colourless beryls from Orissa turned to shades of ye llow-green. On subsequent heating at 300 degrees C, the hue changed to yell ow and upon further heating at controlled conditions they turned to a more desirable sky blue, accounting for value addition. Continued heating above 500 degrees C, however, rendered them colourless. Similar studies were also carried out on natural yellow, blue and green beryls from Orissa, all of w hich acquired a greenish hue on irradiation and turned colourless on heatin g to >5000 degrees C. Electron probe microanalysis of beryls showed iron as the major transition element impurity. ranging in concentration between 0. 5 to 0.7wt%. Investigations using electron spin resonance, optical absorpti on spectroscopy and Mossbauer spectroscopy reveal that: colours produced we re on account of Maxixe-type defect centres as well as radiation-induced ox idation of Fe2+ to Fe3+ ion and associated charge transfer process. It is i nteresting to note that the yellow and blue colour produced on irradiation as well as post irradiation heating showed similar hue to that of the natur al ones, both having formed on account of similar charge transfer process. Thus, it can be inferred that natural yellow, blue and green colours in ber yls comprising low iron impurities are due to prolonged irradiation in natu re.