Colouration in Natural Beryls: A Spectroscopic Investigation

Beryl samples from Badmal mines, Orissa were studied by electron spin resonance, optical absorption and Mossbauer spectroscopy. It was found that on artificial irradiation, colourless beryls from Orissa turned to shades of yellow-green. On subsequent heating at 300°C, the hue changed to yellow and upon further heating at controlled conditions they turned to a more desirable sky blue, accounting for value addition. Continued heating above 500°C, however, rendered them colourless. Similar studies were also canied out on natural yellow, blue and green beryls from Orissa, all of which acquired a greenish hue on irradiation and turned colourless on heating to >500°C. Electron probe microanaiysis 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 absorption spectroscopy and Mossbauer spectroscopy reveal that colours produced were on account of Maxixe-type defect centres as well as radiation-induced oxidation of Fe²⁺ to Fe³⁺ ion and associated charge transfer process. It is interesting to note that the yellow and blue colour produced on irradiation as well as post irradiation heating showed similar hue to that of the natural ones, both having formed on account of similar charge transfer process. Thus, it can be inferred that natural yellow, blue and green colours in beryls comprising low iron impurities are due to prolonged irradiation in nature.