Role Of Beryllium In The Coloration Of Fe and Cr-doped Synthetic Corundum

Thailand is perceived as one of the gemstone refineries of the world. The experts from GIT shares their opinion (s) on the pros and cons of beryllium treatment in natural and synthetic corundum.

Visut Pisutha Arnond, Tobias Hager, Pornsawat Wathankul, Wilawan Atichat, Jitrin Nattachai, Chakkaphant Sutthirat, and Bootawee Sriprasert writes:

X-radiation and Be-diffusion heating experiments were performed on an iron-doped (colorless) synthetic corundum and a chromium-doped (pink) synthetic corundum to evaluate the role of beryllium in causing color in the Be-Fe-Al2O3 and Be-Cr-Al2O3 systems.

The iron-doped corundum, containing around 140-170 ppm by weight of Fe with negligible concentrations of other trace elements, was irradiated with X-rays (60 kV, 53 mA) for 30 minutes, then the color was faded for one hour with a 100-watt light bulb, and finally the sample was heat treated in a crucible with ground chrysoberyl in an electric furnace at 1780ºC in an oxidizing atmosphere for 50 hours. The chromium-doped corundum, containing around 160-210 ppm by weight of Cr with negligible concentrations of other trace elements, was also irradiated with X-rays (80 kV, 4mA) for 4 hours, then faded for 4 hours with a 100-watt light bulb, and subsequently heat treated with ground chrysoberyl at unspecified conditions by a Thai treater. At each stage of the experiments, the samples were photographed and UV-Vis absorption spectra were recorded.

Both the irradiation and Be-diffusion experiments on the iron-doped synthetic corundum created defect centers that had similar UV-Vis absorption curves and produced yellow coloration. The yellow color was unstable when induced by irradiation, but was stable after Be-diffusion.

Experiments on the chromium-doped synthetic corundum produced orange coloration (and similar UV-Vis absorption patterns) by both irradiation and Be-diffusion heating methods. Again, the orange color was unstable when induced by irradiation (and quickly faded to pink), but remained stable after Be-diffusion. These results confirm that divalent Be acts as a stabilizer of defect centers or color centers in iron-doped and chromium-doped synthetic corundum. Hence, the spectrum produced by the irradiation of Fe-doped or Cr-doped synthetic corundum was attributed to metal-related unstable color centers, while that produced in synthetic corundum doped with Be + Fe + or Be + Cr was caused by Be²+ + metal-related stable color centers.