Thermal enhancement of sapphires from deniyaya, Sri Lanka

Abstract

New sapphire deposits have been discovered in Deniyaya, Southwestern Sri Lanka. However, these sapphires are usually low quality in color. Therefore, they were collected for experiments of heat treatment. Twenty nine samples were categorized into 3 varieties including blue, yellow and colorless sapphires. Their internal features are mainly composed of fingerprint, rutile needle, cloud and fracture. Chemical analyses from EDXRF and EPMA, yielded traces of Fe, Ti, Mg, V and Ga. Each sample variety was divided into 2 batches for heat treatments at 1650ºC with 5 hours soaking time under reduction and oxidation conditions, respectively. After heating under reduction condition, most samples turned into blue or deep blue. On the other hand,, heating under oxidation condition introduced yellow and deep yellow in some samples. However, some samples of blue variety may also be intensified to deep blue under oxidizing heat. UV-VIS-NIR absorption spectra and trace element analyses show that interaction between Fe:Ti:Mg is significant cause of color changing. Blue shade appears to have been caused by absorptions of Fe+2/Ti+4 and Fe+3 that are increased after reducing heat. Besides, yellow shade seems to be caused by absorption of Fe+3/ Fe+3 that is increased after oxidizing heat. FTIR spectra of several samples show similarly that AlOOH absorption is disappeared after heating in both conditions. This is because AlOOH may be changed to Al2O3 after heating. Thus, it is a crucial evidence of heated sapphires. In conclusion, Deniyaya sapphires have potential for thermal enhancement at 1650ºC under both reduction and oxidation conditions.