Gemmological Characteristics and Diffusion Mechanism of Disffusion-treated Synthetic Sapphire

Qi Li Jian 1, C. G. Zeng 2 , CAO Shu-min 1

(1.Gemmological Institute, China University of Geosciences, Wuhan 430074, China
2. Nan Yang Gemological Institute, Singapore 228208 )

Abstract: A new type of synthetic sapphires with diffusion treatment has recently appeared on gem markets, which is treated from the flame fusion synthetic colorless sapphires by the diffusion with the compound of iron and titanium at super-high temperatures. The gemological characteristics and diffusion mechanism of diffusion-treated synthetic sapphires are investigated in depth by using EPMA, FTIR , UV-VIS and so on. The results show that the main chemical compositions of infiltrated layer of the diffusion-treated synthetic sapphires consist of w £¨ FeO T £© = 1.69%~1.02% and w£¨ TiO 2 £© = 0.57% to 0.39%, which are not well distributed on transaction and the chemical compositional zoning occurs from the surface to the interior of the samples. The colouration of the diffusion-treated synthetic sapphire is mainly attributed to b band (573 to 578nm) among the VIS absorption spectra, which results from the charge transfer of Fe 2+/Ti 4+ . When the ratio of w £¨ FeO T £© :w£¨ TiO 2 £©is less that 3:1, it is suitable for keeping the charge transfer of Fe 2+/Ti 4+ to form the blue infiltrated layer. A group of weak FTIR absorption bands at 3371, 3309 , 3231cm -1 are attributed the to n (OH) stretching vibration, and the weak absorption at 3186cm -1 is attributed to the n (NH) stretching vibration. The irregular net-like quenched cracks and etching hollows by the high temperature exit in the surface of some samples and a few gas bubbles exit in the interior. The samples are uneven bluish white florescence under SW ultraviolet. The research indicates that the micro-diffusion transference of Fe 2+and Ti 4+ ions in the diffusion-treated synthetic sapphires is controlled by vacancy transference energy, vacancy diffusion co-efficient and vacancy concentration gradient.

Keywords: synthetic colorless sapphire, diffusion treatment, Fe 2+/Ti 4+charge transfer; diffusion mechanism

*Published in Journal of Gems and Gemmology, 2006, Vol 8, No.3, page 4-8.