Effect of heat treatment on the luminescence properties of natural apatite

Abstract

Regarding gemstone identification, the effect of heat treatment under reduction condition to optical and luminescence properties of natural green apatite (Ca5(PO4)3OH, F, Cl) from Madagascar has been investigated. Although electron microprobe analysis (EPMA) shows a luminescence defect of rare earth element such as Ce (0.47%), Pr (0.22%) and Nd (0.15%), however original sample does not show luminescence properties. With consecutive heating process, the color alteration from green to colorless of samples, and also the enhancement of luminescence properties of Ce3+ in natural apatite was observed. Mechanism of fluorescence enhancement and color changes during heating process were discussed using optical spectra and emission spectra. Optical spectra of heated apatite point out that green and blue color assigned to the existence of a photochromic center of Ce3+-SiO3− radical and SO3−, respectively. Consequently, the vanishing of these both bands resulted in the colorless heated apatite at 800 °C. Heated apatite showed emission band at 365 nm because of the deactivation of electron from 4f of Ce3+ to 5d. Optical spectra reveal that the correlation of luminescence enhancement and the fading of yellow tint of heated apatite by means of the descending of Ce3+ to Ce4+ charge transfer process. The ultimate contribution of this work is the finding of the difference of luminescence properties between non-heated and heated apatite which can be used as an effective tool for gem identification. This identification tool is remarkable because it is rapid and non-destructive.