Yellow coloration in heat-treated natural sapphires

Abstract

The cause of colour change in yellow sapphires can be explained through the process of heat treatment experiments performed on differing sets of samples from both 'metamorphic' and 'basaltic' origins. In this study 'metamorphic' yellow sapphires were five simply heat-treated stones obtained from Sri Lanka. These sapphires were re-heated in a reduced atmosphere at 1650 degrees Celsius for 5 hours. After the treatment the samples changed from their strong yellows to colourless. The residue spectra observed from this experiment showed that the colour chang could be identified as that of a "colour centre pattern". Thrace element analysis by LA-ICP-MS showed that these samples had low Fe and Mg > Ti in content. We can deduce from this that the colour change in metamorthic samples occur through the stabilized assistance of Mg in the "colour centres" process or the so-called 'Mg-trapped hole colour centres'. The basaltic origin samples from Bang Kacha area, Chanthaburi Province were grouped into sets based on the colour variety. The first group (seven untreated stones) were found to be high Fe and Ti > Mg of greenish yellow to yellowish green variety, with the other set (two untreated stones) being high Fe and Mg > Ti of yellow variety. Both sets of samples were heated in an oxidized atmosphere at high temperatures. We discovered differing results from the two sets after heat treatment. The set with the higher content of Ti to Mg showed little to no change in colour. The set with Mg < Ti showed a significant colour change to strong brownish yellow. What we can deduce from this experiment is that the set with Ti > Mg had little to no change in colour because the Ti and Mg combine to form MgTiO[subscript3] and no Mg was left to assist any colour centre process. The yellow hues in this set of samples were likely to have been from the spin forbidden transition of Fe[superscript3+] that can occur in 'basaltic' sapphires. The set with high Mg assisted the stabilization of the yellow colour centres and allowed significant colour change to take place in the sapphires.