Effect of Heat Treatment on Sapphire from Changle, Shandong Province

The sapphire from Changle, Shandong Province belongs to the basalt type of genesis. Due to the dark colour of the rough stones, most of sapphires need to be heat-treated. In this study, the gemmological and spectral characterastics, inclusions and chemical compositions of sapphires from Changle, Shandong Province were studied using infrared spectroscopy, Raman spectroscopy, UV-Vis spectroscopy and LA-ICP-MS.The sapphire samples were heat-treated at different temperatures in order to classify and summary the changes of gemmological and spectral characteristics during the heatment process. The main colour of sapphire from Changle, Shandong Province is deep, with black fused crusts attached to its surface, with occasional secondary ferriferous minerals in the fissure, and obvious pleochroism. The types and quantities of inclusions in sapphires are abundant, including mineral inclusions such as sulfur, calcite, dolomite, hematite, tremolite, rutile, amorphous carbon, zircon, etc. In addition, there are rare oriented regular inclusions and dendritic inclusions.The main characteristic peaks in the fingerprint region of the IR spectrum are 483, 630 cm^-1 and 514 cm^-1, and the functional group region is a series of 3 309 cm^-1 peaks caused by Ti-OH vibration in different directions; the Raman spectrum is concentrated between 100 cm^-1 and 800 cm^-1, with the main characteristic peaks at 247, 380, 417, 577 cm^-1 and 750 cm^-1. The results of LA-ICP-MS and electron microprobe tests show that the content of Fe is the highest among the impurity elements and varied widely, with the content proportional to the colour shade. The content of Ti is mostly within 300× 10^-6, with a decreasing trend from the center to the edges of the stones, and is low in the yellow-green to blue-green sapphire samples. In the oxidizing environment, the sapphires were heated with constant temperature time from low to high temperature gradient heat treatment. The experimental results show that the brightness of the sapphire samples is improved, and is more obvious in "Trapiche" sapphire samples; the inclusions have the phenomena of mineral melting and recrystallization; with the increase of temperature, the 3 309 cm^-1series peaks in the functional group region in of IR spectra gradually become smaller or disappear, and the characteristic absorption peak at 630 cm^-1 in the fingerprint region gradually increases and becomes sharp. The change of 630 cm^-1 is considered as a new evidence for identifying basaltic sapphire. Based on the CIE1967-LAB chromatographic model, colour quantification reveals that the luminosity value (L^*) of sapphires after heat treatment is all greater than those before heat treatment. The changes of saturation (C^*) and hue angle (h°) have no obvious pattern and are subject to some errors, which still need further study.