Gemological Characteristics Of Shandong Changle Sapphire And The Influence Of Heat Treatment On It

Shandong Changle sapphires formed in basalt magma,because of the dark color of their rough stones,most of them need to be heat-treated to change their color before they are put on the market.In this paper,the gemological and spectral characterastics,inclusions and chemical elements of Shandong Changle sapphire were tested by infrared spectroscopy,Raman spectroscopy,UV-Vis spectroscopy and LA-ICP-MS,based on which the sapphires were heat-treated at different temperatures.Then the changes of gemological and spectral characteristics during the process were classified and summarized.The main findings are as follows:The main color of Shandong Changle sapphire is deep,with black fused crusts attached to the surface of the rough stones,occasional secondary ferriferous minerals in the fissure,and obvious pleochroism.The sapphires are rich in inclusions,including 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 cm^-1,630 cm^-1 and 514 cm^-1,and the functional group region is a series of 3309 peaks caused by Ti-OH vibrations in different directions;the Raman spectrum is concentrated between 100and 800 cm^-1,with the main characteristic peaks of 247 cm^-1,380 cm^-1,417 cm^-1,577 cm^-1and 750 cm^-1.The UV-visible-absorption spectra show that the color genesis is related to Fe³⁺,Fe²⁺-Ti⁴⁺and Fe²⁺-Fe³⁺.In the yellow-green～blue-green sapphire,the absorption at 450 nm caused by Fe³⁺is stronger than the 564 nm produced by Fe²⁺-Ti⁴⁺charge transfer.The results of LA-ICP-MS and electron microprobe tests show that the content of Fe was the highest among the impurity elements and varied widely,with the content proportional to the color shade.the Ti content was mostly within 300 ppm,with a decreasing trend from the center to the edges of the stones,and the Ti content was low in the yellow-green～blue-green samples.The Shandong Changle sapphires were heated under oxidizing atmosphere with constant time and a gradient from low to high temperature.The experimental results show that the brightness of the samples increased,and was more obvious in the samples with“Trapiche”structure;mineral melting and recrystallization occurred in the inclusions;With the increase of temperature,the 3309 series peaks in the functional group region in the IR spectrum gradually become smaller or disappear,and the characteristic absorption peaks at 630 cm^-1 in the fingerprint region gradually increase and sharp.The change of630cm^-1 is considered as a new type of evidence for the identification of basaltic sapphire.Based on the CIE1967-LAB chromatographic model,color quantification revealed that the luminosity values（L*）of sapphires after heat treatment were all greater than those before heat treatment.The changes of their saturation（C*）and hue angle（h°）have no obvious pattern and are subject to some errors,which still need further study.