High Temperature Heat Treatment Of Spinel From Burma

Due to its wide range of fantastic colors, spinel has became one of the most popular gems in the market over past ten years, and its price is directly related to the variability and saturation of its color. The most commercial and valuable colors for spinel are red, purple-red, purple and blue. For Chinese jewelry market, spinels are mainly from Burma. Although the productivity is considerable, Burmese spinels outputted in market still lack high-quality colors. Consequently, it is necessary to promote the value of Burmese spinels by enhancing the quality of its color.This report chose low cost Burmese spinels as experimental objects and attended to change their color through three stages of high temperature heat treatment. Compared with the effects of color enhancement under different heating environments, this paper would seek for the most ideal color type of Burmese spinels, which had obvious promoting results and the more efficient heating method.The whole experimental results were shown as following:1) The appearance of pink-purple spinels can be improved by being heated and the hue of their color changed redder. These effects appeared much better and more obvious by heating at 1600 ℃, rather than 1200 ℃.2) At a constant temperature 1600 ℃, extending the holding time was more beneficial for changing color. However, it reduced the stability of samples at the same time and resulted in a development of internal fractures which decreased transparency.3) To keep the same heating temperature and holding time but extend the heated times would not cause significant effects on the final appearance of experimental samples, however, reducing holding time was helpful to avoid the emergency of internal fractures.4) The same sample which was repeatedly heating process at a different constant temperature displayed a unlike changing trend which was closely affected by the highest heating temperature, nevertheless, the final enhancing result could be impacted by the whole process.The analysis of samples, which was carried out by using laser-induced breakdown spectroscopy (LIBS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), together with optical absorption spectroscopy (UV-VIS) before heating treatment, showed that the presence of trace Fe concentration was the main reason resulted in their color. The absorption bands appearing in the range 350～780 nm were attributed to spin-forbidden transition of Fe2+at tetrahedral coordination and Fe3+ at octahedral coordination, involving with exchanged-couple pair or intervalence charge-transfer transition between them.Discussions on color change mechanism were as following:1) In the internal structure of spinel sample, the activity of spin-forbidden transition of Fe2+ at tetrahedral coordination was increased after a heating process at a constant temperature 1200 ℃. As a result, Fe3+ moved from octahedral coordination to tetrahedral coordination and its effect with exchanged-couple pair of Fe2+ got enhaced2) However, after heating at a constant temperature 1600 ℃, the length between cation and anion became the main reason of changing color with the increase of structural disorder coefficient. Additionally, the intervalence charge-transfer transition of O2-â†'Fe2+ and O2-â†'Fe3+ enhanced the absorption near ultraviolet area.The research on Raman spectrum of experimental samples after the heat treatment showed that the inner structure of spinel already became disordered. Influenced by that, Raman shift changed as following:1)As the heating temperature rose, the half high width of Raman single F2g(1) (311 cm-1), Eg(407 cm-1), F2g(3) (665 cm-1) and A1g(766 cm-1) had a continuous trend of getting broader, and Eg showed a shoulder peak at a lower wave number side (at position of 385 cm-1).2) The frequency of F2g(1) grandly decreased, while the frequency of F2g(3) and A1g began to life up.3) Raman shift of 725 cm-1 was a proof for heated spinel. Affected by thermal expansions and contractions during the whole experiment, some small feather fractures appeared near the surface of samples. Additionally, due to the different thermal expansion coefficient, solid inclusions had a few fractures surrounding them after heat treatment. Those characteristics could offer a very effective method to identify whether a spinel should be heated or not.