Country Of Origin And Colour Enhancement Mechanism During Heat Treatment Of Red-Orange-Purple Mg-Al Spinels From Myanmar,Tajikistan,Tanzania And Vietnam

Spinel has significantly increased in the market since the turn of the new millennium for various reasons,e.g.its extensive colour ranges from pink to red,orange to violet to pastel,and green to blue and has a high degree of transparency.Since the decline in global ruby production in the 2000s,spinel has become the second most popular and most expensive gemstone after ruby.Heat treatment is an important method that has been conducted to enhance the colour and value of many gemstones（e.g.ruby,sapphire,and zircon）.Although natural spinels are mostly beautiful themselves,small batches of heat-treated spinels are still available in the global gem market.The main aim of this study is to study the cause of colour modification of spinel after heat treatment which is not well understood using advanced analytical techniques,including UV-Vis,FTIR,and XANES,and it is also the first time of using linear discriminant analysis（LDA）for discriminating pink-to-red spinels from four geographic origins.A total of sixty pink-to-red spinels from Myanmar（Mogok）,Tajikistan（Kuh-i-Lal）,Tanzania（Mahenge）,and Vietnam（Luc Yen）were selected for this study.Their gemmological properties fell into the range of natural Mg Al₂O₄ spinel.It can be noted that only the Tajikistan spinels showed their lowest SG and RI values among the three deposits.Predominant internal features of spinels from Myanmar,Tanzania,and Vietnam,were apatites,carbonate minerals,and graphite platelets.Only zircon crystal was observed in the spinels from Tanzania.Octahedral negative crystals and partially healed fractures were present in all studied samples.Linear discriminant analysis was performed using seven elements（Ti,V,Cr,Fe,Zn,Ga,and Sn）,and it provided potential criteria for separating spinels from these for important localities.For the heat treatment experiment,the samples were heat treated twice at 850 and 1000℃ in oxidising atmospheric conditions using the electric furnace,with 5℃/min heating-up rate,and held for two hours at each maximum temperature.It can be observed that heat treatment at 850℃ can reduce slightly to moderately yellow subtle in the stone,while heating higher at 1000℃ can relatively reduce blue subtle.Therefore,pink and red turned more satisfying.Several sophisticated spectroscopic techniques were carried out to explain the cause of colour modification after heat treatment.UV-Visible absorption spectra of all samples evidenced that the Cr³⁺band at 538 nm was broadening and shifted to 550 after heating to 1000℃.It should be pointed out that this change can partially absorb the yellow-orange portion in the visible spectrum,while FTIR spectra of all samples indicated that the Mg and Al cations are disordered in the spinel structure after heating at 850℃ by showing no absorption band at 575 cm^–1.Furthermore,the XANES analysis results combined with the linear combination fitting calculation confirmed that the oxidation state of iron was partially converted from Fe²⁺to Fe³⁺after heating at 850℃,which let the blue subtle reduced to some extent,whereas the oxidation state of vanadium that might cause yellow-orange hue in the spinel was unchanged.In summary,heating at low-temperature of 850–1000℃ can affected the spinel structure.Moreover,some oxidation states of iron have been transformed from Fe²⁺to Fe³⁺.Combining UV-Vis and FTIR results can indicate the spinel has undergone heat treatment.