The Study On Gemological Characteristics And Inclusion Origin Of Spinels In Myanmar

In this paper,different colors of spinel in Burma are studied by using conventional gemological tests and modern testing methods such as electronic probes,Raman spectroscopy,infrared spectroscopy,UV-visible absorption spectroscopy and so on.It is available to have a systematic study on its gemological characteristics and analysis of its internal inclusion origin and color causes.The spinel of Myanmar is a magnesium spinel.Compared with chemical components of other origin spinels,the content of ZnO is high,and the content of V₂O₃and Cr₂O₃is also high.The total contents of Cr₂O₃+V₂O₃in warm color（pink,red and orange）spinels are highe,and Fe makes it dark hue.The orange tones in spinel are related to V and Cr,and V plays a dominant role.The content of Fe in cold color（purple and blue）spinels is higher.The ZnO content in warm color（pink,red and orange）spinels was higher than that in the Si O₂.In cold color（purple and blue）spinels,except for the sample b-11,the Si O₂content in the other samples was higher than ZnO content.The infrared spectra of different colors of spinel are basically similar,and there are four absorption bands in the range of 400～1200cm^-1.The absorption bands are at 535cm^-1,594cm^-1,727cm^-1and 852cm^-1.Raman spectra of different spinel samples are also similar,with four raman shift in the range of 200～900cm^-1,312cm^-1,407cm^-1,and 665cm^-1,respectively.766 cm^-1,of which 407cm^-1is a characteristic Raman shift of the spinel,with a FWHM between8.44～11.35cm^-1and an average value of 10.41cm^-1.The natural spinel can be distinguished by the full width at half maximum of the 407cm^-1Raman shift,heat-treated spinel and synthetic spinel.UV-visible absorption spectroscopy analysis shows that the pink and red spinel are mainly caused by Cr and V.The orange spinel may be caused by the interaction of Cr,V,and Fe.However,due to the lower Fe content,the spectrum is weaker and is overlapped by the spectra of Cr³⁺and V³⁺.Its ultraviolet absorption spectrum shows two large absorption bands in the range of 300～600nm,mainly attributed to spin-allowed transitions and spin-forbidden transitions of Cr³⁺and V³⁺.The color of blue and purple samples are mainly caused by Fe.Their UV-VIS absorption spectra mainly belong to the spin-forbidden electronic transitions of ^TFe²⁺,^MFe²⁺and ^MFe³⁺,and the exchange coupled pair transitions and intervalence charge transfer transitions between Fe²⁺and Fe³⁺.In the transition,the relative intensity of the Fe-related absorption band determines the blue color of the spinel.The main characteristics of octahedral inclusions in Burma’s spinel are as follows:（1）octahedral inclusions are mainly distributed independently or in groups,and have the same direction as the main crystal.The main components of the octahedral inclusions are calcite and/or dolomite.In addition,we can find a dark octahedral inclusions as a pyrite.Black flake graphite inclusions and other forms of carbonate minerals are also seen.（2）Compared with other forms of inclusions,the octahedral inclusions can develop several groups of fractures along the crystallographic edge.（3）Independently distributed octahedral inclusions are primary fluid inclusions.The octahedral inclusions distributed in groups are secondary inclusions.The dark octahedral inclusions are protogenetic inclusions.