Composition and Formation of Octahedral Inclusion in Spinel

At present, there isn't any reasonable conclusion towards the composition of octahedral inclusions in gemmology, and some of the existing opinions are not supported by credible evidence. Therefore, it is necessary to review the available opinions with further evidence. To solve this question, the authors conduct this study based on spinels primarily from Burma with the characteristic octahedral inclusions seen in spinels from Burma and Sri Lanka. After observing 52 pieces of spinels from Burma, the authors chose 4 representative pieces for microscopic observation and laser Raman spectroscopy to find out the composition and formation of octahedral inclusions. From the Raman spectra of the inclusions, 155, 282, 712 and 1 086 cm^-1 peaks of calcite and 177, 300, 1 099 cm^-1 peaks of dolomite were seen. The spatial distribution of the inclusions can provide some evidences of their formation. The authors draw conclusion from its refractive index, special arragenment of the inclusions, polarity and composition that the octahedral inclusions in spinels from Burma have the following features:white or colourless, sharp border, directional alignment, etc., usually indicating that they are syngenetic, secondary or pseudo-secondary negative crystals filled with dolomite or calcite from residual metallogenic hydrothermal solution. However, considering the limit of the researched sample numbers and sources, the existence of other spinel group minerals as octahedral inclusion is not verified in the papers and it needs further work.