Gem mineralogy and inclusion characteristics of gahnite from Nigeria
The conventional gemological properties,chemical composition,crystal structure,and inclusions compo-sition and characteristics of the Nigerian gahnite were analyzed in depth by conventional gemological testing,elec-tron probe,single crystal X-ray diffraction,and Raman spectroscopy.The gahnite exhibits well-preserved octahe-dral crystal morphology,strong vitreous luster,and semi-transparency,with a relative density ranging from 4.48 to 4.61 and a refractive index of 1.792 to 1.794,displaying inert fluorescence under ultraviolet light.The electron probe analysis determined its chemical formula as Zn(0.87～0.92)Fe(0.06～0.07)Al(1.98～2.01)O4.Single-crystal X-ray diffrac-tion revealed a cubic crystal system with space group Fd3m,lattice parameter a=8.089(2)Å,tetrahedral bond length T—O=1.953(4)Å,and octahedral bond length M—O=1.912 9(19)Å.Characteristic Raman absorption peaks for gahnite were identified at 418 cm-1 and 659 cm-1,corresponding to the Eg mode and high-frequency T2g(3)mode,respectively.Raman spectroscopy of inclusions confirmed the presence of various minerals in gahnite,including sphalerite,albite,muscovite,quartz,beryl,zircon,and phenacite.Combined with previous studies,it is believed that gahnite in Nigeria is formed in granite-pegmatite rich in Li,Cs,Rb,Be and Ta.
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