尼日利亚锌尖晶石的宝石矿物学及包裹体特征研究

Gem mineralogy and inclusion characteristics of gahnite from Nigeria

采用常规宝石学测试、电子探针、单晶X射线衍射和拉曼光谱等方法,深入分析了尼日利亚锌尖晶石的常规宝石学特征、化学成分、晶体结构以及包裹体特征。尼日利亚锌尖晶石呈完好的八面体晶形,强玻璃光泽、半透明,相对密度为4.48~4.61,折射率为1.792~1.794,紫外荧光灯下呈惰性。通过电子探针测试,计算其化学式为Zn_((0.87~0.92))Fe_((0.06~0.07))Al_((1.98~2.01))O_(4)。单晶X射线衍射揭示其为立方晶系、空间群Fd3m,晶胞参数a=8.089(2)A,四面体键长T—O=1.953(4)A,八面体键长M—O=1.9129(19)A。锌尖晶石的特征拉曼吸收峰位于418 cm^(-1)和659 cm^(-1),分别对应E_g模式、高频T_(2g(3))模式。包裹体研究显示锌尖晶石含有多种矿物包体,包括闪锌矿、钠长石、白云母、石英、绿柱石、锆石和硅铍石,结合前人研究认为尼日利亚锌尖晶石形成于富Li、Cs、Rb、Be和Ta等元素的花岗伟晶岩中。

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))e(0.060.07)Al_((1.98-2.0))O_(4) Single-crystal X-ray dffrac-tion revealed a cubic crystal system with space group Fd3m,lattice parameter a=8.089(2)A,tetrahedral bond length T-0=1.953(4)A,and octahedral bond length M-0=1.9129(19)A.Characteristic Raman absorption peaksfor gahnite werei denifed a 48cm^(-1) and 659cm^(-1),coreponding he modeand hgh-reueney T_(2g(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.