Study On Gem Mineralogical Characteristics Of Nanshankeng Fluorite In Yiwu

China is the largest fluorite producer in the world and is rich in resources.The fluorite deposit in Yiwu is densely distributed,which is related to volcanic magmatic activity.Fluorite is generally a dense massive aggregate,and there are few fluorite with complete and unique crystal form,but the fluorite block in this area has large particles,high transparency and bright color,with clear and beautiful ribbons and growth textures.It has the characteristics of making gem-grade fluorite and ornamental stone.The Yiwu Nanshankeng fluorite with the above characteristics is a typical representative of the Yiwu fluorite.The predecessors of fluorite in Yiwu studied more on the mineral bed of fluorite,but less on the mineralogical characteristics of fluorite gemstones.In this paper,on the basis of previous studies,through conventional gemmology test,microscopic observation,electron probe test,laser ablation-plasma mass spectrometry,infrared spectrum,and ultraviolet-visible spectrum,Raman spectrum analysis has made a systematic study and analysis on the gemmological characteristics,mineral composition,structure characteristics and chromogenic genesis of the Yiwu Nanshankeng fluorite.The fluorite from the Nanshankeng fluorite deposit in Yiwu is rich in color,with green,purple,pink,dark green and colorless colors,with well-developed ribbons and glassy luster.The crystal form of fluorite is intact,mostly cube,or interspersed with twin crystals or clusters,and some crystals are symbiotic with quartz.The refractive index is 1.433-1.438,Vickers hardness is 161.7-190.7,Morse hardness is 3.68-3.89,and the relative density is 2.7555-3.2600g/cm³.Most of the ultraviolet fluorescence has no fluorescence,that is,it is inert.Some fluorite shows weak purple fluorescence at long wave,no fluorescence at short wave,and lavender cathodoluminescence.Fluorite ores show autogenous-semi-autogenous coarse-grained structure,cataclastic structure,metasomatism structure and strip structure.Fluorite structure is generally block structure,breccia structure,pore structure,comb structure,grape structure,vein structure and strip structure.Fluorite is rich in inclusions.The chemical composition of fluorite in this area is relatively pure,and the content of Ca varies from 50.92%to 52.16%and from 46.12%to 47.51%.Other major impurity elements are S,Si,Fe,Cu,Ni,Zn,and contain rare earth elements.The impurities in fluorite of different colors are mainly Na,Mg,Al,Si,P,S,Cl,K,Mn,Fe,Ti and other elements.Purple fluorite contains higher Na elements,white fluorite contains higher Ti elements,green fluorite contains higher Na,Fe and Ti elements,dark green fluorite contains higher Na and Mg elements,pink fluorite contains higher Na,Mg and Ti elements,and other elements are low or absent in various color fluorite.The results of infrared spectra show that the samples have characteristic absorption peaks between 410-420cm^-1and no obvious infrared peaks in the region of600-4000cm^-1.Some samples have peaks located in 2852cm^-1and 2923cm^-1,which are related to organic clusters,and the infrared reflection spectra of different colors are basically the same.The characteristic reflection peak of purple and pink fluorite is 376cm^-1,590cm^-1,650cm^-1,730cm^-1,750cm^-1,825cm^-1,877cm^-1,943cm^-1.Green fluorite not only has the above characteristic peaks,but also has 306cm^-1,327cm^-1,439cm^-1,480cm^-1,characteristic reflection peaks.White fluorite has the characteristic reflection peak of32cm-17480cm-1.Dark green fluorite has the characteristic reflection peak of306cm-1327cm-1439cm-1480cm-1587cm-1.The results of UV-vis spectra show that the sample has the characteristic absorption of YO₂in the UV spectrum near212cm^-1,262cm^-1and 316cm^-1,and the absorption of different colors deviates slightly;in the visible range,the characteristic absorption peaks of green fluorite are the absorption peaks of 439cm^-1and 607cm^-1,respectively,which are mainly related to the rare earth ion Sm²⁺,which is consistent with the previous element test results;the characteristic absorption peak of purple fluorite is located near 570cm^-1,which is related to the color center produced by colloidal calcium.The absorption peak of306cm^-1in dark green fluorite is related to Ce³⁺.Raman spectra show that the sample has an obvious peak at 1100-1500cm^-1,which is related to rare earth elements such as Pr³⁺,Eu³⁺,Tb³⁺,Dy³⁺,Er³⁺,Ho³⁺and so on.