广西大化墨玉的矿物学及谱学特征研究

Study of Mineralogical and Spectroscopic Characteristics of Black Nephrite from Dahua in Guangxi

近年来大量的墨玉在国内外玉石市场上陆续出现,广西大化墨玉是最新发现的墨玉新品种。为了探究广西大化墨玉的矿物学及谱学特征,针对产自广西大化瑶族自治县的墨玉样品进行了常规检测,以及采用X射线粉末衍射仪、激光拉曼光谱仪、傅里叶红外变换光谱仪和激光剥蚀等离子体质谱仪等现代谱学仪器测试分析,从矿物组成、拉曼光谱和红外光谱以及化学元素组成进行了研究分析。常规宝石学特征测试显示广西大化墨玉的折射率为1.64(点测),比重为3.12。偏光显微镜观察显示广西大化墨玉的主要矿物为阳起石,含量大于98%,结构为显微毛毡状结构。XRD测试明确样品主要成分为阳起石,其特征面网间距为8.498 3和3.145 9。傅里叶红外变换光谱仪测试结果显示样品的红外光谱与透闪石理论值接近,主要的特征峰为1 078,1 026,925,765,703,659,584,485,436cm^(-1),其中1 078,1 026,925cm^(-1)为O—Si—O和Si—O—Si的反对称伸缩振动及O—Si—O对称伸缩振动,765,703,659cm^(-1)为Si—O—Si对称伸缩振动,584,485,436cm^(-1)为Si—O弯曲振动。激光拉曼光谱测试测试结果显示样品的图谱基本集中在3500~3 800和119~1 054cm^(-1)这两个区域内,样品的拉曼光谱119~1 054cm^(-1)的特征峰中1 055,1 029和930cm^(-1)为闪石类矿物特征的Si—O伸缩振动,744和671cm^(-1)为Si—O—Si伸缩振动,且在671cm^(-1)是强度最大的特征峰位,代表硅氧四面体结构单元中桥氧的对称伸缩振动;在3 800~3 500cm^(-1)区间为M—OH伸缩振动区域,反映了M1和M3位置的阳离子与结构中的OH-成键的振动信息,位于3 628,3647,3 664,3 678cm^(-1),这是由于OH-伸缩振动导致。通过激光剥蚀质谱仪测试分析发现样品的主要化学成分为SiO2(52.4%),FeO(21.95%),CaO(12.5%)和MgO(12.4%)。此外还含有少量Al2O3,MnO,Na2O,P2O5,K2O和TiO2,由于样品富含Fe元素,计算Mg/(Mg+Fe)=0.504,因此大化墨玉为软玉中的阳起石玉,并由此推断大化墨玉的黑色由含铁量较高所致。

In recent years,a large amount of black nephrites began to appear in domestic and foreign markets.A new kind of black nephrites was found in Dahua Yao Autonomous County,Guangxi Zhuang Autonomous Region.In order to analyze mineral composition,spectroscopic characteristics and chemical composition of Dahua black nephrites,the standard gemological methods,X-ray powder diffraction,laser Raman spectroscopy,infrared absorption spectroscopy and laser ablation inductively coupled plasma mass spectrometry（LA-ICP-MS）were used.Based on standard gemological testing,the samples showed a spot RI of1.64 and a hydrostatic SG of 3.12.With polarizing microscope,the major constituent mineral for Dahua black nephrites is found to be actinolite（98%）.The main structure of samples is microstructure fibers woven structure.The major constituent mineral of samples is confirmed to be actionolite with XRD testing.The characteristic mesh spacing of actionolite is 8.498 3and 3.145 9.FTIR spectrum of samples are similar to that of tremolite,and main absorption bands are 1 078,1 026,925,765,703,659,584,485,436cm-1.Infrared absorption bands at 1 078,1 026,925cm-1 are induced by O—Si—O antisymmetric stretching vibration,Si—O—Si antisymmetric stretching vibration and O—Si—O symmetric stretching vibration.Infrared absorption bands at 765,703,659cm-1 are induced with Si—O—Si symmetric stretching vibration.Infrared absorption bands at 584,485,436cm-1 are induced with Si—O bending vibration.Laser Raman spectroscopy testing results show that Raman peaks of samples are located at 3 500~3 800 and 119~1 054cm-1.Raman peaks at 1 055,1 029 and 930cm-1 are induced by Si—O stretching vibration which is the characteristic vibration of amphibole.Raman peaks at 744 and 671cm-1 are induced by Si—O—Si stretching vibration.671cm-1 is the strongest Raman peak,which represents symmetric stretching vibration of bridge oxygen in silicon-oxy tetrahedron.Raman peaks in the range of 3 800~3 500cm-1 are induced by M—OH stretching vibration.These Raman peaks reveal bonding vibration information between cations in M1,M3 and OH-.Raman peaks at 3 628,3 647,3 664,3 678cm-1 are induced by OH-stretching vibration.According to LA-ICP-MS,the chemical compositions of samples are SiO2（52.4%）,FeO（21.95%）,CaO（12.5%）and MgO（12.4%）.A spot of Al2O3,MnO,Na2 O,P2O5,K2 O and TiO2 were found.Having high concentrations of Fe is a notable characteristic of Samples.The result of Mg/（Mg＋Fe）is 0.504,suggesting that Dahua black nephrites belong to actinolite jade in nephrite series.High-Fe actinolite is responsible for the color of Dahua black nephrites.