陕西汉中符山石玉的矿物学特征及颜色成因

MINERALOGICAL CHARACTERISTICS AND COLOR GENESIS OF VESUVIANITE JADE IN HANZHONG AREA,SHAANXI PROVINCE

使用光学显微镜和电子探针、紫外一可见光光度计等现代测试仪器,系统研究了陕西汉中符山石玉的矿物学特征,初步探讨了玉石的颜色成因。结果表明:玉石颜色分布不均匀,以绿色调为主,局部伴有白色、浅灰褐色、绿色和深褐色,半透明一微透明,折射率1.70~1.73,相对密度3.32~3.44。粒状镶嵌结构为主,主要组成矿物为半自形粒状的符山石,含量高达90%以上,次要矿物包括方解石、石榴石、透辉石、绿泥石以及铬铁矿等。不同颜色区域中符山石的主要成分基本一致:w(SiO_(2))36.43%~38.72%,w(Al_(2)O_(3))16.93%~19.75%,w(CaO)35.24%~39.09%,次要成分差异明显:w(FeO)0.97%~6.39%,w(MgO)0.11%~3.57%。根据电子探针成分数据和紫外一可见光光谱的分析,符山石晶体中的铁主要为Fe2+,含有极少量的Fe3+。玉石的颜色主要取决于符山石晶体的颜色及其分布特征,次要矿物及其分布特征对玉石的颜色也有一定的影响。玉石中不同色调的绿色、黄色以及褐色与符山石晶体中铁的含量变化密切相关,随着FeO的含量由0.97%递增至6.39%,符山石玉的颜色由绿白色逐渐过渡到深绿色甚至褐色。绿色与符山石晶体中的Fe^(2+)和Fe^(3+)有关,且随着符山石晶体中FeO含量的增大而变深,O2-→Fe^(2+)之间的电荷转移是导致符山石呈黄色的主要原因。玉石中不规则形态的白色团块主要由粒径5μm左右的方解石充填于符山石晶粒间而形成,白色的细脉由呈脉状分布的方解石引起的;形态不规则的粒状石榴石导致玉石局部呈浅灰褐色;深褐色斑块由大量点状、斑状的细分散微晶质铬铁矿聚集所致,环绕深褐色斑块的绿色主要与细小的不规则片状绿泥石有关。

The mineralogical characteristics of the vesuvianite jade from Hanzhong,Shaanxi Province were systematically studied by means of optical microscope,EPMA,UV-Vis spectrophotometer,and other modern testing instruments.Besides,the color genesis of jade was preliminarily explored.The results show that the color of jade is unevenly distributed,dominated by green tones that include green-white,light green,yellowish green,green,dark green,partly accompanied by white,light gray-brown,green and dark brown,featuring semi-transparent to slightly transparent.Refractive index of vesuvianite jade is 1.70 to 1.73,with a relative density of 3.32 to 3.44.The Hanzhong vesuvianite jade possesses granular mosaic texture with more than 90%hypidiomorphic-granular vesuvianite as its main mineral component.Minor minerals include calcite,garnet,diopside,chlorite and chromite,etc.The main compositions of vesuvianite in different color areas are basically the same,including w(SiO_(2))=36.43%-38.72%,w(Al_(2)O_(3))=16.93%-19.75%,and w(CaO)=35.24%-39.09%.While the chemical composition of secondary minerals is significantly different,including w(FeO)=0.97%-6.39%and w(MgO)=0.11%-3.57%.According to the composition data of EPMA and the analysis of UV-Vis spectrum,the vesuvianite crystal is mainly Fe2+and contains very small amounts of Fe3+:The color and distribution characteristics of vesuvianite crystal determine the color of jade,and the secondary minerals and their distribution also have a certain impact on the color of jade.The different shades of green,yellow and brown in vesuvianite jade are closely related to the change of iron content in vesuvianite crystal.With the increase of FeO content from 0.97%to 6.39%,the color of vesuvianite jade gradually change from green-white to dark green and even brown.Thegreen color is related to Fe+and Fe in the crystal of vesuvianite,and becomes darker with the increase of FeO content.The charge transfer between O2--Fe^(2+)is the main reason for the yellow color of Hanzhong vesuvianite.The irregular white masses in the jade are mainly formed by the infiling of calcite with a grain size of about 5μm occurred as veinlets in vesuvianite and the irregular shape of granular garnet results in light gray-brown in some region of jade.The dark brown patches are caused by the aggregation of a large number of punctate and porphyritic fine dispersed microcrystalline chromite.The green color surrounding the dark brown patches is mainly related to fine irregular flaky chlorite.