内蒙古阿拉善地区绿色戈壁玛瑙的宝石学特征与致色成因研究

A study of gemological characteristics and color origin of green Gobi agate from Alxa area,Inner Mongolia

内蒙古阿拉善地区经历数亿年的地质活动,产出颜色丰富且结构致密的戈壁玛瑙。通过常规宝石学测试、偏光显微镜及扫描电镜观察、X射线粉晶衍射、电子探针、红外光谱及紫外-可见光-近红外分光光度计等测试分析方法对绿色戈壁玛瑙的宝石矿物学特征及致色成因进行了深入研究。肉眼观察,阿拉善绿色玛瑙呈深绿色至褐绿色,微透明至不透明,相对密度、折射率、摩氏硬度等均符合石英质玉石的特点。偏光显微镜观察,绿鳞石富集于表层,并向内部呈放射状生长;方解石与石英均为隐晶质结构。扫描电镜观察,绿鳞石呈颗粒状分布于石英及方解石之间。X射线衍射分析结果表明,绿色戈壁玛瑙的物相组成主要为石英、方解石和绿鳞石。电子探针分析结果表明绿鳞石的主要化学组成为SiO_(2)、FeO、Al_(2)O_(3)、K_(2)O和MgO。红外光谱分析也显示存在绿鳞石对应基团的特征峰。表层绿鳞石在紫外-可见光-近红外分光光度计下显示出Fe^(2+)与Fe^(3+)的特征光谱,Fe^(2+)与Fe^(3+)之间的电荷转移是其呈现绿色的主要原因。绿鳞石是阿拉善绿色戈壁玛瑙的主要致色矿物。

The Alxa area in Inner Mongolia has experienced hundreds of millions years of geological activities,producing Gobi agate with rich colors and compact structure.The gem mineralogical characteristics and coloring causes of green Gobi agate were studied by conventional gemological test,polarizing microscope and scanning electron microscope observation,X-ray diffraction,electron microprobe analysis,infrared spectrometer and ultraviolet-visible near infrared spectrophotometer.With naked eyes,the agate is dark green to brown green,translucent to opaque The relative density,refractive index and Mohs hardness are all in line with the characteristics of quartzite jade stone.Through microscopic observation,celadonite is enriched in the surface layer and grows radially inward.Both calcite and quartz have cryptocrystalline texture.Through scanning electron microscope,celadonite is distributed between quartz and calcite in granular form.X-ray diffraction analysis shows that the mineral phase composition o green Gobi agate is quartz,calcite and celadonite.The EPMA results show that the main chemical composition o celadonite are SiO_(2),FeO,Al_(2)O_(3),K_(2)O and MgO.Infrared spectrum analysis also shows that there are characteristic peaks of corresponding groups of celadonite.The test results of surface celadonite under UV-Vis-NIR spectrophotometer show the characteristic spectra of Fe^(2+)and Fe^(3+).The charge transfer between Fe^(2+)and Fe^(3+)is the main reason for its coloration.Celadonite is the main chromic mineral of green Gobi agate.