印度龙蛋石的宝石矿物学及谱学研究

The Gemological,Mineralogical,and Spectral Characteristics of Indian Longdan Stone

印度“龙蛋石”因其与青田龙蛋石具有相似的多色材质,作为雕刻石引进国内。然而目前市场对印度“龙蛋石”的谱学特征和成分结构还不清楚。选取了极具代表性的样品,利用偏光显微观察、X射线粉晶衍射仪(XRD)、扫描电子显微镜(SEM)、电子探针(EPMA)、傅里叶红外光谱仪(FTIR)对该样品的宝石矿物学特征、颜色成因以及谱学特征进行了深入的研究。印度“龙蛋石”以具有“白肉红心”为特征。偏光显微镜显示,“印度龙蛋石”为隐晶质鳞片变晶结构,“红心”区域出现明显的红褐色斑块物质,并从中心红色区域到边缘黄绿色区域呈现出由密集到松散分布的规律,与外观颜色变化相符。X射线粉晶衍射结果显示,淡黄绿色“白肉”和“红心”区域衍射图谱一致,均显示三个强的10.00、4.99和3.33?衍射峰,且在2.86、2.99、3.20、3.49和3.73?处均可见伊利石衍射峰,无其他矿物相存在,表明样品为纯度较高的2M1型伊利石。XRD在10.00?峰的半高宽为0.092°Δ2θ,表明伊利石有序度和结晶度较好。电子探针测试进一步证实该印度“龙蛋石”主要为伊利石,其平均层间阳离子含量为0.824 p.u.f.,并含有0.05%~0.08%Wt的结构铁。扫描电镜背散射成分图像显示红褐色斑块状物质具有明显的高亮衬度,但普遍表现为伊利石鳞片状形貌。能谱测试表明该区域铁平均含量为0.48%Wt,高于伊利石中结构铁含量一个数量级,表明红褐色斑块状含铁物质可能是印度“龙蛋石”的致色物质。扫描电子显微镜下发现立方形貌的KCl晶体,指示伊利石可能直接结晶于富K流体中。傅里叶红外光谱测试结果表明,样品在3630 cm^(-1)为OH伸缩振动峰;830 cm^(-1)为四面体内Al—O振动;756 cm^(-1)的吸收峰与Al取代Si参与四面体配位有关,为四面体内Si—O—Al振动的表现。位于3625 cm^(-1)附近的OH伸缩振动吸收峰与825和750 cm^(-1)双指纹吸收峰为伊利石矿物的特征傅里叶红外吸收峰,印证了该印度“龙蛋石”主要矿物为伊利石。对印度“龙蛋石”的研究丰富了对雕刻石材质的宝石学和谱学特征的认识,其红外光谱特征可作为雕刻石样品快速无损测试的鉴定依据。

Indian Longdan stone was introduced into China as a carving stone because of its similar colors and materials to Qingtian Longdan stone.However,the spectral,mineral compositional,and structural characteristics of Indian Longdan stone are still unclear.In this paper,the gemmological characteristics,color genesis and spectral characteristics of a representative sample were studied by polarizing microscopy,X-ray Diffractometer(XRD),Scanning Electron Microscope(SEM),Electron Probe Micro Analyzer(EPMA)and Fourier Infrared Spectrometer(FTIR).Indian Longdan stone is characterized by“white meat and red heart”.The polarizing microscope shows that the illite is a cryptocrystalline lepidoblastic texture.Obvious red-brown spotted materials are aggregating in the“red heart”area,presenting as dense to loose from the central red area to the edge yellow-green area,consistent with the color change.XRD results show that the light yellow-green“white meat”and“red heart”diffraction patterns are the same,showing three strong diffraction peaks at 10.00,4.99 and 3.33.Clear peaks can be observed at 2.86,2.99,3.20,3.49 and 3.73 and no other mineral phasecan be detected,indicating that the sample is pure 2M1 type illite.The full width at half maximum of XRD at 10.00 peak is 0.092°Δtwo theta,indicating that the order and crystallinity of illite are well.EPMA further confirms that the Indian Longdan stone is mainly illite,with an average cation content of 0.824 p.u.f.and a structural iron content of 0.05%~0.08%.SEM backscattering composition images show that the red-brown patchy material has obvious bright contrast but generally shows similar morphology of illite.The energy spectrum analyses show that the average content of iron in this area is 0.48%wt,which is one order of magnitude higher than the content of structural iron in illite,indicating that the red-brown patchy iron-bearing material may be the chromogenic material of Indian Longdan stone.SEM observation reveals KCl crystals with cubic morphology,indicating that the illite may be directly crystallized in K-rich fluid.The results of FTIR show that the samples have an OH stretching vibration peak at 3630 cm^(-1) and an Al—O vibration peak in tetrahedron at 830 cm^(-1).The absorption peak at 756 cm^(-1) is related to the substitution of Al in tetrahedral coordination instead of Si,which is characteristic of Si—O—Al vibration in the tetrahedron.The absorption peaks of OH stretching vibration near 3625 cm^(-1) and at 825 and 750 cm^(-1) double fingerprints are the characteristic infrared absorption peaks of illite minerals,which confirms that the main mineral of Indian Longdan stone is illite.The study of Indian Longdan stone enriches the understanding of the gemmological and spectral characteristics of carved stone material,and the infrared spectral characteristics can be used as the identification basis for rapid,nondestructive testing of carved stone samples.