金膏玉的岩石矿物学特征及颜色成因

Mineralogical Characteristics and Color Origin of Jingao Jade

  • 摘要: 陕西省洛南县产出的金膏玉以其独特的金黄色而得名,目前关于金膏玉颜色和矿床成因的研究尚处于起步阶段,尚未形成统一认识,对颜色成因等研究的不完善制约了金膏玉作为宝玉石开展无损检测。为进一步明确金膏玉的颜色和矿床成因,选取不同颜色金膏玉,利用偏光显微镜、X射线衍射仪、电感耦合等离子体发射光谱/质谱仪、电子探针、扫描电镜-能谱仪、激光拉曼光谱仪、漫反射光谱仪、稳定同位素质谱仪等研究其矿物学特征及颜色成因。结果表明,金膏玉主要由白云石组成,白云石晶胞参数和化学成分均与理想白云石接近。全岩化学分析及白云石单矿物电子探针分析结果均表明,黄色金膏玉中Fe含量明显高于白色者。黄色金膏玉的一阶导数漫反射光谱呈现出归属于针铁矿的特征吸收(526~538nm、435~437nm),且黄色色调与针铁矿含量呈正相关关系;拉曼光谱中白云石主峰的规律性偏移表明,白云石中的Fe可能也对黄色有所贡献。全岩化学分析结果支持金膏玉基本未受到陆源组分影响,可以反映生成时期的环境及变化特征。金膏玉δEu平均值为0.956,δCe平均值为0.818,指示弱氧化的沉积环境。基于碳氧同位素(金膏玉δ13C平均值为−0.190‰,δ18O平均值为−4.21‰)的成因模式分析表明,金膏玉可能属于埋藏成因白云岩。使用碳氧同位素数值,依据经验公式计算Z值来判断灰岩来源,得到金膏玉Z值平均值为142.746,远超过海相灰岩判别阈值(Z>120为海相灰岩,Z<120为淡水灰岩),支持金膏玉由具显著海相成因特征的灰岩改造而来,并且在成岩后未经受可能使碳氧同位素显著降低的地质事件(如海平面降低、淡水环境等),反映了稳定的海洋沉积环境。

     

    Abstract: The Jingao jade produced in Luonan County, Shaanxi Province, is named for its distinctive golden color. Currently, the research on the color and genesis of the gold paste jade deposit is still in its infancy and has not yet reached a consensus. The lack of completeness in the research on the origin of color restricts the non-destructive detection of gold paste jade as a gemstone. In order to further clarify the color origin and genesis of Jingao jade, different colored samples were selected and studied by using polarizing microscope, X-ray powder crystal diffractometer (XRD), inductively coupled plasma-optical emission spectrometry (ICP-OES), inductively coupled plasma-mass spectrometry (ICP-MS), electron microprobe (EPMA), scanning electron microscope-energy spectrum (SEM-EDS), laser Raman spectrometer (LRS), diffuse reflectance spectroscopy (DRS), and stable isotope mass spectrometer. Data demonstrate that Jingao jade mainly consists of dolomite. The unit cell parameters and chemical composition of the dolomite are very close to those of the ideal dolomite. The results of the full rock chemical analysis and the EPMA analysis of dolomite single mineral in Jingao jade were both consistent in showing that the Fe content in the yellow jade was significantly higher than that in the white one. In the first-order derivative DRS of Jingao jade, characteristic absorption attributed to goethite (526-538nm, 435-437nm) is observed, and the yellow color is positively correlated with the content of goethite. The regular shift of the main peak of dolomite in Raman spectroscopy also indicates that the Fe in dolomite might also contribute to the yellow color. Full rock chemical analysis indicate that Jingao jade have not been significantly influenced by terrestrial components, and can reflect the environment during the formation period. The average values of δEu and δCe of Jingao jade are 0.956 and 0.818, respectively, indicating a weakly oxidized sedimentary environment. Based on the results of the isotopic analysis of carbon and oxygen (with the average δ13C value of Jingao jade being -0.190‰ and the average δ18O value being -4.21‰), the formation model analysis indicates that Jingao jade may belong to buried-type dolomite. Based on the carbon and oxygen isotope values, the Z value is calculated using empirical formulas to determine the source of the limestone. The average Z value of Jingao jade is 142.746, which is far beyond the discrimination threshold for marine limestone (Z>120 indicates marine limestone, while Z<120 indicates freshwater limestone). This supports the conclusion that Jingao jade was formed by the transformation of limestone with distinct marine origin characteristics, and it did not undergo any geological events that might have significantly reduced the carbon and oxygen isotopes (such as sea level drop, freshwater environment, etc.) after diagenesis, reflecting a stable marine sedimentary environment.

     

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