Mineralogical Characteristics and Color Origin of Jingao Jade

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.