Coloring And Metallogenic Mechanisms Of Different Colors In Qinghai Nephrite

The Qinghai nephrite (QN) deposit, which was first found in the early 1990s, has at least 1600 tons in proven reserves and, as a result, is considered a large-scale nephrite deposit. Compared with nephrites from other localities, QN has good transparency and is rich in color. In addition to a white series, other colors in QN include green, brown, yellow, and purple, while special color varieties include blue-violet and viridis. Due to its high quality, QN is becoming increasingly sought after, and its market value has increased significantly since it was used to make models for the 29th Olympic Games in Beijing. QN has become an important and globally recognized nephrite deposit. However, despite its economic worth and broad scientific interest, it remains not fully understood. In this paper, eight colors QN samples collected and analyzed in the study possess representative color, fine-grained and compact microstructures.To coloring elements and mechanisms of QN, the paper aims to distinguish essential coloring elements and to discuss the QN coloring mechanisms by analyzing eight samples of different colors (white, green-white, yellow, brown, blue-violet, viridis, green, and azure-green). X-ray diffraction (XRD) and mineralogy identification indicated that the mineral main component in eight colors QN is tremolite which accounts for over 95%. The XRD data also show all QN samples have high crystallinity (on average of 96.61%). In addition, cell parameters ao, bo, co and ? of eight samples are deviated from the standard values of tremolite, which indicates elemental replacements are notable. In particular, with the exception of the green and azure-green QN, the bo of other colors QN are all less than the standard bo of tremolite. This indicates the presence of high electrovalence ions with small ionic radius (e.g., Fe3+, Mn3+, Mn4+, Ti4+, V4+), which have substituted for Mg2+in octahedral Mi, M2, and Ms sites. However, for azure-green QN, low electrovalence ions of larger ionic radius (e.g., Fe2+, Ti3+, Cr3+, V3+, Ni2+) raised in Mg2+ sites, leading to larger bo. The transition metal element contents of eight colors QN are measured by X-ray fluorescence spectroscopy (XRF) and inductively-coupled plasma-mass spectrometry (ICP-MS). The results suggest that the coloring elements of QN may be Fe2+, Fe3+, Mn and Ti.Electron paramagnetic resonance (EPR) spectra of all QN samples are measured at 293 K and 93K. For all the samples investigated, the EPR spectra reveal three high-spin Mn2+ centers and one high-spin Fe3+ center. The former is readily identified by three sextets with splitting of-85 Gs at the effective g= 2.001, corresponding to the characteristic 55Mn hyperfine structuresand suggesting that Mn2+is present in three closely related sites Mi, M2, and M3.The rhombic Fe3+ center at g= 4.331 can be probably attributed to high-spin (S= 5/2) Fe3+ ions at the octahedral sites [Mi, M2 and M3] only.However, the absorption intensities of Mn2+ and Fe3+ EPR spectra are not consistent with Mn and Fe3+ contents as measured by XRF and ICP-MS. The main reason is that Mn occurs also in higher valence states that minor amounts of Fe3+ substitute for Si4 +at the tetrahedral sites (Ti and T2).Under the condition of oxidation, heating to 500? and 800? leads to fading of some QN colors, including the yellow, brown, and blue-violet, but darkening of the green-white, viridis, green, and azure-green colors. Ultraviolet-visible spectra (UV-vis spectra) of the faded samples show that the absorption bands near 550 nm or 560 nm gradually decrease with heating (and may disappear completely), and the absorption bands near 437 nm or 450 nm enhanced in which the color darkens. In detail, the yellow colors of QN are caused primarily by O2- ? Fe3+ ligand-to-metal charge transfer (LMCT) and Mn4+4A2 ? 4T2 (F), the blue-violet colors by Fe2+ ? Ti4+ inter-valence charge transfer (IVCT), and the brown color by the electron transitions 4A2? 4T2 (F) of Mn4+. Color fading in response to heating is due to the oxidation of Fe2+or Mn4+, which results in reducing concentrations of these ions. Meanwhile, the darkening of the QN colors in response to heating is due to the oxidation of Fe2+, leading to increasing Fe3+ concentration. In these cases, the viridis color is caused mainly by electron transitions:Cr3+ 4A2?4T2, Cr3+4A2?4T1+2E, and Fe3+6A1?4E+4Ai (4G); the green-white color results from the electron transition Fe3+6A1?4E+4A1 (4G); and the green and azure-green colors are caused mainly by IVCT of Fe2+?Fe3+ and Fe2+(5T2)+ Fe3+(6A1)?Fe2+(5E)+Fe3+(6A1).Electronic microprobe analysis showed that all QN samples are mainly composed of tremolite and minor accessory minerals, such as diopside, calcite, serpentinite, and magnetite. According to the cation coefficients, the crystallo-chemistry-genesis illustration demonstrates that all QN deposits are contact metasomatic. Depending on the mole percent of Fe2+(3+)/(Mg2++Fe2+(3+)) and the content of Cr, Co, and Ni in all QN samples measured by X-ray fluorescence spectroscopy (XRF) and inductively coupled plasma-mass spectrometry (ICP-MS), green and azure-green QNs are characterized as serpentinite-related contact metasomatic deposit (S-type), whereas white, green-white, brown, blue-violet, yellow, and viridis QNs are dolomite-related contact metasomatic deposit (D-type). The assemblages and chemical composition of accessory minerals of the eight-color QN samples show evident characteristics, which reveal four possible ore-forming processes:(i) dolomitic marble ? tremolite, (ii) dolomitic marble ? diopside ? tremolite, (iii) dunite ? serpentinite? tremolite, (iv) pyroxenes? serpentine? tremolite.We also measured trace and rare earth elements (REEs) in these samples through ICP-MS to deduce the origin of and the changes in metallogenic conditions. The chondrite-normalized REE patterns of D-type QN exhibit moderately negative Eu anomalies with moderate light REE enrichment, flat heavy REE (HREE), and low ?EE concentrations, similar to dolomitic marble. Green QN samples of S-type show enrichment in HREE and moderately negative Eu anomalies, which is consistent with characteristics of dunite. Whereas azure-green QN samples of S-type exhibit a right-dipping V-type curve with severe depletion in Eu (5Eu= 0.36-0.47), in accordance with the characteristics of gabbro from Yushigou ophiolite in North Qilian mountains. Furthermore, this finding is consistent with the content of trace elements and the petrographic analysis results. On the basis of several significant differences in the characteristic elements, which may have been affected by the metallogenic environment, we inferred the differences in oxygen fugacity and basicity of mineralization environments in different-colored QNs. According to the ratios of Fe2+/Fe3+, ?Ce, Sr/Ba, Zr/Hf and Nb/Ta in all QN samples, we suggested that oxygen fugacity gradually decreased in the order of green-white?white?-yellow?viridis?brown? blue-violet? azure-green?green QN, and the basicity of the mineralization environments gradually increased in the order of yellow ? viridis ? azure-green? blue-violet? green? brown ?green-white?white QN.