Qi Chen

A recently discovered turquoise deposit in the Fangshankou area of Dunhuang, Gansu Province, has been relatively understudied compared to turquoise from other sources due to its short mining history. Currently, no relevant research literature on this deposit has been identified. Therefore, a systematic mineralogical and spectroscopic study of Dunhuang turquoise samples was conducted using conventional gemological testing methods, combined with techniques such as X-ray powder diffraction (XRD), electron probe microanalysis (EPMA), Fourier transform infrared spectroscopy (FTIR), laser Raman spectroscopy, ultraviolet-visible spectroscopy (UV-Vis), and X-ray fluorescence (XRF) mapping. The test results indicate that the turquoise samples from this area have a density ranging from 2.40 to 2.77 g/cm³ and a refractive index between 1.59 and 1.65. The samples generally exhibit a cryptocrystalline structure, with some displaying spherulitic radial and radial fibrous structures. The texture is relatively dense and hard, with particle diameters less than 10 μm. Chemically, the turquoise samples from this region are characterized by high Fe and Si content and relatively low Cu content. Samples contain, in addition to the turquoise mineral, other minerals such as quartz, goethite and alunite, etc. The oxide content ranges are as follows: w(P₂O₅) between 23.83% and 33.66%, w(Al₂O₃) between 26.47% and 33.36%, w(CuO) between 5.26% and 7.91%, w(FeO) between 2.46% and 4.11%, and w(SiO₂) between 0.97% and 10.75%. In the infrared absorption spectra of Dunhuang turquoise, the bands at 3510 cm⁻¹ and 3464 cm⁻¹ are attributed to ν(OH)⁻ stretching vibrations, while the bands near 3308 cm⁻¹ and 3098 cm⁻¹ are assigned to ν(M-H₂O) stretching vibrations. The infrared absorption bands near 1110 cm⁻¹ and 1058 cm⁻¹ are due to v[PO₄]³⁻ stretching vibrations, and the bands near 651 cm⁻¹, 575 cm⁻¹, and 485 cm⁻¹ are attributed to δ[PO₄]³⁻ bending vibrations. A clear correlation exists between the Raman spectral features and the infrared spectra of this turquoise. The hue and chroma of the turquoise from this area are primarily influenced by the mass fractions of Fe³⁺, Cu²⁺, and Fe²⁺, as well as their bonding modes with water molecules. The ultraviolet-visible spectra are attributed to O²⁻–Fe³⁺ charge transfer, the ⁶A₁–⁴E_g + ⁴A₁ transition of Fe³⁺ ions (D⁵ configuration) in hydrated iron ions [Fe(H₂O)₆]³⁺, and the spin-allowed ²E_g–²T_2g transition of Cu²⁺ ions in hydrated copper ions [Cu(H₂O)₄]²⁺. Associated minerals include goethite, alunite, jarosite, and quartz. Fine-grained quartz often exists as secondary micron-sized independent mineral phases, which have a certain impact on the quality of the turquoise.

Hydrogen and oxygen isotopes of ore-forming fluid of nephrite deposits have always been changing due to mixings between different fluids and oxygen isotope exchanges between the ore-forming fluid and country rocks, resulting in that the tremolite (or actinolite) has to constantly re-establish new isotope fractionation equilibriums with the dynamic fluid, which is of great significance to understand the genesis of hydrogen and oxygen isotopes of nephrite. Based on this, Taylor’s closed model and fluid mixing model are used to unravel the control of multi-stage evolution of ore-forming fluid on the δD and δ¹⁸O of nephrite. Although Taylor’s closed model is conducive to interpreting the genesis of nephrite with light δD and δ¹⁸O, such as Vitim nephrite, Russia, and Chuncheon nephrite, South Korea, it is unable to be effectively used in other nephrite. The fluid mixing model can quantitatively constrain proportions of different fluids during different ore-forming stages. Multiple solutions of ore-forming fluids of carbonate rock-related nephrite result from the absence of external constraints, such as isotope compositions of intrusive rocks, carbonate rocks, and meteoric water. Due to the generally heavy δ¹⁸O of country rocks, a small amount of meteoric water that enters the hydrothermal system in the later ore-forming stage is insufficient to offset the δ¹⁸O increment of nephrite caused by the oxygen isotope exchange between country rocks and water, which should be responsible for the abnormal heavy δ¹⁸O of Luodian nephrite, Dahua nephrite, Sanchakou nephrite, Xiaomeiling nephrite, etc., and not metamorphic water dominating their formation.

Hydrogen and oxygen isotopes of ore-forming fluid of nephrite deposit have always been changing due to mixings between different fluids and oxygen isotope exchanges between the ore-forming fluid and country rocks, resulting in that tremolites (or actinolite) have to constantly re-establish new isotope fractionation equilibriums with the dynamic fluid, which is of great significance to understand the genesis of hydrogen and oxygen isotopes of nephrites. Based on this, Taylor closed model and fluid mixing model are used to unravel the control of multi-stage evolution of ore-forming fluid on the δD and δ18O of nephrite. Although Taylor closed model is conducive to interpreting the genesis of nephrite with light δD and δ18O, such as Vitim nephrite and Chuncheon nephrite, it is unable to be effectively used in other nephrites. The fluid mixing model can quantitatively constrain proportions of different fluids during different ore-forming stages. Multiple solutions of ore-forming fluids of carbonate rock-related nephrites result from the absence of external constraints, such as isotope compositions of intrusive rocks, carbonate rocks and meteoric water. Due to the generally heavy δ18O of country rocks of carbonate rocks, a small amount of meteoric water that enters the hydrothermal system in the later ore-forming stage is insufficient to offset the δ18O increment of nephrite caused by the oxygen isotope exchange between country rocks and water, which should be responsible for the abnormal heavy δ18O of Luodian nephrite, Dahua nephrite, Sanchakou nephrite, Xiaomeiling nephrite, etc, not metamorphic water dominating their formation.