Mineralogical and Spectroscopic Investigation of Turquoise from Dunhuang, Gansu

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 6A1–4Eg + 4A1 transition of Fe3+ ions (D5 configuration) in hydrated iron ions [Fe(H2O)6]3+, and the spin-allowed 2Eg–2T2g 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.