Study on Gemmological Characteristics of Turquoise from Dian’anshan,Anhui Province

Currently turquoises have been discovered in the mining areas of Dian'anshan, Anhui Province.Turquoise samples from Dian'anshan were analyzed by using electron microprobe (EMPA), polarizing microscope, scanning electron microscope (SEM), Fourier transform infrared spectrometer (FTIR) and ultraviolet-visible spectrometer (UV-Vis).The chemical compositions, mineral components and microstructure of the turquoises were also studied in this paper.The turquoise mines from Dian'anshan are located in a mountainous region of Ma'anshan, Anhui Province, where geological formation activities are abundant.The turquoises are found as concretion form and vein within the cracks of dioritic pophyrite and compresso-crushed zone.The structure of turquoises is compact in deeper area.Near the earth's surface, they are in the form of fine vein and thin film with loose texture.The colour of the turquoise are from blue to green. The electron microprobe analysis indicates that the chemical compositions of blue turquoises from Dian'anshan deviate from theoretical value.The w (Al₂O₃) varies between 37.47%and 38.31%, w (P₂O₅) between 34.92%and 35.81%, and w (CuO) between 7.84% and 9.10%, while the green turquoise cantains less contents of Al₂O₃ and CuO.The blue turquoise cantains about 2.0% FeO_T and the green turquoise cantains 9.40% FeO_T.The colour of turquoise is gradually from blue to green with the increase of rate of FeO_T/CuO.Compared to the other turquoise mines in China, the turquoise from Dian'anshan is characterized by poor Si and rich Fe. The polarizing microscope investigations show that the turquoises from Dian'anshan are microcrystal and scales structures, some of which are radial fiber, granular and crust structures.The turquoise crystals tend to be radial growth.The crust structure shows that the turquoise deposits from Dian'anshan are of weathering-leaching characteristics. The SEM results show that the turquoise crystals are mainly leaf shape in form of tight pile.The turquoise crystal growth is like micro-hole, short columnar and plate structure, which tends to radial spherolitic structure. The infrared spectrum bands of the turquoises are caused by vibration of constitution water, crystal water and phosphate group.The stretching vibrations of constitution water are recorded at 3 510, 3 464cm^-1 with a sharp band.The stretching vibrations of crystal water are recorded at 3 294, 3 080cm^-1 with a moderate band.In addition, weak bands at 837, 788cm^-1 are caused by the bending vibration of constitution water and crystal water.Three bands assigned to the stretching vibrations of phosphate group are approximately recorded at 1 108, 1 191and 1 056cm^-1.Three bands assigned to the bending vibrations of phosphate group are approximately recorded at 650, 570and 484cm^-1. The UV-Vis spectra of the turquoise samples are different.A strong and narrow band centered at 430nm is caused by Fe³⁺ d-d electronic transition, and a broad absorption band centered at 676nm is caused by Cu²⁺ d-delectronic transition in all samples.However, a band centered at 852nm in the green turquoise sample is caused by Fe²⁺ d-d electronic transition.The band appearing at 776nm in the blue turquoise sample may be caused by the mixture effect of Cu and Fe cation.