Infrared Spectral Characteristics and Composition of the “Oily Turquoise” in Zhushan, Hubei
CHEN Quan-li1, DING Wei1, XU Feng-shun1, LIU Xian-yu2*, WANG Hai-tao3
1. Gemology Institute, China University of Geosciences, Wuhan 430074, China
2. School of Jewelry, Shanghai Jian Qiao University, Shanghai 201306, China
3. College of Creative Design, Shenzhen Technology University, Shenzhen 518118, China
Abstract：In recent years, a relatively unique turquoise variety has been found in the Xiaolinpa ore district in Qingu Town, Zhushan County, Shiyan City of Hubei Province. The color of this turquoise variety is mostly light green, light yellow green, or light apple green. The rough material is slippery feel and brittle. As a result, it is called “oily turquoise” by a local citizen. Compared to the turquoise with similar structure fineness, the density of turquoise is generally lower, and the hardness is smaller, with the Mohs hardness ranges from 3 to 4. This type of turquoise has not been significantly improved in hardness and structural density after being filled with a traditional organic binder, and cannot be used as a gem-quality turquoise material for jewelry. The conventional gemological instruments, infrared absorption spectrometers, X-ray powder crystal diffractometers, electron probes microscopic analysis, and environmental scanning electron microscopes were used to study the chemical composition and microstructure characteristics of the “oily turquoise”. The results show that the specific gravity of the “oily turquoise” range 2.04 from 2.22, which was lower than that of typical turquoise. It shows inert fluorescence under long-wave and short-wave UV light. The infrared absorption spectrum of the “oily turquoise” is mainly distributed in the range of 3 700 to 3 090 and 1 638 to 466 cm-1, of which the peaks at 3 509 cm-1± and 3 462 cm-1± have sharp OH-induced absorption spectra, and the broader crystallization water-induced absorption spectra at 3 277 cm-1± and 3 090 cm-1 ± are consistent with the absorption characteristics of the turquoise functional group region. It has weak OH-induced infrared absorption peaks in kaolinite or montmorillonite at high frequencies of 3 700 and 3 622 cm-1. Relatively broad absorption band around 1 638 cm-1, with moderate intensity agree with the absorption peak of the bending vibration induced by H2O in turquoise. The shape and position of the absorption peaks in the fingerprint region of the “oily turquoise” are quite different from those of typical turquoise; they are mixed absorption spectrum peaks of Si—O and P—O. The main chemical composition of the “oily turquoise” is SiO2, Al2O3, FeOT, P2O5 and CuO, with small amounts of MgO, CaO and Cr2O3. The content of SiO2 is 25.60% to 30.90%, and the content of Al2O3 is 26.55% to 28.29%, FeOT content is 5.35% to 5.90%, P2O5 content is 22.00% to 23.52%, and CuO content is 5.10% to 5.87%. Compared with another typical natural turquoise, the “oily turquoise” contains a higher content of SiO2, which is higher than 25%. The main constituent mineral of the “oily turquoise” is turquoise, and it contains a certain amount of montmorillonite and montmorillonite-kaolinite. The low hardness and slippery characteristics of the “oily turquoise” are relevant to montmorillonite that results in the insignificant optimization effect.
Key words：Turquoise；Oily；Infrared absorption spectrum；X Ray diffraction；Montmorillonite
