湖北竹山“油松”的红外光谱特征及其成分研究

doi:10.3964/j.issn.1000-0593(2021)04-1246-07

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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, FeO_T, 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%, FeO_T 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

Received: 2020-03-25 Accepted: 2020-08-05

ZTFLH:

P575

Corresponding Authors: LIU Xian-yu E-mail: liuxianyu@gench.edu.cn

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https://www.gpxygpfx.com/EN/10.3964/j.issn.1000-0593(2021)04-1246-07 OR https://www.gpxygpfx.com/EN/Y2021/V41/I04/1246