热处理棕褐色电气石光谱学特征与颜色成因初探

doi:10.3964/j.issn.1000-0593(2021)08-2524-06

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Abstract Tourmaline group belongs to the trigonal system and contains a series of Boro-Aluminosilicate minerals. It can be subdivided into lithium tourmaline, magnesium tourmaline, and sodium-manganese tourmaline. Gem grade tourmalines show various colors, due to the occurrence of different trace elements and color centers. Brown tourmalines are selected to be modified into attractive colors by 3～4 hours(h) heating treatment under oxidizing or reducing environment. We obtained such results of 250～600 ℃ step heating-treatment experiments in brown tourmalines: (1) the color of samples changed successively from brown, greenish-brown to brownish-green in 250～350 ℃; (2) the brown hue continuous faded as the transparency improved in 450～500 ℃ which indicated the optimum heating temperature; (3) the fracture in all samples enlarged when heated above 600 ℃; (4) after heating treatment, the dichroism of samples showed green and brown on the direction parallel to c-section, while brown perpendicular to c-section. The color modification mechanism of brown tourmalines before and after heating treatment were investigated in this study by mid-near infrared absorption spectroscopy (IR), X-ray fluorescence spectroscopy (XRF), and ultraviolet-visible spectrophotometry (UV-Vis). The result of XRF indicated that all tourmaline samples belonged to the lithium tourmaline group which were rich in Mn and Fe. The mid-IR absorption peaks in natural brown samples were located at 3 800～3 400, 1 350～1 250, 1 200～800 cm^-1 and below 800 cm^-1, while the near-IR located at 4 720, 4 597, 4 537, 4 441, 4 343, 4 203, and 4 170 cm^-1. The absorption peaks between 3 800～3 400 cm^-1 attributed to bending and stretching vibration of M—OH (M can be replaced by Al, Mg, Fe, Mn etc.), which decreased after heating treatment and vanished at 600 ℃. The water loss in heating treatment caused the weakening of bending vibration of structural water. The UV-Vis-spectra in natural brown samples showed 715, 540, and 417 nm absorption bend on the direction parallel to c-section, caused by Fe²⁺ d—d (⁵T_2g→⁵E_g), Fe²⁺→Fe³⁺ inter valence charge transfer (IVCT), and Fe²⁺→Ti⁴⁺ (IVCT) respectively. In this contribution, all samples contain high Mn content. The presence around 417 nm absorption is possibly influenced by the superposition of 413/414 nm absorption, which attributed to spin-allowed transitions of Mn²⁺in d—d orbits (⁶A_1g→⁴A_1g, ⁴T_Eg). After heating treatment, Mn³⁺ was reduced into Mn²⁺, which led to an augment in 414 nm absorption. Simultaneously, the absorption of 520 nm vanished as the content of Mn³⁺ decreased. The presence of 520 nm absorption might be a reason to form asymmetrical absorption in 540 nm band. After heating treatment above 450 ℃, the absorption band of 715 and 417 nm remained unchanged, while 540 nm vanished. The vanishment of 540 nm absorption band could be caused by partial Fe³⁺→Fe²⁺ charge transference in heating treatment, which led to the reduction of Fe²⁺→Fe³⁺ (IVCT) in the direction parallel to the c-section. The vanishment of 540 nm absorption band induced transmittance increase for the green-light region, which could be the reason of green color existence after heating treatment.

Key words：Brown tourmaline; Heating Treatment; X-ray fluorescence spectrum; Infrared spectrum; Ultraviolet-visible spectrum

Received: 2020-07-19 Accepted: 2020-11-20

ZTFLH:

P575.4

Corresponding Authors: YAN Xiao-xu E-mail: yanxiaoxu@gcu.edu.cn

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http://www.gpxygpfx.com/EN/10.3964/j.issn.1000-0593(2021)08-2524-06 OR http://www.gpxygpfx.com/EN/Y2021/V41/I08/2524