热处理棕褐色电气石光谱学特征与颜色成因初探

电气石属三方晶系的硼铝硅酸盐,主要有铁电气石、锂电气石、镁电气石、钠-锰电气石等品种,因含不同的过渡元素或色心而呈绿、蓝、黄、红、粉、棕和黑色.选取棕褐色电气石样品在还原和中性气氛加热3 h,结果显示,600℃晶体出现大量裂隙;500和450℃棕褐色调减弱,透明度大大提升,500℃裂隙稍多;350℃加热,样品变绿黄棕色...

Spectroscopic Characteristics and Coloring Mechanism of Brown Tourmaline Under Heating Treatment

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.