Spectroscopic Characteristics and Coloring Mechanism of Brown Tourmaline Under Heating Treatment
YUE Su-wei1, 2, YAN Xiao-xu1, 2*, LIN Jia-qi1, WANG Pei-lian1, 2, LIU Jun-feng3
1. School of Jewelry, Guangzhou City Institute of Technology, Guangzhou 510800, China
2. Institute of Jewelry, Guangzhou City Institute of Technology, Guangzhou 510800, China
3. Chow Tai Fook Jewellery & Gold (Shenzhen) Co., Ltd., Shenzhen 518081, China
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 (5T2g→5Eg), 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 (6A1g→4A1g, 4TEg). 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.
岳素伟, 剡晓旭, 林佳淇, 王沛炼, 刘军锋. 热处理棕褐色电气石光谱学特征与颜色成因初探[J]. 光谱学与光谱分析, 2021, 41(08): 2524-2529.
YUE Su-wei, YAN Xiao-xu, LIN Jia-qi, WANG Pei-lian, LIU Jun-feng. Spectroscopic Characteristics and Coloring Mechanism of Brown Tourmaline Under Heating Treatment. SPECTROSCOPY AND SPECTRAL ANALYSIS, 2021, 41(08): 2524-2529.
[1] Castaneda C, Eeckhout S G, Da Costa G M, et al. Physics and Chemistry of Minerals, 2006, 33(3): 207.
[2] Cano N F, Gundu Rao T K, Ayala-Arenas J S, et al. Journal of Luminescence, 2018, 205: 324.
[3] Pezzotta F, Laurs B M. Elements, 2011, 7(5): 333.
[4] Shigley J E, Cook B C, Laurs B M, et al. Gems and Gemology, 2001, 37(4): 260.
[5] Ayuso R A, Brown C E. Canadian Mineralogist, 1984, 22(5): 327.
[6] WANG Jin-jun, TAO Xiao-feng, WANG Wu-jun(王进军, 陶晓风, 王武军). Acta Petrologica et Mineralogica (岩石矿物学杂志), 2005, 24(4): 319.
[7] Maneewong A, Seong B S, Shin E J, et al. Journal of the Korean Physical Society, 2016, 68(1): 83.
[8] Liu X, Feng X, Fan J, et al. Chinese Optics Letters, 2011, 9(8): 90.
[9] Thongnopkun P, Naowabut P. Journal of Applied Spectroscopy, 2018, 85(4): 616.
[10] Phichaikamjornwut B, Pongkrapan S, Intarasiri S, et al. Vibrational Spectroscopy, 2019, 103: 1.
[11] Laurs B M, Simmons W B S, Rossman G R, et al. Gems & Gemology, 2007, 43(4): 314.
[12] Laurs B M, Zwaan J C, William C B, et al. Gems & Gemology, 2008, 44(1): 4.
[13] Ahn Y, Seo J, Park J. Vibrational Spectroscopy, 2013, 65(2013): 165.
[14] da Silva S F, Moura M A, Queiroz H de A, et al. Journal of Geosciences (Czech Republic), 2018, 63(2018): 155.
[15] Faye G H, Manning P G, Gosselin J R, et al. Canadian Mineralogist, 1974, 12(6)：370.
[16] Da Fonseca-Zang W A, Zang J W, Hofmeister W. Journal of the Brazilian Chemical Society, 2008, 19(6): 1186.
[17] Rossman G R, Mattson S M. American Mineralogist, 1986, 71(3): 599.
