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；250 ℃加热样品略微变浅，仍为棕褐色调；加热后∥c轴切面见明显绿色与棕色二色性，垂直c轴切面，即{0001}面，为棕色；综合显示，最佳变色温度在450~500 ℃。利用X射线荧光光谱(XRF)、红外吸收光谱(IR)和紫外-可见光吸收光谱(UV-Vis)对热处理前后样品进行分析，样品属于富Mn和Fe的锂电气石。样品中红外特征吸收峰在3 800~3 400，1 350~1 250，1 200~800与800 cm-1，近红外光谱有4 720，4 597，4 537，4 441，4 343，4 203和4 170 cm-1特征峰。热处理后，由M-OH(M为Al，Mg，Fe和Mn等)伸缩和弯曲振动所致的3 800~3 400 cm-1吸收峰减弱，600 ℃消失，与加热失水行为导致的结构水弯曲/伸缩振动减弱有关；近红外光谱4 170和4 720 cm-1吸收消失。棕褐色电气石在∥c轴切面的可见光范围内具有715，540和417 nm吸收带，依次为Fe2+ d-d(5T_2g→5E_g)跃迁、Fe2+→Fe3+(IVCT)、Fe2+→Ti4+(IVCT)所致。样品具有高的Mn含量，417 nm附近的吸收可能存在Mn2+ d-d (6A_1g→4A_1g, 4T_Eg)自旋禁阻跃迁产生的413/414 nm叠加。热处理使Mn3+还原成Mn2+，Mn2+增加导致414 nm吸收峰增强，因此417 nm附近吸收带变化不大。同时，热处理后与Mn3+有关的520 nm吸收也同时消失，520 nm吸收带的存在也可能是540 nm吸收带呈非对称吸收峰的原因。450 ℃以上热处理后，715和417 nm吸收带变化不大，位于绿光区的540 nm吸收带消失，分析认为加热使得部分Fe3+还原为Fe2+，导致Fe2+→Fe3+(IVCT)减少，在∥c轴切面上540 nm吸收显著减弱。540 nm吸收带在绿色光区域，其消失导致绿色光透过，样品呈绿色。

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.