热处理棕褐色电气石光谱学特征与颜色成因初探  

Spectroscopic Characteristics and Coloring Mechanism of BrownTourmaline Under Heating Treatment

作　　者：岳素伟 剡晓旭 林佳淇 王沛炼 刘军锋 YUE Su-wei;YAN Xiao-xu;LIN Jia-qi;WANG Pei-lian;LIU Jun-feng(School of Jewelry,Guangzhou City Institute of Technology,Guangzhou 510800,China;Institute of Jewelry,Guangzhou City Institute of Technology,Guangzhou 510800,China;Chow Tai Fook Jewellery&Gold(Shenzhen)Co.,Ltd.,Shenzhen 518081,China)

机构地区：[1]广州城市理工学院珠宝学院,广东广州510800 [2]广州城市理工学院珠宝研究所,广东广州510800 [3]周大福珠宝金行(深圳)有限公司珠宝检验中心,广东深圳518081

出　　处：《光谱学与光谱分析》2021年第8期2524-2529,共6页Spectroscopy and Spectral Analysis

基　　金：国家自然科学基金项目(41403032);广东省教育厅特色科研类项目(CQ180002);华南理工大学广州学院优博项目(YB1700001)资助。

摘　　要：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3800~3400,1350~1250,1200~800与800 cm^(-1),近红外光谱有4720,4597,4537,4441,4343,4203和4170 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3800~3400 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4170和4720 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Fe^(2+)d—d(5 T 2g→5 E g)跃迁、Fe^(2+)→Fe^(3+)(IVCT)、Fe^(2+)→Ti^(4+)(IVCT)所致。样品具有高的Mn含量,417 nm附近的吸收可能存在Mn^(2+)d—d(^(6)A_(1g)→^(4)A_(1g),^(4)T_(Eg))自旋禁阻跃迁产生的413/414 nm叠加。热处理使Mn^(3+)还原成Mn^(2+),Mn^(2+)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Mn^(3+)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Fe^(3+)还原为Fe^(2+),导致Fe^(2+)→Fe^(3+)(IVCT)减少,在∥c轴切面上540 nm吸收显著减弱。540 nm吸收带在�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 3800~3400,1350~1250,1200~800 cm^(-1) and below 800 cm^(-1),while the near-IR located at 4720,4597,4537,4441,4343,4203,and 4170 cm^(-1).The absorption peaks between 3800~3400 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 Fe^(2+)d—d(5 T 2g→5 E g),Fe

关 键 词：棕褐色电气石 热处理 X射线荧光光谱 红外光谱 紫外-可见光谱 

分 类 号：P575.4[天文地球—矿物学]