基于颜色计算和原位成分分析对紫色玉髓颜色成因的探究

产于印度尼西亚的紫色葡萄状玉髓，具有特殊的球粒状外观和浓郁的紫罗兰体色，其双面抛光片在反射光照射下为紫色，透射光下则为棕黄色，且颜色浓集于球粒中心。为探究其颜色成因，进行了偏光显微镜和扫描电子显微镜结构观察，显微紫外-可见光谱，热处理以及LA-ICP-MS原位成分分析。玉髓具有纤维状核心--粗粒石英外壳的特殊结构，粗粒石英外壳粒度500 μm左右，隐晶质部分则主要由粒度小于1 μm的形状不规则的SiO₂颗粒组成。紫外-可见光谱显示紫色主要来源于540 nm左右的吸收峰，而黄色则由于谱线“左倾”产生的近紫外区及蓝光区强烈吸收所致。紫外-可见光谱使用塞尔迈耶尔方程修正表面反射误差、减去无吸收波段强度矫正仪器误差，并用最小二乘平滑扣除基线得到540 nm吸收峰的强度信息。计算玉髓在反射光下的紫色调及透射光下的黄色调的L*a*b*值和E*值定量表征颜色。热处理实验中，玉髓的紫色调在350 ℃左右开始褪去，紫外-可见光谱390和540 nm吸收峰消失，反射光和透射光下颜色差异减小，都呈现黄色调。随温度升高至400 ℃后，棕色调加深，出现478 nm左右的吸收峰。热处理过程中谱线吸收强度升高，“左倾”加剧，峰位“红移”。该现象与铁/二氧化硅纳米粒子（Fe/SiO₂ NPs）生长过程中的谱形变化相似，有可能与玉髓内部与Fe有关的微细结构或包裹体在热处理过程中的变化有关。颜色参数结合原位成分分析，将数据采用标准分数（Z-score）归一化处理，比对紫色调的E*值与540 nm吸收峰强度及元素含量之间的关系，发现540 nm吸收峰强度可很好的反映紫色的浓集程度，但紫色调与过渡金属元素含量的线性相关性却并不显著，黄色调的E*值则与Fe元素含量具有近似的负相关性。Fe并不以杂质矿物的形式存在，元素含量这一因素也并不能完全决定玉髓的颜色，可能还受到Fe在玉髓中的存在形式，内部微细结构或包裹体等因素的影响。

Based on Color Calculation and In-Situ Element Analyze to Study the Color Origin of Purple Chalcedony

Purple Grape Chalcedony from Indonesia has a special spherical appearance and violet color. Its double sides polishing waferis purple under the reflected light and brownish-yellow under the transmission light, while the color is concentrated in the center of the spherules. To investigate the origin of its color, a polarizing microscope, Scanning Electron Microscope, Microscopic UV-Vis Spectroscopy, heat treatment and LA-ICP-MS in situ composition analysis were performed. Chalcedony has a structure of fibrous core and micro quartz shell. The micro quartz shell has a particle size of 500 m, while the cryptocrystalline part is mainly composed of irregular SiO₂ particles with particle sizes less than 1 m. UV-Vis spectra show that the purple is mainly from the absorption of about 540 nm, while the yellow is due to the strong absorption of near-ultraviolet area and blue light generated by the “left-leaning” the spectrum. In the UV-Vis spectrum, the surface reflection error was corrected by the Selmeier equation, and the instrument error was subtracted from the intensity of the unabsorbed band. The intensity information of the 540nm absorption peak was obtained by deducting the baseline with the least square smoothing. Calculate the L*a*b* and E* values of chalcedony in purple tone under reflected light and yellow tone under transmitted light. Thus the color can be quantified. In the heat treatment experiment, the purple hue of chalcedony began to fade at about 350 ℃, and the absorption peaks of UV-Vis spectra at 390 and 540 nm disappeared, while the color difference between reflected and transmitted light decreased; both of them are yellow. As the temperature rises to 400 ℃, the brown tone deepens, and the peak of about 478 nm appears. The intensity of spectral increases during the heat treatment, the “left-leaning” intensification, and the peak “redshift”. This phenomenon is like the change of spectral of Fe /SiO₂ nanoparticles (Fe /SiO₂ NPs) during its growth.It may be related to the change of Fe-related fine structure or inclusions in chalcedony during heat treatment. Color parameter was combinate with in-situ composition analysis, the data were normalized by standard score (Z - score), compared the relationship of the value of E* of purplecolor, the intensity of 540 nm peak and element content, found that 540 nm peak intensity can well reflect the concentration of purple. However, the linear correlation between color and the transition metal elements content is not significant. The E* value of yellow tone has an approximately negative correlation with Fe content. Fe does not exist in the form of impurity minerals, and the color of chalcedony is not determined by the element content independently but is affected by Fe’s existence in chalcedony, fine internal structure or inclusion.