青海三岔河灰紫色软玉颜色定量表达与紫色成因研究

摘要

颜色是衡量宝玉石品质、价格的重要因素,对宝玉石颜色的量化研究一直是宝石科学关注的热点.在透射光条件下观察宝玉石颜色特征是珠宝行业中十分重要的手段.本研究致力于依赖光谱和光谱分析技术,建立一套依照透射法实现软玉颜色定量表达、颜色复原和确定致色机制的方法.以来自中国青海格尔木地区、具有渐变颜色特点的灰紫色软玉为例,采用紫外-可见吸收光谱(UV-Vis)定量测量、结合国际照明委员会1976 CI E L*a*b*色度学模型转化及计算机软件处理(Adobe Photoshop),实现对特定厚度(10 mm)灰紫色软玉样品颜色的定量表达和颜色复原.样品浅色和深色区域复原的颜色与样品透射光下肉眼观察颜色较为接近,存在的色差可能与玉石样品的半透明多晶质结构有关.而对于紫色的致色机理,通过激光剥蚀电感耦合等离子质谱仪(LA-ICP-MS),我们发现微量元素Mn含量随颜色加深呈现递增规律.在较深颜色区域,光致发光光谱(PL)中检测到的以585 nm为中心的发射峰、UV-Vis光谱中以530 nm为中心的吸收峰,以及电子顺磁共振光谱(EPR)的六重超精细分裂峰都在指示Mn2+是导致灰紫色软玉中紫色调的主要原因.本工作对于研究透射光条件下宝玉石样品颜色定量观测和表达提供了确切的实验方法依据,并为采用光谱学确定致色元素以及致色机理提供了有价值的实验信息.%Color is one of the most important factors in evaluating the quality and price of jewelry .Quantitative research on color of jewels has been a hotspot in gemological science .Whether for jewelry industry or gems research,observing and describing the gems' color characteristics under transmission light is an essential method .This study focuses on building a research method to quantitatively characterize nephrite color,and to determine their color origin based on transmitted spectroscopy techniques .Natural gray-purple nephrite of Sanchahe mining,Qinghai,China was chosen as a typical subject due to its gradual-change color characteristic .We first quantitatively expressed and replicated the different color region on a gray-purple nephrite sample with given thickness (10 mm) with UV-Visible absorption spectra and 1976 CIE L*a*b* colorimetric parameters,as well as Adobe Photoshop software .The replicated color of light and dark color regions were both close to the transmitted color observed by naked eyes .It is inferred that the subtle color differences between naked eyes observation and transmitted spectroscopy replication may from the multiple effects of incident light in translucent polycrystalline structure,such as absorption,refraction,diffraction,scattering,and so on .As for the purple color origin,Laser Ablation Inductively Coupled Plasma Mass Spectroscopy (LA-ICP-MS) showed an increase of the concentrations of manganese (Mn) as the nephrite color becomes darker .Moreover,the emission peak at 585 nm on Photoluminescence (PL) and absorption peak at 530 nm on Ultraviolet Visible (UV-Vis),and the sextet Mn2+ resonance peaks on Electronic Paramagnetic Resonance (EPR) provide solid support to prove that Mn2+ should be the main factor contributing to the purple color .This work provides a specific experimental method on quantitative observing and describing the color of gems under transmitted light,and it also offers valuable information on determining the chromophores and color origin .