两类南红玛瑙仿制品的光谱学特征

Spectral Characteristics of Two Kinds of Nanhong Agate Imitations

南红玛瑙是我国珠宝市场上常见的红色玛瑙品种之一。南红玛瑙市场火热且仿制品层出不穷,但对于南红玛瑙仿制品在材质和谱学特征上与天然南红玛瑙差异的研究较少。选取市场中出现的两类南红玛瑙仿制品和天然南红玛瑙作为研究对象,对常规宝石学特征、显微特征、紫外-可见光谱、拉曼光谱以及红外吸收光谱进行了测试。研究结果表明:两类南红玛瑙仿制品主要成分与天然南红玛瑙相同,均为石英,其折射率、密度、硬度、颜色、光泽等物理性质与南红玛瑙相近,短波和长波紫外光下均呈惰性。第一类仿制品(FZP-1)仿制对象为樱桃红手链,紫外可见光光谱240~570 nm处存在一宽吸收带,呈浅橙红色。显微镜下观察到FZP-1呈粒状结构,见红色染料沿裂隙分布,呈“丝瓜瓤”状,是典型染色石英岩的结构特征;第二类仿制品(FZP-2)仿制对象为柿子红手链,紫外可见光光谱从240~540 nm处存在一宽吸收带,呈橙黄色。FZP-2呈隐晶质结构,表面可观察到条带状和“指甲纹”状构造,切面可见三层不同的颜色分带且明显沿打孔处分布,指示该仿制品经过染色处理与热处理。天然南红玛瑙(TR)为隐晶质结构,内部含有红色点状赤铁矿,与两类仿制品的内部结构具有明显的差别。两类仿制品与天然南红玛瑙红外光谱均显示仿制品具有石英的典型光谱特征,在1100~1250,600~800和300~600 cm^(-1)范围内均存在吸收峰,分别归属于Si—O—Si的非对称伸缩振动、Si—O—Si的对称伸缩振动和Si—O—Si的弯曲振动。两类样品800 cm^(-1)峰均存在分裂,指示结晶程度良好。FZP-1的显微红外光谱测试发现颗粒间隙在2800~3200 cm^(-1)范围内存在有机物相关峰位,以2916和2848 cm^(-1)强峰为特征,由C—H的伸缩振动引起。拉曼光谱测试结果显示,两种样品中除石英的有关峰位外,FZP-1在915和1337 cm^(-1)等处存在与其有机染料相关的峰,由饱和C—H的弯曲振动引起,与显微红外光谱结果一致。FZP-2的拉曼光谱显示在502 cm^(-1)存在与斜硅石相关的峰。根据斜硅石与石英特征峰强度比值计算两者的相对含量,结果显示斜硅石与石英的相对含量比约为0.15~0.16,远高于天然南红玛瑙。根据国家标准第一类仿制品应定名为染色石英岩玉,第二类样品应定名为玛瑙。

The Nanhong agate is one of the most common red agates in the Chinese jewelry market.The market of the Nanhong agate is thriving,and Nanhong agate imitations are flourishing,while few studies have been conducted on the differences of materials and spectral characteristics between the natural Nanhong agate and its imitations.In this paper,the conventional gemological instruments,Microscope,UV-Vis spectrometer,Raman spectrometer and Fourier infrared absorption spectroscopy(FTIR)were employed to study two kinds of the Nanhong agate imitations and the natural Nanhong agate.The results show that the two kinds of the Nanhong agate imitations are both composed of quartz,similar to the natural Nanhong agate,and have a similar refractive index,density,hardness,color,luster,and other physical properties relative to the natural Nanhong agate,with inertia under both short-wave and long-wave ultraviolet light.The first imitation(FZP-1)imitates a cherry red bracelet,exhibiting a light orangey-red as indicated from the spectrum of the UV-Vis showing a broadened absorption band between 240 and 570 nm.The FZP-1 displays granular texture and a pulp luff-like shape,with red dyestuff filling along the edge of the quartz granular,a typical structural characteristic of a dyed quartzite.The second imitation(FZP-2)imitates a persimmon red bracelet with a yellow orange red color as demonstrated by an absorption band from 300,240 nm to 550,540 nm under UV-Vis.The natural Nanhong agate(TR)showed an absorption band from 440 to 560 nm under UV-Vis.The FZP-2 shows a cryptocrystalline structure,whose banded and nail-like structures can be observed on the surface,and whose different colors are shown in different layers,indicative of dyeing and heating treatments of the FZP-2.The nature Nanhong agate exhibits cryptocrystalline texture and contains spot-like hematite,which is remarkably different from the internal structures of two imitates.The FTIR spectrum reveals quartz’s typical spectrum characteristics for both of the two kinds of Nanhong agate imitations and the natural Nanhong agate.The absorption peaks existed in the range of 1100~1250 and 600~800 cm^(-1) are attributed to the So—O—Si’s asymmetric and symmetric stretching vibration,respectively.The peaks at 300~600 cm^(-1) are assigned to the bending vibration of Si—O—Si.The peaks around 800 cm^(-1) in FTIR patterns are splitting in both samples,indicating a good crystalline degree of quartz.Peaks between 2800~3200 cm^(-1) are detected at the particle clearance in the FZP-1 under micro-infrared spectroscopy,which is related to organic dyeing agent,especially at 2916 and 2848 cm^(-1) to the stretching vibration of C—H.In addition to the peak position of quartz,the Raman spectrum of the FZP-1 show peaks at 915 and 1337 cm^(-1),due to the bending vibration of the saturated C—H,which are related to organic dyeing agent,in good agreement with the result of micro-infrared spectroscopy.The peak at 502 cm^(-1) in the Raman spectrum indicates the existence of moganite in the ZFP-2.The ratios of relative content of moganite and quartz are calculated spanning 0.15~0.16,based on the ratios of characteristic peaks of moganite and quartz in the Raman spectrum,which is higher than the natural Nanhong agate.Rather than,the peaks relate to hematite is inexistent in both of the two kinds of imitations.The first and second imitations should be named as dyed quartzite and agate,respectively,according to the national standard of Gems-Nomenclature.