基于色度学-拉曼光谱特征鉴定北红和南红玛瑙

Identification of Beihong Agate and Nanhong Agate from China Based on Chromaticity and Raman Spectra

产于黑龙江的“北红玛瑙”与四川凉山、云南保山的“南红玛瑙”是我国珠宝市场上常见的红色玛瑙,然而相应的产地特征研究较少,结合色度学、拉曼光谱、X射线粉晶衍射分析对三个产地的73件红色玛瑙的色度学、矿物学、光谱学特征进行了对比分析。结果表明,“北红玛瑙”的主要物相组成为α-石英和斜硅石,次要矿物组成为针铁矿、赤铁矿;四川凉山与云南保山“南红玛瑙”的主要物相组成为α-石英,次要矿物为赤铁矿、针铁矿、方解石等,少量样品含有斜硅石。黑龙江“北红玛瑙”的颜色主波长范围为574~605nm,集中于[580,590]区间,对应黄色-橙黄色-橙色色调,CIE1976Lab色空间中a≤6.2,b≤6.3;四川凉山“南红玛瑙”的主波长范围为589~624nm,云南保山“南红玛瑙”主波长范围为589~599nm,两个产地的“南红玛瑙”主波长均集中于[590,600]区间,对应橙色-橙红色色调,大部分样品a>6.2或b>6.3,整体而言相比“北红玛瑙”颜色色调偏红,其彩度和亮度总体上也高于“北红玛瑙”。拉曼光谱中,斜硅石Si—O—Si对称伸缩-弯曲振动引起的501cm-1峰在“北红玛瑙”中的强度高,在两个产地的“南红玛瑙”中不存在或者强度弱。斜硅石与α-石英特征峰强度比(I501/I463)和面积比(A501/A463)结果基本一致,在研究及鉴定过程中可以根据实际情况灵活选择。拉曼光谱粉末法测得的斜硅石与α-石英特征峰强度比(I501/I463)和面积比(A501/A463)结果位于大量随机点测的范围内,在日常鉴定中可以用多次无损点测的方法来获得接近粉末法的结果。黑龙江“北红玛瑙”的特征峰面积比(A501/A463)稳定在0.15~0.36,而四川凉山与云南保山“南红玛瑙”稳定在0.00~0.08,指示了两个产地“南红玛瑙”的斜硅石相对含量比“北红玛瑙”少,推测是两地的“南红玛瑙”在初期形成后都经历了较强的脱水和重结晶作用过程,使斜硅石转化成低温的α-石英所致。可以综合利用色度学特征及拉曼光谱,结合斜硅石与α-石英谱特征峰强度比(I501/I463)或面积比(A501/A463),对产于黑龙江的“北红玛瑙”以及四川凉山、云南保山的“南红玛瑙”进行区分,这也对玛瑙的产地鉴定、出土文物溯源等具有重要意义。

The "Beihong agate" from Heilongjiang province and the "Nanhong agate" from Liangshan(Sichuan province) and Baoshan(Yunnan province) are the most common gem quality red agate in the jewelry market of China. However, few researches have been performed to differentiate theirprovenance. In this paper, 73 red agate samples from three different areas were investigated by chromaticity, Raman spectrum and X-ray diffraction(XRD) to obtain their chromaticity, mineralogy and spectra characteristics. The results show that the "Beihong agate" is mainly composed of α-quartz and moganite, with minor amount of goethite and hematite responsible for the agate’s red color. The mineral phase composition of the "Nanhong agate" is α-quartz with hematite, goethite, calcite and moganite as auxiliary minerals. The domain wavelength of "Beihong agate" ranges from 574 to 605 nm and is concentrated at [580, 590] interval, which accounts for yellow to orange tune. In the CIE1976 Lab color space,"Beihong agate" shows a≤6.2 and b≤6.3 characteristics. The domain wavelength of "Nanhong agate" from Sichuan province ranges from 589 to 624 nm, while "Nanhong agate" from Yunnan provinceranges from 589 to 599 nm, both are concentrated at [590, 600] interval with orange to orange-red tune, and the majority of samples are a>6.2 or b>6.3 in the CIE1976 Lab color space. Generally,"Nanhong agate" exhibits deeper red tone, higher chromaand brightness appearance than "Beihong Agate". In Raman spectra, the peak near 501 cm-1 of the Si-O-Si symmetry stretching and bending vibration in "Beihong agate" is stronger than those in "Nanhong agate". The results of peak intensity ratio(I501/I463) consistent with the results of area ratio(A501/A463). Thepeak intensity ratio(I501/I463) and area ratio(A501/A463) measured by powder method are located in the results measured by random points method. Compared with powder method, random points method does not require destruction of samples. Therefore, random points method is suitable for agate’s qualityanalysis. The peak area ratio(A501/A463) of "Beihongagate" is stable from 0.15 to 0.36, while "Nanghong agate" is stable from 0.00 to 0.08. We hypothesize that "Nanhong agate" have experienced a strong dehydration and recrystallization progress after the formation of primary agate, which caused the transformation of moganite intoα-quartz. The comprehensive utilization of chromaticity characteristics, Raman spectra and the peak intensity ratio(I501/I463) or area ratio(A501/A463) can be used to distinguish "Beihong Agate" from "Nanhong Agate", which is also important to the identification ofagate’s origin and the tracing of unearthed relics.