龙溪玉的光谱学特征及颜色成因

Spectroscopic Characteristics of Longxi Nephrite From Sichuan and Its Color Genesis

龙溪玉产自四川省汶川县龙溪乡地区,是古蜀玉文化中重要的和田玉料。为研究龙溪玉的基本特征以及颜色成因,初步探索其对古玉料色和沁色的指示意义,结合岩石学薄片分析、激光拉曼光谱、 X射线荧光光谱和XRF微区面扫技术,对不同颜色的龙溪玉样品开展检测和数据对比。根据龙溪玉矿点野外地质勘查和采集的玉石样品,龙溪玉普遍发育有清晰的色带,颜色空间分布规律为灰黑色—青色—青白色—青色—灰黑色。矿物成分分析结果显示,不同颜色的龙溪玉均以透闪石为主,含少量白云母、方解石和磷灰石等副矿物,其中浅色层带多含方解石脉,深色层带副矿物相对较多。不同颜色龙溪玉样品均显示典型的透闪石拉曼光谱,即174、 228、 376、 392、 674、 935、 1 027和1 061 cm^(-1)处的特征峰,在3 600~3 700 cm^(-1)内仅显示3 675 cm^(-1)处1个羟基峰。各色龙溪玉未显示阳起石或石墨的特征峰,指示深色龙溪玉并非矿物致色。XRF测试结果显示,各色龙溪玉样品的SiO_(2)含量在55.20%~57.94%, MgO含量在24.10%~25.00%, CaO含量在12.60%~13.80%, Al2O3含量在0.39%~1.77%,铁氧化物含量在0.25%~0.42%, Fe-VOL05滴定法测定铁氧化物主要为FeO,但Fe2+含量与龙溪玉颜色深浅未见相关性。μ-XRF面扫结果显示,Si、 Mg含量在黑灰色龙溪玉中相对较高,V、 Cr含量在青绿色龙溪玉中相对较高,Fe、 Mn、 Cu等宝玉石中的常见致色元素含量在龙溪玉不同色带中相对均匀。因此,综合各分析结果显示,V和Cr是青绿色龙溪玉的致色元素。基于无损鉴定的谱学特征和致色机制分析能够更好地应用于古玉器玉料的对比研究,为进行古蜀玉文化的玉料溯源和玉文化溯源提供科学依据。

Sichuan Longxi nephrite is important in ancient Shu(ancient Sichuan)jade culture.In order to get the basic mineral,chemical and spectroscopic characteristics of Sichuan Longxi nephrite with different colors,exploring its significant implications for identifying the jade color and alteration color of nephrite unearthed in Sichuan,lithologic thin section analysis,laser Raman spectroscopy,X-ray fluorescence spectroscopy andμ-XRF scanning were conducted on Longxi nephrite samples with different colors.The systematic field geological survey and sample collection shows that the color bands of Longxi nephrite outcrop have clear boundaries of grey black-green-white-green-gray black.The Longxi nephrite mainly comprises tremolite,containing a small amount of muscovite,calcite,apatite and other accessory minerals.The light bands contain calcite veins,and the dark bands contain relatively more accessory minerals.The Longxi nephrite samples in different colors have almost the same Raman spectra of tremolite with Raman peaks at 174,228,376,392,674,935,1027,1061 cm^(-1) and the single peak of hydroxyl at 3675 cm^(-1) in the range of 3600~3700 cm^(-1).None,characteristic Raman peaks of actinolite or graphite,indicate that the dark color of Longxi nephriteis not from dark-color minerals.The XRF analysis reveals that the main chemical compositions of Longxi nephrite samples are SiO_(2)(55.20%~57.94%),MgO(24.10%~25.00%),CaO(12.60%~13.80%),Al 2O 3(0.39%~1.77%),and iron oxide(0.25%~0.42%).FeO is the main iron oxide tested by the Fe-VOL05 titration method,and there is no significant correlation between the Fe 2+content and the colors of Longxi nephrite.Theμ-XRF scanning results indicate that Si and Mg are relatively high in black-deep gray Longxi nephrite,and the contents of V and Cr are relatively high in green-white Longxi nephrite.The contents of common coloring elements in jade,such as Fe,Mn and Cu,do not vary in different color bands of Longxi nephrite.It further proves that V and Cr lead to the green color in Longxi nephrite.The analysis of spectral characteristics and coloring mechanism based on nondestructive identification can be fully applied to the comparative raw material study of ancient jade arte facts,providing scientific data for provenance research on jade material and jade culture of ancient Shu culture.