苏门答腊“根珀”的宝石学和谱学特征

Gemmological and Spectral Characteristics of "Root Amber" from Sumatra

  • 摘要: 根珀因其独特的根状外观而得名,在中国琥珀市场中备受青睐,但天然根珀主要产于缅甸,产量有限。近期市场上出现了一种外观与缅甸根珀相似的苏门答腊“根珀”。本研究通过显微观察、傅里叶变换红外光谱、拉曼光谱、三维荧光光谱及扫描电子显微镜等多种分析手段对苏门答腊“根珀”进行了系统研究。结果表明,苏门答腊“根珀”样品在颜色、微观结构、谱学特征与荧光特征等方面均与缅甸根珀存在显著差异。苏门答腊“根珀”颜色呈乳白色至棕褐色,主要分为乳白色透明与褐色不透明部分,长波紫外光下呈蓝白色至蓝紫色荧光,不透明区域荧光强于透明区域,并具有明显的黏手触感和树脂气味,显微镜观察显示透明部分有大小不一的红褐色斑点,在不透明区域可见海绵状结构,疏松多孔,分布较多气泡以及受应力挤压形成的流淌纹;扫描电子显微镜观察揭示其透明部分和不透明部分的结构差异,透明部分光滑平整,而不透明部分疏松多孔,推测为气泡和空管所致;红外光谱分析显示,苏门答腊“根珀”在中红外范围内的特征吸收峰与缅甸根珀有显著差异,具体表现为2 957 cm-1、2 932 cm-1处独特的烷烃CH2反对称伸缩振动吸收峰,且在888 cm-1处有C=C双键上C-H面外弯曲振动特征峰,与婆罗洲柯巴树脂高度相似,从而确定为Ⅱ型柯巴树脂,柯巴树脂较低的成熟度是其黏手触感的原因;Ⅱ型树脂的身份容易导致当基于拉曼光谱吸收峰强度比值(I1 645 cm-1/I1 450 cm-1)分析成熟度时,将苏门答腊“根珀”错判为琥珀,需要警惕;三维荧光光谱分析表明,其特征荧光峰集中在445、478 nm和516 nm附近,发光范围局限于蓝紫区,最佳激发波长(385~418 nm)因样品氧化程度存在细微差异,与缅甸琥珀相比,其荧光发光中心更少、发光范围更窄,且无紫外区发射峰。本研究不仅揭示了苏门答腊“根珀”的谱学和微观特征,还为区分其与缅甸根珀提供了科学依据。

     

    Abstract: Root amber gets its name from the unique root-like appearance and is highly favored in the Chinese amber markets. However, natural root amber is mainly produced in Myanmar with limited output. Recently, a type of "root amber" with an appearance similar to root amber from Myanmar has emerged in the markets. This study systematically investigated "root amber" samples from Sumatra using multiple analytical methods, including microscopic observation, Fourier transform infrared spectroscopy, Raman spectroscopy, three-dimensional fluorescence spectroscopy, and scanning electron microscopy. The results showed that "root amber" samples from Sumatra exhibit significant differences from root amber from Myanmar in terms of colour, microstructure, spectral characteristics, and fluorescence properties. "Root amber" from Sumatra ranges in colour from milky white to brown, mainly divided into milky white transparent parts and brown opaque parts. Under long-wave ultraviolet light, it emits blue-white to blue-purple fluorescence, with the opaque areas showing stronger fluorescence than the transparent areas. It also has a distinct sticky texture and a resinous odor. Microscopic observation revealed reddish-brown spots of varying sizes in the transparent parts, while the opaque areas displayed a spongy, porous structure with numerous bubbles and flow lines formed by stress extrusion. SEM observation revealed structural differences between the transparent and opaque parts: The transparent parts are smooth and flat, while the opaque parts are porous, presumably due to bubbles and hollow tubes. FTIR analysis showed significant differences in mid-infrared characteristic absorption peaks between "root amber" from Sumatra and root amber from Myanmar. Specifically, "root amber" from Sumatra has unique alkane CH2 antisymmetric stretching vibration peaks at 2 957 cm-1 and 2 932 cm-1, as well as a characteristic C-H out-of-plane bending vibration peak of C=C double bonds at 888 cm-1, which is highly similar to copal from Borneo. According to experimental data and relevant literatures, it was concluded that the purported "root amber" from Sumatra is actually copal resin, exhibiting gemmological characteristics of typical type Ⅱ resin fossils. The low maturity of copal explains its sticky texture. The type Ⅱ resin fossil can easily lead to the misidentification of it as amber in maturity analysis based on the ratio of Raman spectral absorption peak intensities (I1 645 cm-1/I1 450 cm-1), which requires vigilance. Three-dimensional fluorescence spectroscopy analysis indicated that "root amber" from Sumatra emits light in the blue-purple region, differing from the fluorescence characteristics of amber from Myanmar. Its characteristic fluorescence peaks are concentrated around 445, 478 nm and 516 nm, with the emission range limited to the blue-purple region. The optimal excitation wavelength varies slightly (385-418 nm) depending on the oxidation degree of the samples. Compared with amber from Myanmar, it has fewer fluorescence emission centers, a narrower emission range, and no ultraviolet emission peaks. This study not only revealed the spectral and microscopic characteristics of "root amber" from Sumatra, but also provided a scientific basis for distinguishing with root amber from Myanmar.

     

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