天然与合成紫晶的红外和偏振拉曼光谱鉴定特征

天然紫晶与合成紫晶的鉴别是国内外珠宝鉴定实验室的一个难题,前人主要从双晶、色带、包裹体、红外吸收光谱特征等方面开展了研究。在利用红外光谱鉴别天然紫晶与合成紫晶时,不同的学者尚对3595cm-1或3543cm-1吸收峰作为诊断性还是指示性的判据存在不同认识。本文系统采集了典型的天然紫晶与合成紫晶样品,研究了利用红外光谱测试技术鉴别天然紫晶与合成紫晶的局限性,并尝试将偏振拉曼光谱应用于紫晶成因鉴别。结果表明:利用3595cm-1、3543cm-1红外吸收峰进行紫晶鉴别仅具有指示性意义,不能作为决定性的判定依据,偏振拉曼光谱可作为重要的补充。天然紫晶的偏振拉曼光谱(偏振方向:HH)均出现400cm-1的拉曼峰,而该峰在合成紫晶偏振拉曼光谱中缺失;合成紫晶的偏振拉曼光谱(偏振方向:HH)均具有795cm-1、448cm-1的拉曼峰,而这两个峰在天然紫晶偏振拉曼光谱中缺失。偏振拉曼光谱产生差异的原因可能与天然紫晶和合成紫晶内部晶格变形程度的不同有关。本文揭示的400cm-1、448cm-1和795cm-1偏振拉曼峰可作为鉴别紫晶成因的新依据。

Identification Characteristics of Natural and Synthetic Amethyst by Infrared and Polarized Raman Spectroscopy

BACKGROUND:The identification of natural and synthetic amethyst is still a challenge for gemological laboratories either at home or abroad. Previous studies focus on twinning, color band, inclusions and the infrared spectrum. Controversy still exists on whether the 3595cm^-1 and 3543cm^-1 absorptions are indicative or conclusive clues when employing infrared spectroscopy to separate natural amethyst from synthetics.
OBJECTIVES:To characterize the features of natural and synthetic amethyst.
METHODS:Typical natural and synthetic amethyst samples were collected. The limitations of infrared spectroscopy to identify natural and synthetic amethyst were studied. The polarized Raman spectroscopy was applied to the identification of amethyst genesis.
RESULTS:The results showed that there were some limitations in the origin determination of amethyst by using the infrared absorption peaks of 3595cm^-1 and 3543cm^-1 as criteria. These peaks were indicative, but cannot be used as a decisive basis. The polarized Raman spectrum was complementary to infrared spectroscopy. The polarized Raman spectrum (the direction of both incident laser and Raman signal set as horizon-HH) peak of 400cm^-1 were observed in all the natural amethyst, but this peak was absent in the synthetics. The polarized Raman spectrum (direction:HH) peaks of 795cm^-1 and 448cm^-1 appeared in all the synthetic amethyst, but were not detected in natural amethyst.
CONCLUSIONS:The difference of polarized Raman spectra may be related to different degree of crystal lattice deformation in natural and synthetic amethyst. The polarized Raman peaks of 400cm^-1, 448cm^-1 and 795cm^-1 can be used as a new criterion for source determination of amethyst.