俄罗斯翠榴石的化学成分及光谱学表征

翠榴石为石榴石家族中最为贵重的亚种，以其漂亮的外观和稀有性深受欢迎。尤其是俄罗斯所产翠榴石更是国内外收藏家最为追捧的对象。前人从不同角度对石榴石族矿物研究较多，但关于翠榴石的研究较少。为了探究俄罗斯翠榴石的化学成分及光谱学特征，运用LA-ICP-MS，IR，Raman和UV-Vis，对俄罗斯翠榴石进行系统研究，旨在获得其化学成分尤其是稀土元素特征、光谱学特征，分析致色原因，为其品种鉴定及产地溯源提供重要数据。化学成分研究表明，俄罗斯翠榴石几乎为纯的钙铁榴石(Andradite>96.39 Mol.%)。次要成分中，Cr₂O₃含量较高，平均0.502 Wt%，除此以外还含少量Al，Mn，Ti和V。其中Cr和V均为石榴石中致绿色的元素。稀土元素含量总体不高，∑REE平均4.85 μg·g-1；且轻稀土元素明显富集，∑LREE平均4.56 μg·g-1；重稀土元素相对亏损，∑HREE平均0.29 μg·g-1, ∑LREE/∑HREE=5.35~100.48。多数样品显示Eu正异常。主要拉曼位移为994.5，873.5，841.5，815，576，552，515，492，451，369，351，323，310.5，295，263，234.5和172 cm-1。拉曼光谱仅可作为翠榴石品种鉴定的手段之一，对其产地溯源作用不大。红外光谱研究表明，指纹区红外反射光谱可以有效鉴别翠榴石，红外光谱官能团区显示结构水的吸收峰，表明俄罗斯翠榴石含有少量结构水，这与其形成过程与热液交代作用有关。紫外-可见吸收光谱研究显示，俄罗斯翠榴石在384和440 nm处具明显吸收峰，436 nm见弱吸收峰，620 nm附近出现宽缓吸收带，从500 nm附近至紫外区强烈吸收。分析认为440 nm吸收带归于八面体位Fe3+的6A₁→4A_1g+4E_g(G)跃迁所致；620 nm吸收带归于八面体位Cr3+的4A_2g(F)→4T_2g(F) d-d跃迁所致，Fe和Cr同为致色元素，O-Fe荷移带及440 nm强吸收带使得钙铁榴石产生黄色、黄绿色，Cr3+的加入，产生620 nm宽缓吸收带，吸收橙黄色光，使得宝石颜色向绿色端偏移，显示纯正的绿色。拉曼光谱、红外光谱指纹区特征可以用于准确鉴定翠榴石；稀土元素特征及中红外光谱官能团区结构水特征，可以为其产地溯源提供重要信息。

Chemical Constituents and Spectra Characterization of Demantoid from Russia

Garnet is the most valuable subspecies in garnet family, and is popular for its beautiful appearance and rarity. In particular, demantoid from Russia is the most sought after object of domestic and foreign collectors. Previous studies were more on garnet minerals from different perspectives, but there are few studies on the demantoid. To study the chemical composition and spectroscopic characteristics of the Russian demantoid, the systematic research is conducted by using LA-ICP-MS, IR spectrum, Raman spectrum, UV-Vis absorption spectrum, so as to obtain the chemical components, especially rare earth elements and spectroscopic characteristics, and to analyze the causes of colour, and provide important data for its variety identification and traceability of origin. Chemical research shows that the demantoid is almost pure Andradite (Andradite>96.39 Mol.%). Among the secondary components, Cr₂O₃ content is relatively high, with an average of 0.502 Wt%. Cr and V are chromophore which causes the green color in garnet. Content of rare earth elements generally is low, ∑REE with an average of 4.85 μg·g-1; And the light rare earth elements are significantly enriched, ∑LREE with an average of 4.56 μg·g-1; Heavy rare earth elements relative loss, ∑HREE with an average of 0.29 μg·g-1, ∑LREE/∑HREE=5.35~100.48. Most samples show Eu positive anomaly. The main Raman shifts are 995, 874, 841, 815, 576, 552, 515, 492, 451, 369, 351, 323, 311, 295, 263, 235, and 172 cm-1. Raman spectra can only be used as one of the methods for the identification of variety, and have little effect on determination of its origin. The infrared spectroscopy studies show that the infrared spectra of the fingerprint region can be used to identify the demantoid, and the functional region shows the absorption peak of the structure water, which indicates that the Russian garnet contains a small amount of structural water, which is relate to its formation with hydrothermal process. Studies on UV-Vis absorption spectra show that Russian demantoid has an obvious absorption peak at 384 and 440 nm, a weak absorption peak at 436 nm, a wide absorption band near 620 nm, and a strong absorption from 500 nm to the ultraviolet region. The 440 nm absorption band was attributed to the 6A₁ to 4A_1g+4E_g(G) d-d transition of Fe3+ in octahedral site. The 620 nm absorption band was attributed to the 4A_2g(F) to 4T_2g(F) d-d inhibiting transition of Cr3+ in octahedral site. Fe and Cr are both chromogenic elements, and the O-Fe Charge transfer band and the 440 nm strong absorption band produce yellow and yellow-green garnet. The addition of Cr3+ produce a 620 nm wide absorption band, which absorbs orange light and makes the gem color shift to the green, producing the pure green demantoid. The fingerprint characteristics of Raman spectrum and infrared spectrum can be used for the accurate identification of demantoid. The characteristics of rare earth elements and the structural water features of functional groups in the middle infrared spectrum can provide important information for its origin determination.