巴基斯坦Swat矿区祖母绿的宝石学及谱学特征

巴基斯坦Swat矿区祖母绿以饱和绿色和高净度著称，其价格逐年攀升，如何有效鉴别Swat矿区祖母绿的特征成为当前的研究热点。采用常规宝石学仪器、电感耦合等离子质谱、红外光谱、拉曼光谱和紫外-可见吸收光谱对该矿区祖母绿的宝石学及谱学特征进行研究，并探讨其颜色成因。结果显示：(1)Swat矿区祖母绿晶体常为六方柱状，且净度较高。样品颜色呈明亮的绿色，具有中等至强的二色性（黄绿/蓝绿色）。折射率偏高，约1.588~1.599，与晶体碱金属含量高有关。(2)晶体中部分Al与Mg，Fe，Cr等发生类质同象置换。其中替换最多的是Mg元素，含量达11 402~12 979 ppma（平均为12 446 ppma）；Fe和Cr元素次之（平均含量分别为2 390和2 199 ppma）。样品中碱金属元素(Na，K，Rb，Cs)含量很高，总量约14 201~16 136 ppma，平均为15 183 ppma。(3)红外光谱显示指纹区1 312，1 152，983，838，701，616和559 cm-1处的吸收峰，由Si₆O₁₈]等基团振动所致。近红外区可见较强的Ⅱ型水振动相关吸收峰(7 074 cm-1)，说明样品中存在较多Ⅱ型水，与晶体中碱金属离子含量较高相符。拉曼光谱显示324，399，516，685和1 067 cm-1等祖母绿的拉曼位移，并检测到气液包裹体相关的H₂O (3 595 cm-1)、CO₂(1 284和1 284 cm-1)和N₂(2 327 cm-1)以及伴生矿物滑石和磁铁矿的拉曼位移。(4)紫外可见-吸收光谱(UV-Vis)测试结果显示有Cr3+(688～641 nm)，V3+(610 nm)，Fe2+(860 nm)和Fe3+(375 nm)相对应的吸收峰。样品中Fe元素含量为1 124~3 561 ppma，Cr元素含量为730~3 077 ppma，V元素含量较少，为28.01~263.9 ppma。不同样品中Cr元素含量差距较大，V元素含量差距小，Cr和V离子数比值约3.43~60.05。随着样品颜色饱和度增加，Cr元素含量急剧增大，V元素含量增加极少，Fe元素含量反而降低。推测Swat矿区祖母绿主要致色元素为Cr，其次为V，其色调和荧光可能受Fe元素影响。

Gemological and Spectral Characteristics of Emeralds From Swat Valley,Pakistan

Emeralds from Swatin Pakistan are well known for their vivid greenish color and high clarity. How identify the characteristics of emeralds from Swat valley has becomes a hot topicsince their prices have increased every year. Therefore, in this paper, we investigated the gemological andspectral characteristics of emeralds by using conventional gemological tests, LA-ICP-MS, infrared spectroscopy, Raman spectrometer and UV-Vis spectroscopy. Meantime, wealso discussed the color origin of emeralds from Swat. The results show: (1) Most of the rough emeralds from Swat valley are hexagonal-prismsforms. These crystals normally show brightly green and high transpar- ency. There is also a moderate to strong (yellow-green/blue-green) dichroism effect. Most emeralds have a higher refractive index of 1.588～1.599. (2) In the crystals, a part of Al hasbeenreplaced by Mg, Fe or Cr. The most substituted element is Mg, with a content of 11 402~12 979 ppma (avg. 12 446 ppma). The second most substitute- ed elementsare Fe and Cr (their average contents are 2 390 and 2 199 ppma respectively). Concentrations of alkaline cations (Na, K, Rb, Cs) are very high, ranging from 14 201 to 16 136 ppma (avg. 15 183 ppma). (3) FTIR spectra show the fingerprint’s absorption peaks at 1 312, 1 152, 983, 838, 701, 616 and 559 cm-1, caused by the vibration of Si₆O₁₈] groups. The peak at 7 074 cm-1 is significantly strong, indicating a significantly high quantity of type Ⅱ H₂O in the crystals. It is related to the higher content of alkaline cations (Na, K, Rb, Cs). Raman shifts at 324, 399, 516, 685 and 1 067 cm-1 are all related to the vibration of emerald. Raman shifts related to H₂O (3 595 cm-1), CO₂ (1 284 and 1 388 cm-1) and N₂ (2 327 cm-1) are also detected in the gas-liquid inclusions. The associated minerals on crystalsare talc and magnetite. (4) UV-Vis spectra show absorption peaks corresponding to Cr3+(688～641 nm), V3+(610 nm)，Fe2+(860 nm) and Fe3+ (375 nm). In the samples, the content of Fe is 1 124~3 561 ppma, the content of Cr is 730~3 077 ppma, while the content of V is less, which is 28.01~263.9 ppma. In addition, the content of Cr in different samples changes greatly, the content of V changes little, and the ratio of Cr and V ions is about 3.43~60.05. With the increase of color saturation, the content of Cr increases sharply, the content of V changes very little, and the content of Fe decreases. We can infer that the main chromogenic element of emeralds from Swat valley is Cr, followed by V, while the hue and fluorescence may be affected by Fe.