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| Gemological and Spectral Characteristics of Emeralds From Swat Valley, Pakistan |
| CAO Su-qiao1, DAI Hui1*, WANG Chao-wen2, YU Lu1, ZUO Rui1, WANG Feng1, GUO Lian-qiao1 |
1. Anhui Geological Experiment Institute(Hefei Mineral Resources Supervision & Testing Center of the Ministry of Land & Resources), Hefei 230001, China
2. Gemmological Institute, China University of Geosciences (Wuhan), Wuhan 430074, China
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Abstract 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.
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Received: 2021-09-03
Accepted: 2022-02-23
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Corresponding Authors:
DAI Hui
E-mail: daihui9809@sina.com
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