Explaining Color Change in Gem-Quality Andradite Garnet
The homomorphic substitution of the garnet group is common in nature. Two rare color-changing andradite garnets are studied in this paper. One color changes from yellowish-green in the presence of daylight to maroon under incandescent light; the other color changes from brownish yellow to brownish r...
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MDPI AG
2024-02-01
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| Series: | Crystals |
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| Online Access: | https://www.mdpi.com/2073-4352/14/2/180 |
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| author | Jia-Hong Xu Xiao-Yan Yu Mei Shen Ying Yan Guang-Ya Wang |
| author_facet | Jia-Hong Xu Xiao-Yan Yu Mei Shen Ying Yan Guang-Ya Wang |
| author_sort | Jia-Hong Xu |
| collection | DOAJ |
| description | The homomorphic substitution of the garnet group is common in nature. Two rare color-changing andradite garnets are studied in this paper. One color changes from yellowish-green in the presence of daylight to maroon under incandescent light; the other color changes from brownish yellow to brownish red. In this study, conventional gemological instruments, infrared (IR) spectroscopy, ultraviolet–visible–near infrared (UV–Vis–NIR) spectroscopy, Raman spectroscopy, and electron probe microanalysis (EPMA) were used to explore the gemology and coloration mechanisms of color-changing garnets. Experiments revealed that the color-changing gemstones in the study are andradite garnets. There are two transmission windows in the UV–Vis spectrum: the red region (above 650 nm) and the green region (centered at 525 nm). The chemical compositional analysis indicates that the samples are very low in Cr (<1 ppm) and high in Fe<sup>2+</sup> (from 2.31 wt.% to 4.66 wt.%). The combined spectra and chemical compositional analysis show that Fe<sup>2+</sup> is the main cause of the color change. Based on the IR spectrum (complex water peaks), UV–Vis–NIR spectrum (similar to that of Namibian andradite garnet), and chemical compositional analysis (low Cr content), it is concluded that color-changing andradite may be related to skarn rock genesis. |
| first_indexed | 2024-03-07T22:37:01Z |
| format | Article |
| id | doaj.art-e58ec0b9198548049e178a62c671b243 |
| institution | Directory Open Access Journal |
| issn | 2073-4352 |
| language | English |
| last_indexed | 2024-03-07T22:37:01Z |
| publishDate | 2024-02-01 |
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| series | Crystals |
| spelling | doaj.art-e58ec0b9198548049e178a62c671b2432024-02-23T15:13:18ZengMDPI AGCrystals2073-43522024-02-0114218010.3390/cryst14020180Explaining Color Change in Gem-Quality Andradite GarnetJia-Hong Xu0Xiao-Yan Yu1Mei Shen2Ying Yan3Guang-Ya Wang4School of Gemology, China University of Geosciences Beijing, 29 Xueyuan Road, Beijing 100083, ChinaSchool of Gemology, China University of Geosciences Beijing, 29 Xueyuan Road, Beijing 100083, ChinaTaiwan Gemmological Research Center and Laboratory, 6th Floor, No. 92, Section 1, Zhongshan North Road, Zhongshan District, Taipei 10444, TaiwanSchool of Gemology, China University of Geosciences Beijing, 29 Xueyuan Road, Beijing 100083, ChinaSchool of Gemology, China University of Geosciences Beijing, 29 Xueyuan Road, Beijing 100083, ChinaThe homomorphic substitution of the garnet group is common in nature. Two rare color-changing andradite garnets are studied in this paper. One color changes from yellowish-green in the presence of daylight to maroon under incandescent light; the other color changes from brownish yellow to brownish red. In this study, conventional gemological instruments, infrared (IR) spectroscopy, ultraviolet–visible–near infrared (UV–Vis–NIR) spectroscopy, Raman spectroscopy, and electron probe microanalysis (EPMA) were used to explore the gemology and coloration mechanisms of color-changing garnets. Experiments revealed that the color-changing gemstones in the study are andradite garnets. There are two transmission windows in the UV–Vis spectrum: the red region (above 650 nm) and the green region (centered at 525 nm). The chemical compositional analysis indicates that the samples are very low in Cr (<1 ppm) and high in Fe<sup>2+</sup> (from 2.31 wt.% to 4.66 wt.%). The combined spectra and chemical compositional analysis show that Fe<sup>2+</sup> is the main cause of the color change. Based on the IR spectrum (complex water peaks), UV–Vis–NIR spectrum (similar to that of Namibian andradite garnet), and chemical compositional analysis (low Cr content), it is concluded that color-changing andradite may be related to skarn rock genesis.https://www.mdpi.com/2073-4352/14/2/180color-changing andraditecoloration mechanismschemical compositionspectroscopic analysis |
| spellingShingle | Jia-Hong Xu Xiao-Yan Yu Mei Shen Ying Yan Guang-Ya Wang Explaining Color Change in Gem-Quality Andradite Garnet Crystals color-changing andradite coloration mechanisms chemical composition spectroscopic analysis |
| title | Explaining Color Change in Gem-Quality Andradite Garnet |
| title_full | Explaining Color Change in Gem-Quality Andradite Garnet |
| title_fullStr | Explaining Color Change in Gem-Quality Andradite Garnet |
| title_full_unstemmed | Explaining Color Change in Gem-Quality Andradite Garnet |
| title_short | Explaining Color Change in Gem-Quality Andradite Garnet |
| title_sort | explaining color change in gem quality andradite garnet |
| topic | color-changing andradite coloration mechanisms chemical composition spectroscopic analysis |
| url | https://www.mdpi.com/2073-4352/14/2/180 |
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