New Insights into Coloration Mechanism in Violet-Red Pyrope-Almandine

Yang, Puyue; Guo, Ying

doi:10.3390/cryst12030379

Cited by 4 publications

(3 citation statements)

References 21 publications

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“…When the transmittance of the two regions is almost the same, the color of the gem is determined by the external light source. The absorptions at 573 nm and 569 nm correspond to the spin-forbidden transitions ( 5 E g → 3 T 1g and 5 E g → 3 E 1g , respectively) of eightfold-coordinated high-spin Fe 2+ , and Fe 3+ may have a contribution to these absorption bands [25,[28][29][30][31]. The unusual high-intensity spin-forbidden optical transitions of eightfold-coordinated Fe 2+ cause many optical transitions in the green-yellow part of the visible spectrum and leave two strong transmission windows in the green and red spectral regions.…”

Section: Uv-vis-nir Spectroscopymentioning

confidence: 99%

“…Very small amounts (<1 ppm) of Cr may be present in the samples. The weak absorption bands at 620 nm and 621 nm are both ascribable to Fe, and the spin-forbidden transitions of Cr 3+ ( 4 A 2g → 4 T 1g ) may have a slight contribution to these absorption bands [5,13,26,30,32]. The Fe 2+ spin-forbidden transition ( 5 E(D) → 3 T 1 (H)) produces a broad absorption band in the visible-light range 550-650 nm, leading to two transmittance regions on both sides, which show a yellowish green to brown alexandrite-like effect along with the change in the spectral power distribution of the light source.…”

Section: Uv-vis-nir Spectroscopymentioning

confidence: 99%

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Explaining Color Change in Gem-Quality Andradite Garnet

Xu,

Yu,

Shen

et al. 2024

Crystals

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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 green in the presence of daylight to brown under incandescent light; the other color changes from yellow–green to orange. 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 (centered at 525 nm) and the green region (above 650 nm). The chemical compositional analysis indicates that the samples are very low in Cr (<1 ppm) and high in Fe2+ (from 2.31 wt.% to 4.66 wt.%). The combined spectra and chemical compositional analysis show that Fe2+ 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.

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Section: Uv-vis-nir Spectroscopymentioning

confidence: 99%

Section: Uv-vis-nir Spectroscopymentioning

confidence: 99%

Explaining Color Change in Gem-Quality Andradite Garnet

Xu,

Yu,

Shen

et al. 2024

Crystals

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“…Qiu et al examined the causes of the color change in pyrope-spessartine garnets and discussed the color of garnet samples that different kinds of light source cause [18]. Yang et al found the new coloration mechanism of violet-red pyrope-almandine [19]. Lv et al analyzed how gemstones' composition affects their color parameters, finding the genesis of the body color of opal [20].…”

Section: Introductionmentioning

confidence: 99%

New Insights into the Coloration Mechanism in Spessartines and the Impact of Munsell Neutral Grey Backgrounds

Zhu,

Guo

2023

Crystals

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In this study about spessartines,13 spessartine samples with proper conditions are used for FTIR spectrum and UV-Vis spectrum tests to find the gemological characteristics and coloration mechanism. In the UV-Vis spectrum, spessartines’ special color has a relationship with the absorption bands at about 460 nm and 480 nm. Based on the CIE 1976 L*a*b* colour system, we come to the conclusion that both color coordinates a* and b* control the value of chroma C*, and color coordinate b* mostly controls the hue angle h°. We also explore how the different chromogenic ions FeOtot and MnO and their ratio FeOtot/MnO influence spessartines’ color, finding that the color of spessartine samples is influenced by both Mn and Fe. By analyzing the FTIR spectrum, we discovered that as the content of Mn decreases, and the A, C and D peaks move to the position of a longer wave, the color of spessartine samples also changes significantly. By using the standard light source D65, we find that N9.5 Neutral Grey Background is the best background to grade the color of spessartines.

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Almandine jewelry garnet from the Kitelya deposit (Karelia): composition and spectroscopic properties

Lyutoev¹,

Makeyev

Terekhov

2023

Lithosphere

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Research subject. The chemical composition and spectroscopic properties of almandine jewelry garnets from the Kite-lya deposit in the Northern Ladoga region (Karelia). Materials and methods. The chemical composition, impurity elements, mineral inclusions and spectroscopic properties of almandine jewelry crystals were studied using IR and Mössbauer spectroscopy. Results. Garnet crystals were found to exhibit a weakly pronounced zonal composition, varying from Alm75Pir15Sps7Grs3 in the center to Alm80Pir14Sps4Grs2 at their edges. Therefore, the Ca and Mn contents decrease towards the grain edges. This zonality of garnets is characteristic of the processes of progressive metamorphism of their host rocks. The garnet crystals feature small inclusions of quartz, chlorite, mica FACI (biotite), ilmenite, rutile, monazite, zircon and pyrrhotite. The composition of chlorite, biotite and zircon was established. The parameter of the cube unit cell ao = 11.522 ± 0.003 Å was calculated. The IR absorption spectra of 995, 966, 901, 878, 638, 568, 528, 476 and 455 cm–1 are characteristic of the pyrope-almandine difference. Mössbauer spectroscopy revealed an insignificant admixture of trivalent iron (Fe3+) in the structure of Kitelya garnets (≈1 % of the amount of isomorphic iron). The obtained optical absorption spectra of garnet plates in the visible light spectrum indicate that Fe2+ ions in dodecahedral positions, to a lesser extent dodecahedral Mn2+ ions, as well as possibly octahedral Fe3+ ions are responsible for the bright red-crimson color of pyrop-almandine from the Kitelya deposit. Conclusions. A “portrait” of typomorphic features (composition and properties) of the pyrope-almandine jewelry garnet from the Kitelya deposit was obtained. This portrait can be used when analyzing the historical finds of faceted or cabochonized differences of almandine in jewelry, church utensils in both Russia and Europe (where this jewelry material was exported during the 17th century). The preservation of garnet jewelry differences in the host rock is due to the presence of thin amorphous kelefite shells or soft minerals (sericite, chlorite, kaolinite etc.).

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New Insights into Coloration Mechanism in Violet-Red Pyrope-Almandine

Cited by 4 publications

References 21 publications

Explaining Color Change in Gem-Quality Andradite Garnet

Explaining Color Change in Gem-Quality Andradite Garnet

New Insights into the Coloration Mechanism in Spessartines and the Impact of Munsell Neutral Grey Backgrounds

Almandine jewelry garnet from the Kitelya deposit (Karelia): composition and spectroscopic properties

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