Most gem-quality feicui on the market comes from Burma and has long been favored by consumers. In recent years, feicui from Guatemala has also been continuously entering the chinese market.

Recently, to investigate the gemological properties, spectroscopic characteristics, and mineral compositions of green feicui from Burma and Guatemala, provide a basis for origin determination of green feicui, and help regulate the feicui market, GUILD gemologists conducted an in-depth and detailed study in the basic gemological properties, microscopic observation, infrared spectroscopy, ultraviolet-visible spectroscopy, Raman spectroscopy, and X-ray fluorescence spectroscopy with a large number of green feicui samples from these two origins.

What Is Feicui?

According to the Chinese national standard GB/T 16553-2017, the dominant mineral of feicui is jadeite, or jadeite together with other sodic and sodic-calcic pyroxenes (such as omphacite and kosmochlor), and small amounts of amphibole, feldspar, chromite, and other minerals can also present. Feicui is a cryptocrystalline to polycrystalline mineral aggregate formed mainly of pyroxene-group minerals [1].

Figure 1: A piece rough of Guatemalan feicui. Photo courtesy of GUILD Gem Laboratory.

Occurrence and Origins of Feicui

Feicui forms under high-pressure, low-temperature geological environments within tectonic subduction zones and through complex processes such as plate subduction and multi-stage fluid metasomatism [2]. Feicui deposits occur primarily in Burma, but are also found in Guatemala, Russia, Kazakhstan, Japan, the United States, and elsewhere [3].

Feicui deposits in Burma are divided into primary and secondary deposits. Primary deposits are mainly distributed in serpentinized peridotite bodies in northern Burma, whereas secondary deposits are mainly located in the upper reaches of the Uru River basin [2]. In Guatemala, feicui occurs in serpentinite mélanges on opposite sides of the Motagua fault[4].

Basic Gemological Properties

Note: The data in the table are measured values for the research samples.

In this study, 17 Burma feicui samples and 20 Guatemalan feicui samples were selected. Visual observation showed the following:

Burma feicui shows green to deep green, with relatively high saturation and brightness. Green color concentrations are visible, and some color centers appear as transparent green crystals under transmitted light. The material shows fibrous interwoven structure, vitreous luster, and translucent to transparent appearance.

Guatemalan feicui displays deep grayish green and deeper than Burma green Feicui. Black dotted minerals can be seen on the surface with the naked eye. In some samples, white flocculent or vein-like distributions are visible on the surface under reflected light, while these materials appear dark green under transmitted light. Overall, it shows mylonitic structure, vitreous luster, and translucent to transparent appearance.

Figure 2. Green Burma feicui samples. Photo courtesy of GUILD Gem Laboratory.

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Microscopic Observation

The green areas in Burma feicui are distributed in vein-like, scattered dot-like, or patchy forms. White minerals occur in snowflake-like patterns interwoven with the main crystals. Brown flaky minerals and dark prismatic minerals are occasionally observed.

In Guatemalan feicui, white minerals commonly appear in vein-like distributions, showing white under reflected light and gray under transmitted light. Conical, clustered, irregular blocky, and platy dark minerals with strong metallic luster are observed, and these dark minerals were not observed in the Burma Feicui samples.

Figure 4. Microscopic features of Burma feicui: (a) green color band with vein-like distribution; (b) white minerals with snowflake-like distribution; (c) exposed brown flaky mineral; (d) scattered deep green dots; (e) dark prismatic minerals and fibrous interwoven texture; (f) patchy green distribution.

Figure 5. Microscopic features of Guatemalan feicui: (a) abundant white clustered minerals on the surface; (b) exposed irregular dark minerals; (c) irregular dark minerals within the sample; (d) interwoven light-green prismatic minerals; (e) white vein-like minerals on the surface; (f) clustered dark minerals with metallic luster; (g) conical dark inclusions; (h) regular dark inclusions; (i) deep platy omphacite; (j) irregular dark minerals with metallic luster; (k) white flocculent jadeite; (l) white snowflake-like material.

Infrared Spectroscopic Analysis

Pyroxene-group minerals are chain silicates in which SiO4 tetrahedras are linked by two shared corners to form chains extending along the c-axis. Two kinds of interchain sites are present: the octahedral M1 site and the irregular octahedral M2 site. The pyroxene chemical formula can be expressed as M2M1[T2O6], where M2 is mainly occupied by Na2+, Ca2+, Mn2+, Fe2+, Mg2+, and Li+, while M1 is mainly occupied by Mn2+, Fe2+, Mg2+, Fe3+, Cr3+, Al3+, and Ti4+, and T is primarily Si4+ with subordinate Al3+.

The infrared spectroscopic peaks of jadeite are mainly concentrated at 1170, 1087, 960, 585, 530, and 470 cm-1, and that of omphacite are mainly concentrated at 1062, 960, 889, 650, 560, 520, 450, and 417 cm-1, with the typical characteristic feature that the 1062 cm-1 and 960 cm-1 peaks form mirror-image peaks.

Infrared reflection spectra showed that both Burma and Guatemalan feicui samples belong to the pyroxene group, with peak positions shifted due to differences in the metal cations present. The principal constituent mineral of green Burma feicui is jadeite. By contrast, the infrared peak positions of Guatemalan feicui shift toward lower wavenumbers. In some samples, the principal constituent mineral is omphacite, while some others is an intermediate phase between omphacite and jadeite.

Table 1 Assignment of infrared spectral vibration peaks and peak positions in different regions

UV-Visible Absorption Spectroscopic Analysis

According to previous studies [2], different colors of feicui all shows an absorption peak at 437 nm (related to Fe3+) in the UV-visible spectrum. The green color of feicui in Burma results from Cr3+ substituting for Al3+ in jadeite, and the Cr content affects the intensity of the green tone.

The UV-Vis absorption spectra of green Burma feicui show peaks at 370 nm or 382 nm, which may also occur simultaneously; a strong absorption peak at 437 nm; an occasional absorption peak at 450 nm; strong absorption peaks near 635, 660, and 690 nm (related to Cr); and occasionally a relatively weak absorption band at 860–900 nm.

The UV-Vis absorption spectra of green Guatemalan feicui mainly fall into two types.

One type resembles spectra of green Burma feicui and shows absorption peaks at 379 nm or 382 nm (related to Fe3+), a absorption peak at 437 nm, distinct strong absorption peaks at 635, 660, and 690 nm (related to Cr2+ ), and a relatively weak absorption band at 940 nm.

The other type UV-Vis spectra is characterized mainly by partial absorption in the red and yellow-green regions, concentrated in broad bands around 600–620 nm, 730–760 nm, and 940 nm, and may even show complete absorption from the red region to the yellow-green region. When Fe2+ undergoes electronic transitions in an octahedral crystal field, it usually forms a broad absorption band extending from the infrared edge into the red region, and even into the yellow-green region [7].

Raman Spectroscopic Analysis

Multiple points on different parts of each sample was tested by Raman spectrometer, as Feicui is a polycrystalline aggregate. Previous studies have shown that the principal Raman shifts of jadeite occur at 1036, 991, 777, 698, 574, 525, 427, 375, 328, and 206 cm-1. which shows strongest peaks at 1036 cm-1 and 698 cm-1. The 1036 cm-1 peak corresponds to Si-O stretching, and the 698 cm-1 peak corresponds to the Si-O-Si bending band [9]. The principal Raman spectrum peaks of omphacite [8] dispalays at 1020, 678, 369, and 212 cm-1, which shows the strongest peaks at 1020 cm-1 and 678 cm-1.

Green Burma feicui is dominated by jadeite, though omphacite can be detected in some areas. By contrast, green Guatemalan feicui is dominated by omphacite and may contain jadeite in some parts. Other secondary minerals were also detected at different points in different samples.

Burma Feicui

Multiple-point Raman spectrometer testing was carried out on Burma feicui samples, especially on different colored parts. Raman spectrum analysis of sample BJ-01 showed typical jadeite characteristic peaks.

Raman spectrum showed that the light-colored and deep-green parts of green Burma feicui sample BJ-02 may be omphacite. Raman shift at 1082 and 278 cm-1 were also observed in the light-colored areas. Comparison with reference data showed that the main Raman shift of calcite occur at 1085, 711, 281, and 154 cm-1, suggesting the presence of calcite in this sample.

Overall, the principal constituent mineral of green Burma feicui is jadeite, and the secondary minerals include omphacite and calcite.

Guatemalan Feicui

Green guatemalan feicui commonly contains irregular dark minerals with metallic luster appearing in clusters or dots. Raman spectrum of sample GO-05 identified these dark minerals as amorphous carbon.

Deep-green prismatic crystals in sample GO-10 were identified as omphacite by Raman spectrometer. The Raman spectrum of other parts of the sample also revealed the typical peaks of both jadeite and omphacite, indicating that both minerals are present in this sample GO-10.

Raman spectrum of the green Guatemalan feicui sample GO-16 revealed typical omphacite characteristic peaks, along with a weak Raman shift at 204 cm-1 attributable to jadeite. The white flocculent part in the sample was identified as jadeite with a Raman shift at 695 cm-1.

Raman 3D Mapping

Raman 3D mapping technology elevates the application of Raman spectroscopy to a new level. It can display the minute differences in specific selected areas of the feicui in a more vivid and intuitive manner. By combining the micro-Raman spectrometer with a three-dimensional movable sample stage, the position of the confocal laser beam can be precisely adjusted for in-depth testing of selected inclusions. At the same time, 3D mapping can be used to scan selected areas in greater detail and obtain comprehensive information on the internal mineral distribution of feicui.

To further observe the mineral composition of Guatemalan feicui, a flat and smooth surface of the sample GO-16 was selected for 3D scanning.

The variation in characteristic peaks between 550 and 850 cm-1 showed the following: (1) within this range, a single peak appears at 679 cm-1 at first; (2) the initially strong 679 cm-1 Raman peak gradually weakens and shifts toward higher frequency, while the 688 cm-1 peak gradually appears and strengthens; (3) the 679 cm-1 peak gradually disappears and the 689 cm-1 peak exists as a single peak; (4) the 689 cm-1 peak gradually weakens and shifts toward lower frequency, while the 679 cm-1 peak gradually reappears and strengthens, and finally the 689 cm-1 peak disappears.

At Z = 27 μm in the Raman test, the characteristic peaks of both omphacite and jadeite can be observed simultaneously according to the 679 cm-1 and 689 cm-1 peaks are the typcal peaks of omphacite and jadeite respectively. Reconstructed structural data for this area of the sample show that the green region represents jadeite and the red region represents omphacite. This indicates that omphacite and jadeite are interwoven in guatemalan feicui. According to the vibrational spectral assignment of chain silicates, the 689 cm-1 Raman shift (related to jadeite) and the 679 cm-1 Raman shift (related to omphacite) belong to the Si-O-Si bending band. The Raman shift toward lower frequency is inferred to result from isomorphic substitution involving Fe, Mg, and Ca, where replacement of Na and Al by Fe, Mg, and Ca increases the Si-O bond length, lowers the force constant K, and thus decreases the vibrational frequency.

X-Ray Fluorescence Spectroscopic Analysis

Feicui samples from Burma and Guatemala were tested by X-ray fluorescence spectroscopy. This method can excite elements such as Al, Ca, Fe, and Cr. Two to five different positions were tested on each sample. Comparison showed obvious differences in Al, Ca, Ti, Cr, Fe, and Ni between the two origins.

Table2 Analysis of element Contents in feicui from burma and guatemala

Burma feicui contains distinctly higher Al, reaching up to 86,360 ppm, whereas guatemalan feicui contains distinctly higher Ca, reaching up to 37,770 ppm. Compared with Guatemalan feicui, Burma feicui has relatively lower Ti, Cr, Fe, and Ni contents. The two origins can be distinguished by comparing Ca/Al ratios.

Reports

A rich and comprehensive database is the scientific guarantee for the accuracy and reliability of Feicui origin determination. Following extensive, detailed, and systematic origin studies, GUILD Gem Laboratory is currently able to issue origin reports for feicui from Burma and Guatemala.

EPILOGUE

Advanced detection technologies have always been one of the crucial factors driving research and detection of gem stones. The detailed and in-depth research conducted by GUILD Gem Laboratories on feicui origins provides a scientific basis for the origin determination of green feicui, offers strong protection for consumers, and contributes to the standardization of the feicui market and the healthy development of the feicui industry.

References

[1] Ouyang Qiumei. Mineral Composition of Jadeite Jade. Journal of Gems and Gemmology, 1999, 1(1): 18-23.

[2] Li Yali. Gemology Course. China University of Geosciences Press, 2006: 287.

[3] Shi Guanghai, Lei Weiyan. Origins and Characteristics of Feicui in the World. Forbidden City, 2018, No. 280(05): 44-55.

[4] Hargett D. Jadeite of Guatemala: A Contemporary View. Gems & Gemology, 2008, 26(2): 134-141.

[5] Ouyang Qiumei, Qu Yihua. Characteristics of Western Sayan Jadeite Jade deposite in Russia. Journal of Gems and Gemmology, 1999, 1(2): 5-11.

[6] Zheng Ting. The Study on Gemological and Mineralogical Characteristics of Guatemala Green Feicui. China University of Geosciences (Beijing), 2015: 50-52.

[7] Yuan Xinqiang, Qi Lijian, Du Guangpeng, et al.UV-VIS-N IR Spectrum of Jadeite Jade from Burma. Journal of Gems and Gemmology, 2003, 4(5): 1-6.

[8] Lin C L, He X M, Lu Z Y, et al. Phase Composition and Genesis of Pyroxenic Jadeite from Guatemala: Insights from Cathodoluminescence. RSC Advances, 2020, 10(27): 15937-15946.

[9] Chen Quanli, Yin Zuowei, Bu Yuewen, et al. Raman Spectroscopy study on the Mineral Composition of Guatemalan Jade. Spectroscopy and Spectral Analysis, 2012, 32(9): 2447-2451.