Bicoloured Jadeite-Omphacite Jade from Guatemala
Traditionally, jadeite-omphacite jade (fei cui) from Myanmar prevails in the Chinese market, although in recent years material from Guatemala has drawn considerable attention. The increasing quality and availability of this material has quickly expanded its market share. Guatemalan jade has been shown to consist predominantly of the pyroxenes jadeite (NaAlSi2O6) and/or omphacite [(Na,Ca,Fe2+,Mg)(Al,Fe3+,Fe2+,Mg)(Si2O6)] (Liu et al. 2024).
Recently, Guild Gem Lab obtained a parcel of Guatemalan jade that contained an interesting bicoloured piece, which underwent Raman and chemical analyses to assess how mineralogical and compositional variations correlated with its colour zoning. The sample consisted of a slab weighing 42.17 g (82.18 x 13.33 x 3.05 mm), of which about 65% was near-colourless and the rest exhibited a highly saturated green colour (Figure 8). No indications of bleaching or dyeing were evident, which was confirmed by microscopic observation and Fourier-transform infrared (FTIR) spectroscopy. In the near-colourless areas, many small white shiny flecks (sometimes known as 'fly wings') were visible, even to the unaided eye, which were produced by reflections from mineral cleavages. However, these reflections were not seen in the green areas, possibly because the minerals there were different from those in the near-colourless areas.
Several points on both the near-colourless and green areas were analysed by Raman spectroscopy. The resulting spectra (Figure 9) showed that the near-colourless material was predominantly jadeite and the green areas were mainly omphacite. These two clinopyroxenes can be differentiated by their Raman peak position in the 670-700 cm-1 region, which is due to Si-O bridging-stretching vibrations. The peak position is linearly related to the multivalent isomorphism of Na+Al3+-Ca2+Mg2+, with only minor orientation effects (Zhang & Shi 2022). The diagnostic Raman peak occurs at about 700 cm-1 for jadeite and 678 cm-1 for omphacite (Eaton-Magana et al. 2021). Within one green area, Raman analysis also identified several white particles as albite and some colourless grains as jadeite, indicating that some jadeite may be present in the omphacite-dominant green areas of the sample.
Figure 8: This slab of Guatemalan jade (82.18 x 13.33 x 3.05 mm) displays both near-colourless and green areas, as seen in (a) ambient light and (b) transmitted light. The use of transmitted light shows the diaphaneity of the sample and enhances the colour saturation of the green portions. Photos by Fei Hu.
To better understand the mineral distribution and variations in the piece, two-dimensional (2D) Raman mapping was conducted in selected areas, each measuring 2.0 x 3.6 mm (e.g. Figure 10). The analyses required 36 hours, during which a total of 3,600 spectra were collected. By using the characteristic peaks of jadeite and omphacite at 697 and 680 cm-1, respectively, a 2D Raman map was created. The false-colour image shows a clear boundary between jadeite (green area of the map) and omphacite (red area), consistent with the eye-visible near-colourless and green colour distribution of the analysed areas. However, within the jadeite-dominant near-colourless portion, several areas of omphacite were identified. A similar condition applied to the omphacite-rich green portion, where several jadeite grains were sparsely distributed.
Figure 9: The Raman spectrum of a green area of the sample (grey trace; point 1 in Figure 10) identifies it as omphacite, while the spectrum of a near-colourless area (red trace; point 2 in Figure 10) shows it is jadeite.
Chemical analysis by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was conducted on selected points, and the results showed that the green omphacite-rich area of the sample was rich in Cr (613-3,620 ppmw, with an average of 1,540 ppmw for 17 points), while the near-colourless portion was poor in Cr (2.33-59.7 ppmw; average of 23.4 ppmw for 13 points). This difference in Cr content agrees with the colour zoning, since Cr is responsible for the green colouration of both jadeite and omphacite.
The combination of Raman spectral mapping and chemical analysis proved to be very helpful for characterising the mineral composition and distribution in this sample in a quasi-non-destructive way (several tiny ablation spots were required for LA-ICP-MS analysis), as compared to destructive methods such as cutting thin sections and performing X-ray powder diffraction. The interesting colour distribution in the sample is apparently related to localised variations in Cr content during jade formation, which occurred independently of the distribution of jadeite and omphacite in the sample. Similarly, Eaton-Magana et al. (2021) used Raman mapping to reveal the jadeite and omphacite distribution in homogeneously coloured (green) jade samples, illustrating the usefulness of this technique for jade characterisation.
Figure 10: These images show a portion of the sample measuring 2.0 x 3.6 mm that underwent Raman mapping. (a) A photomicrograph of the analysed area shows that it is green in the top portion and near-colourless in the bottom area. (b) Several traverses of point analyses (green dots) were done for Raman mapping, as shown on this reflected-light image. (c) The resulting false-colour 2D Raman map shows a clear boundary between jadeite-rich and omphacite-rich portions of the sample, which are seen here as green and red areas, respectively. The two labelled analysis points correspond to the Raman spectra in Figure 9. Images by Ziyun Zhang.
Yujie Gao, Tiantian Huang(candice.huang@guildgemlab.com),Ziyun Zhang and Wenli LiaoGuild Gem LaboratoriesShenzhen, China
References
Eaton-Magana, S., Breeding, C.M., Palke, A.C., Homkrajae, A., Sun, Z. & McElhenny, G. 2021. Raman and photoluminescence mapping of gem materials. Minerals, 11(2), article 177, https://doi.org/10.3390/min11020177.
Liu, S.I, Man, K.Y.A., Seneewong-Na-Ayutthaya, M., Jakkawanvibul, C. & Lee, A.T.-Y. 2024. Geographic origin determination of high-quality green jadeite-omphacite jade (fei cui) from Myanmar, Guatemala and Italy using statistical processing coupled with spectroscopic and chemical analyses. Journal of Gemmology, 39(2), 124-144, https://doi.org/10.15506/JoG.2024.39.2.124.
Zhang, Y. & Shi, G. 2022. Origin of blue-water jadeite jades from Myanmar and Guatemala: Differentiation by non-destructive spectroscopic techniques. Crystals, 12(10), article 1448, https://doi.org/10.3390/cryst12101448.
