Fayalite Inclusion in Guatemalan Jadeite-Omphacite Jade
Figure 1: The jadeite-omphacite cabochon in this pendant (29.40 x 20.87 mm) was found to contain some interesting inclusions. Photo by Siqi Luo.
Jadeite-omphacite jade (fei cui) enjoys immense popularity-especially in the Chinese market-and in recent years material sourced from Guatemala has seen a marked surge in demand.
A jade pendant was recently submitted by a reliable Guatemalan jade supplier to Guild Gem Laboratories for provenance analysis (Figure 1). The client asserted that the specimen originated from Guatemala. The Raman spectral features were consistent with a combination of jadeite and omphacite. The stone exhibited a richly saturated green hue with even colour distribution, and microscopic examination revealed a fine-grained texture punctuated by several tiny, dark, irregular inclusions-a characteristic that is diagnostic of Guatemalan jadeite (Zhen et al. 2023).
Figure 2: This surface-reaching dark inclusion in the jadeite-omphacite was identified as fayalite with amorphous carbon. Photomicrograph by Kaiyin Deng; image width 2.0 mm.
Figure 3: The Raman spectrum obtained from the dark inclusion in Figure 2 is consistent with features characteristic of fayalite. In addition, peaks due to amorphous carbon are present at about 1330 and 1570 cm-1.
One of the dark inclusions reached the surface (Figure 2), and its Raman spectrum exhibited features of the olivine-group mineral fayalite (Fe2+2 SiO4, which forms a solid solution with forsterite, Mg2SiO4). The Raman spectral characteristics included four lattice-mode peaks at approximately 106, 137, 168 and 196 cm-1, a pronounced SiO4 tetrahedral bending peak at 448 cm-1, and a high-intensity Si-O stretching doublet at 807 and 835 cm-1 (Figure 3). The low-frequency features arise from translational and rotational vibrations of the (Mg,Fe)-O framework, and the 448 cm-1 peak reflects the v2 bending mode of the SiO4 tetrahedron-both hallmarks of olivine structure (Chopelas 1991). The prominent doublet corresponds to the symmetric (v1) and asymmetric (v3) Si-O stretching modes, which decrease in position with increasing Fe content. While Mg-rich forsterite shows this doublet near 824/857 cm-1, its observed positions at 807/835 cm-1 indicate a composition close to the fayalite end member (Chopelas 1991; Kuebler et al. 2006). Moreover, the peak widths and relative intensities align with trends documented for Fe-Mg olivine solid solutions (Kolesov & Geiger 2004), confirming that this inclusion is Fe-dominant (fayalite-rich) olivine.
Part of the dark inclusion was locally carbonised under the Raman laser, giving rise to broad bands at about 1330 and 1570 cm-1 characteristic of amorphous carbon (again, see Figure 3). This observation suggests that the inclusion was not a single phase but rather a composite aggregate in which fayalite-rich olivine predominated.
Non-destructive investigations of such inclusions-as demonstrated here-can deepen our insight into the genesis of Guatemalan jadeite-omphacite jade and help with provenance testing in the gemmological laboratory.
Tiantian Huang, Yujie Gao (peter.gao@guildgemlab.com) and Wenli Liao
Guild Gem Laboratories
Shenzhen, China
References
Chopelas, A. 1991. Single crystal Raman spectra of forsterite, fayalite, and monticellite. American Mineralogist, 76(7-8), 1101-1109, http://www.minsocam.org/ammin/AM76/AM76_1101.pdf.
Kolesov, B.A. & Geiger, C.A. 2004. A Raman spectroscopic study of Fe-Mg olivines. Physics and Chemistry of Minerals, 31(3), 142-154, https://doi.org/10.1007/s00269-003-0370-y.
Kuebler, K.E., Jolliff, B.L., Wang, A. & Haskin, L.A. 2006. Extracting olivine (Fo-Fa) compositions from Raman spectral peak positions. Geochimica et Cosmochimica Acta, 70(24), 6201-6222, https://doi.org/10.1016/j.gca.2006.07.035.
Zhen, C.L., Gao, Y.J. & Li, K. 2023. Identification characteristic of green feicui from Myanmar and Guatemala. Journal of Gems & Gemmology, 25(5), 17-28.
