Locality determination of inky black omphacite jades from Myanmar and Guatemala by nondestructive analysis

Abstract

AbstractLocality identification of high‐quality inky black omphacite jades (inky jades) from Myanmar and Guatemala is becoming increasingly urgent. In this study, Raman, Fourier transform infrared (FTIR), and energy‐dispersive X‐ray fluorescence spectroscopy (EDXRF) were performed to distinguish samples of inky jade from the two localities. The Raman spectra revealed the presence of several types of minor minerals in Guatemalan samples, including titanite, albite, orthoclase, taramite, celsian, and cymrite, whereas the Myanmar samples consisted almost entirely of omphacite. Taramite and lath‐like celsian occurred only in Guatemalan samples and had significance in locality tracing. The Raman and FTIR reflectance spectra of omphacite shifted to low wavenumbers in proportional to isomorphic substitution Na+Al3+ → Ca2+(Mg2+, Fe2+). Semiquantitative calculations based on the Raman (~680 cm−1) and IR modes (~658, ~576, and ~424 cm−1) showed consistent jadeite end‐member components (~43% to ~64%) in the Myanmar samples, whereas those (~23% to ~87.5%) of the Guatemalan samples were wider and generally lower. In addition, the full width at half maximum values (FWHMs) of Raman shifts near 680 cm−1 in the Myanmar samples (avg. 15.03 cm−1) were less than those (avg. 16.81 cm−1) of the Guatemalan specimens. Linear discriminant analysis (LDA) based on Raman spectroscopy identified most samples with accuracy of 85.5%. Combined with results by EDXRF, these analyses revealed the characteristics of a relatively narrow range of Na+Al3+ → Ca2+Mg2+ isomorphism for the Myanmar jades and a more developed Na+Al3+ → Ca2+(Mg2+, Fe2+) isomorphism for the Guatemalan jades. This study might have implications for commercial applications, archeological research, and judicial expertise.