Application of infrared external reflection spectroscopy in the identification of emerald provenance

Throughout the history, emerald has always had an economic and religious importance, as evidenced by its widespread diffusion even far from its mines. Emerald belongs to beryl-group minerals with a general formula Be3Al2Si6O18, where tetrahedra form six-member rings filled by Si, whereas Al and Be ions are located at the octahedral and tetrahedral sites, respectively, which link together the columns of rings of six Si-O tetrahedra. The green color is due to the presence of Cr and/or V which replace Al at the octahedral site (Aurisicchio et al., 2018). Recent studies have proved that spectroscopies can be a useful non-invasive approach in the identification of emerald provenance. In particular, thanks to the shape and position of vibrational modes associated with the Si-O stretching (main peak ~1200 cm-1 and shoulder at ~1140 cm-1) and the bands associated with the OH stretching mode of water (in the range 3500-3800 cm-1) of FTIR spectra, it could be possible define the genetic model for each deposit (Giuliani et al., 2019) and consequently their provenance. The present work reports for the first time the results of infrared external reflection spectroscopy on emerald samples from the main deposits exploited in antiquity such as Malyshevsk (Russia), Jos (Nigeria), Pajshir Valley (Afghanistan), Habactal (Austria), Sikait, Zabara (Egypt) and Swat Valley (Pakistan). The results are compared with FTIR spectra acquired in transmission mode to identify spectroscopic “markers” which could help in the reconstruction of the provenance.