New insights of emerald geographic origin determination based on the infrared spectroscopy of D2O and HDO molecules

Abstract

Emerald geographic origin determination in laboratories typically relies on the microscopic features of inclusions, ultraviolet through visible and near-infrared (UV-Vis-NIR) spectroscopy, and trace element chemistry measured by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS). Infrared (IR) spectroscopy, a non-destructive technique, has played a minor role in this process. This work conducts a systematic examination of emerald samples from twelve origins, encompassing spectroscopic, and trace element analyses. For the first time, IR absorptions related to D2O and HDO molecules within the 2600–2850cm–1 range for from multiple origins (333 emerald samples) were recorded, assigned, and classified. These IR absorptions, controlled by the concentrations of deuterium (D) and alkali metals (primarily sodium) within channels, reveal three distinct patterns and derived four groups that serve as strong evidence for origin determination. Emeralds from Zambia group display the most prevalent HDO-dominant IR-pattern I characterized by the pronounced absorption peak of Type II HDO at 2672cm–1; Panjshir (Afghanistan) and Swat (Pakistan) emeralds exhibit the transitional type IR-Pattern II with overall five peaks and obvious 2808cm–1 peak; D2O-dominant IR-Pattern III is featured by marked absorption band at 2740cm–1, and can be further subdivided into two subtypes (IIIa and IIIb). IR-Pattern IIIb is currently only found in Nigerian emeralds, characterized by strong broad band of Type I D2O due to enriched deuterium and obvious absorption of Type I HDO at 2685cm–1 due to depleted sodium. Furthermore, this research updates the classification of UV-Vis-NIR spectra, unveils the trace element features, and presents effective compositional diagrams. This contribution enriches and expands existing techniques and provides new insights for emerald origin determination.