Historiography and FTIR spectral signatures of beryl crystals from some ancient Roman sites in the Eastern Desert of Egypt

Beryl (mostly emerald) was mined in Egypt since Antiquity, particularly the so-called “Cleopatra’s Mines” in the Eastern Desert were a major source of emerald in the world during the Roman era in northeast Africa. Emerald from the Nugrus belt is located at Sikait, Um Sleimat, Um Addebbaa and Um Kabu areas where the ancient mines are confined structurally and genetically to “schist-type” deposits as a part of the Neoproterozoic ophiolitic mélange. At Homret Mikpid and Homr Akarem occurrences, non-gem beryl occurs in greisinized granites, pegmatites and quartz veins. Fourier transform infrared (FTIR) spectroscopy is able to distinguish Egyptian emeralds from non-gem beryls. Different types of H₂O molecules and OH-groups can be identified in all especially at ~ 3435–3480 cm⁻¹ as an indication of X-site occupancy of Na and vacancy (□) and to the Y-site M²⁺ and M³⁺ cation distribution. High wavenumber bands > 3600 cm⁻¹, e.g. at ~ 3865–3988 cm⁻¹ are characteristics of non-gem beryl only. Structural CO₂ molecules are seen in both emerald and beryl at ~ 2360 cm⁻¹. About 97–99% (at a wavelength range of 335–611 nm.) infrared reflectance of non-gem beryl is much higher (97–99%) whereas it never exceeds ~ 88% (at ~ 360 nm) in emerald.