New insights into chemical and spectroscopic characterization of beryl mineralization related to leucogranites in the west Wadi El Gemal area, southern Eastern Desert of Egypt

Abstract

Our combined mineral chemistry and spectroscopic studies enabled us to envisage details about composition and genesis of beryl from the Eastern Desert of Egypt. Two types of beryl were investigated including emerald from ancient workings in the schist-related type whereas the other type is non-gem and granite-related. On the basis of FeOt-Cr2O3-MgO ternary classification, the Wadi El Gemal emeralds (alkali-type, sodic) are Mg-bearing while the Mikpid-Akarem beryls (normal-type) are Fe2+-bearing. Intense green colour of emerald results in a 323 cm−1 Raman shift due to ∼0.77 wt% Cr22O3 derived from serpentinite. In the Mikpid-Akarem district (belonging to the granite-related type), beryls are either faint blue or white and lack Cr, Mg and AlIV. On the other hand, they are relatively enriched in Fe and AlVI compared with the Wadi El Gemal emeralds (up to 0.13 & 1.97 apfu and 0.04 & 1.68 apfu, respectively), which is a function of the peraluminous composition of the granite host. Al3+ is substituted in the Y-site by di- to tetravalent cations (Mg2++Fe2++Cr3++Ti4+) and this is paired with a vacancy substitution by Na+. The Wadi El Gemal emeralds have H2O & K+ in the 2a-site and H2O & Na+ in the 2b-site. The latter site is vacant in the Mikpid-Akarem beryls.The FTIR spectra of our beryls and emeralds show a series of stretching and bending vibrations due to the presence of type I H2O (OH-Na band at 3657 cm−1) and type II H2O (stretching at 3591–3657 cm−1 & bending at 1631 cm−1). Lacking of Li+ leads to the absence of OH-Li band that is indicative of type III H2O. Hypothetical H2O in the Wadi El Gemal emerald (green beryl) amounts 2.24–2.92 wt% whereas it amounts 1.21–1.25 wt% in the Mikpid-Akarem non-green beryl. The asymmetric stretching vibration at 2921–2923 cm−1 is related to CH due to the presence of CH4, and possibly CO2, in the fluid inclusions trapped in the analyzed crystals. Some beryls, e.g. the Um Kabu emerald, has a Raman shift at 672 cm−1, which indicates maximal cation substitutions in the octahedral sites and minimal H2O in the channel sites. An uncommon shift at 147 cm−1 can be attributed to defect structure due to presence of radioactive zircon and monazite inclusions. Fe2+ substitution in the octahedral sites is appreciable and results in an UV–Vis-NIR band at 836 nm.