Spectroscopic Studies on Natural Gem Opal Hosted in Ethiopia: South Gondar

Abstract

The representative sample of gem-opal was collected from south Gondar mines and energy bureau. This sample was identified and characterized by different physical property measuring devices and advanced spectroscopic techniques. In order to verify that the sample was indeed opal a refractometer, heavy liquid method and "MOHS HARDNESS PENCILS" were used. The performed physical analysis shows that the hardness value is between 5.5- 6; the refractive index value is in the range between 1.437-1.445 and the specific gravity value is between 2.02-2.3; the measured values are comparable with literature data which verify that the samples were indeed opal. The contents of trace elements; (Mg, Cu, Mn, Ca, Fe and Zn) were analyzed by Flame Atomic Absorption Spectroscopy (FAAS) technique. The metal level observed in investigated sample is comparable with their corresponding worldwide report values. It is observed that the intensity of the orange color (from yellow to orange to red to "chocolate"-brown) correlates with the large amount of iron concentration and white color correlate for large amount of calcium concentration. The appearance of the light brown color on a sample is associated with small contents of Mn and blue color is associated with the presence of Cu, Mg and Zn silicate inclusion. The characteristic FTIR and XRD spectra data have significant contribution for the identification and characterization of the investigated sample. The FT-IR spectra analysis was characterized by spectra features of molecular water and silanol (Si-OH) groups, in which broad absorption bands centered around 3460 cm-1, due to the O-H stretching vibration of water molecules and water bending vibration occur around between 1640 cm-1-1625 cm-1. In the low frequency region from 2000 cm-1-500 cm-1the sample has three bands centered around 520 cm-1, 800 cm-1 and 1020 cm-1, these bands are due to the absorption of silicate(Si-O) frame work vibration. The performed XRD analysis show that the sample has five resolved peaks around 20.65°, 21.8°, 36.05°, 44.5° and 56.85° and has two weak peaks at ~28.5° and ~31.4° with 2Ѳ values between 10° to 70°. The five resolved peaks and the two weak peaks are characteristic peak positions for opal-CT and opal-C opaline minerals respectively. The results are comparable to literature data.