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.
