Chemistry of Opal from Ethiopia

In 2008, a new opal deposit was discovered near Welo Province, Ethiopia. The opals from Ethiopia found in volcanic rocks with vivid play-of-colour are very different from those opals from other localities. LIBS, XRF, FTIR and TG—DSC were used to study the chemical composition and the water speciation in opal from Ethiopia. Comparing with opals from Australia and Mexico, research indicates that the concentrations of Ca, Sr and Ba in opal from Ethiopia are higher than those from other localities, which can be used as an indicator of opal from Ethiopia. The infrared spectrum of opal from Ethiopia shows a peak at 791 cm^-1. It indicates that opal from Ethiopia is not completely amorphous. This feature is the same as Mexican opal with the similar geological origin, and is different from Australian opal with epigenetic origin. Opal from Ethiopia, Australia and Mexica show the same TG-DSC testing result, and two obvious endothermic valleys (80-100 ℃ and 200-600 ℃) indicate the loss of absorbed water and crystal water and structurally bounded respectively. The content and these two types of water show no difference in opals from all three localities.