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Blue Opal From Peru

Qi Lijian 1 Yang Meizhen 1 Hu Yongbing 1 Lu Yongqing 2 (1 Gemmological Institute, China University of Geosciences, Wuhan, 430074, China; 2 Yongqing Gemstone Company, Taibei, Taiwan, China)  
Blue opal from Peru is a new variety of gemstone. The comprehensive research on the gemmological features of this kind of blue opal shows that it is composed mainly of opal and minorly of α christobalite and α tridymite. At high temperature (1100℃), the opal, α christobalite and α tridymite in the blue opal are thermally transformed into β tridymite. The water in the blue opal occurs mainly in the form of planar water and structural water, with its Cu 2+ impure ions tend to combine with H 2O molecules into [Cu 2+ (H 2O) 4] 2+ hydrated ions. The presence of 761nm 2E g→ 2T 1g spin permitted transition absorption band in the octagonal crystal field indicates that the colouring of the blue opal is attributed mainly to [Cu 2+ (H 2O) 4] 2+ hydrated ions.
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