Study On The Gemological And Mineralogical Characteristic Of Blue Opal From Li Shui,Zhe Jiang

The Blue Opal near Daishi village in Jinyun,Lishui,Zhejiang,belongs to the ordinary opal,and its opal vein is in the fissure of the zeolite rock formed by the eruption of volcanic rocks.In this paper,the samples were tested by conventional gemological properties.The spectral characteristics were studied by means of Infrared spectroscopy(IR)and laser Raman spectroscopy(Raman).The phase composition and microscopic morphology were investigated by X ray powder diffraction(XRD)and observed by field emission Scanning electron microscopy(FE-SEM).In addition,we used the X-ray fluorescence spectroscopy(XRF)and electron microprobe analysis(EPMA)to analyzed chemical composition of samples,and the colour mechanism of the Blue Opal was judged by Ultraviolet-visible absorption spectroscopy(UV-VIS).The results show that the basic gemological properties of the Blue Opal in Li Shui,Zhe Jiang are in accordance with the opal minerals,their refractive index and specific gravity are larger,and the opal have a certain water absorption.The IR shows that the absorption spectrum of blue and white opals are very different from the amorphous opal.The absorption peaks are obvious,the degree of crystallization is higher,and a certain amount of water is contained.Raman confirmed that the blue and white parts are opals,and the dark yellow green part belongs to quartz mineral.The XRD image shows that the blue and White Opal diffraction peaks are sharp.The main components of the Blue Opal are ?-cristobalite,a small amount of ?-tridymite and quartz,with a high degree of crystallization,and overall belong to Opal-C type;the white opal is Opal-CT type,and most of the brown wall rock is the zeolite mineral.The FE-SEM observe that the Blue Opal part include Opal-A,Opal-CT and Opal-C type of opal.Opal-A is the internal regular arrangement and the Si O2 like spheroid with the size of 30?50nm.Opal-CT is the long strip of paralleled ?-tridymite.Opal-C is the Si O2 of a lump of micron to millimeter.The White Opal contains a small amount of fibrous palygorskite.The main chemical compositions of blue and white opal are Si O2 and a little Fe2O3 and Al2O3 by XRF.After the EPMA,the content of Si O2 in blue opal is 94?96%,which contains about 0.03% of Fe O;the Si O2 of white opal is about 95%,with almost void Fe.The brown wall rock is zeolite,and the water content of the zeolite is larger and contains feldspar,quartz and other impurities.The blue body color of samples is caused by Fe,which may be related to Ti.