Study On The Color Genesis And Structural Characteristics Of Nanhong Agate From Gansu Province

Nanhong agate is a kind of cryptocrystalline quartz jade mainly with red color.At present,the research on Nanhong agate is mainly concentrated in the two producing areas of Baoshan in Yunnan Province and Liangshan in Sichuan Province.However,it is rarely discussed in Nanhong agate from Gansu Province.In order to explore the gemmological and mineralogical characteristics of Gansu Nanhong agate,and then to analyze its structural characteristics and color formation mechanism,the samples were examined by conventional gemological testing,gemological microscope and polarizing microscope,infrared spectroscopy and ultraviolet-visible spectroscopy,Raman spectroscopy,X-ray diffraction(XRD),X-ray fluorescence spectroscopy(XRF),Scanning Electron Microscope(SEM),cathodeluminescence under scanning electron microscopy(SEM-CL).The study found that the color of Gansu Nanhong agate are mainly dark red or orange-red,and some samples show colorless,white or yellow bands.The relative density is in the range of 2.59~2.65.And the matrix of the samples mostly appears as long or short fibres,partly showing as fine grains or macrocrystalline quartz,with concentric ring-shaped,horizontal band-like or vein-like structures.The main mineral composition is ?-quartz,with a small amount of water.And most of the fibrous matrix also contains moganite,with a content of approximately 31 to 44%;but the matrix of macrocrystalline quartz and fine grains does not contain or contain a very small amount of moganite.The diffraction tracings of 68° quintuplet for the samples are sharp,and the crystallinity index is roughly in the range of 5.95 to 7.38.In addition,XRF test results show that the samples also contain some trace elements such as Fe,Al,Ca,K,Mn,Cr,etc.Under SEM,the microscopic morphology of the matrix mainly presents as subspherical grains,idiomorphic-hypidiomorphic grains and flakes,and all have obvious pores.Under the cathode ray,except for the quartz vein in individual samples presents medium-intensity purple luminescence,most samples have no obvious luminescence.It is speculated that the matrix was formed in a low-temperature environment,and the quartz vein was formed by the filling of high-temperature liquid along the crack in the later period and then cooling and crystallizing quickly.The red color of Gansu Nanhong agate is mainly caused by inclusions.Under the polarizing microscope,it can be seen that the inclusions appear in the form of spherical grains,irregular grains,and flakes.Raman spectroscopy shows that the main component of the inclusions is hematite,and some of them also contain a small amount of goethite and lepidocrocite.The first-order derivative spectra of the UV-Vis absorption spectra also shows characteristic peak of hematite at 586 nm.Based on previous studies,it is speculated that the color formation mechanism is mainly due to endogenous origin,and the process is roughly as follows: The Si-rich hydrothermal fluid extracts iron-bearing minerals from the surrounding rocks.Later,under the influence of conditions such as temperature and pH,the conversion between iron oxides and oxyhydroxides occurs,and inclusions dominated by hematite eventually form.