Formation, Identification and Colour Grading of Nanhong Agates from the Southwestern of China  

作　　者：Chaowen Wang Huimin Huang Haotian Wang Lingyue Yang 王朝文

机构地区：[1]Gemmological Institute,China University of Geosciences,Wuhan 430074,China [2]Hubei Provincial Jewelry Quality Supervision and Inspection Center,Hubei Geological Research Laboratory,Wuhan 430034,China [3]School of Earth Sciences,China University of Geosciences,Wuhan 430074,China

出　　处：《宝石和宝石学杂志（中英文）》2024年第S01期133-134,共2页Journal of Gems & Gemmology

基　　金：国家自然科学基金(42072056);中国地质大学(武汉)珠宝学院科研基金项目(CIGTXM-03-202306)。

摘　　要：Nanhong agate is widely welcomed in China because of its bright red colour.The pursuit of the red agate is the inheritance of Chinese red agate culture,which can be dated back to more than 3000 years ago.Field survey showed Nanhong agate geographically distributed in Yunnan and Sichuan provinces,the Southwestern of China(Fig.1).Geologically,Nanhong agate can be either produced as fillers in cracks and pores of the Late Permian E’meishan basalts,or as pebbles in the Early Triassic conglomerates and mudstones.Petrographic analyses reveal the formation of agates could be linked to the alteration of basalts by the silicon releasing in the process of transformation from pyroxene to chlorite.The formation of Nanhong agates could have experienced multi-stage alterations and crystallizations,which are supported by the occurrence of multi-stage chlorites and different homogenization temperatures of fluid inclusions bearing in the coarse grained quartz.The Nanhong agates are characterized by diffusely distributed hematite and/or goethite,causing the red to reddish orange in colour.These iron oxides occurre as aggregates showing dominantly spherical shapes and minor visible ellipsoid,hemisphere,radial and veil shape,with a size generally between 1.20μm and 43.21μm.Raman spectra,scanning electron microscopy,and high resolution transmission electron microscopy confirmed the present of hematite with single hematite particles smaller than 100 nm.Wide market survey of the selling Nanhong agates presented the quantified colour parameters,including hue,saturation,and brightness,obeying normal distribution in the Hue-Saturation-Brightness(HSB)colour space.After standardization,the hue(H)of Nanhong agate is mainly distributed in a range between[3.20°,17.52°]at a 95%confidence level.The saturation(S)and brightness(B)are mainly within[63.82%,92.69%]and[42.97%,75.37%],respectively.The Nanhong agates could be statistically classified as different grades by the Two Step Cluster analysis,which is objective compared to subjective vis

关 键 词：E’meishan basalt HEMATITE HSB Nanhong agate Yunnan Province 

分 类 号：TS933[轻工技术与工程] P619.28[天文地球—矿床学]