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

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 visual evaluation.Factors causing the colour difference of Nanhong agates are attributed to the colour properties and distribution characteristics of hematite and goethite.The hue of Nanhong agate is dependent on the colour of the impurity minerals,hematite or goethite.Saturation is affected by the area ratio of impurity minerals.As the area ratio of impurity minerals is both affected by the distribution density and the average particle size,the saturation of Nanhong agate is the largest when the average particle size is near 10μm.No obvious correlation is observed between the brightness of Nanhong agates and their related properties and characteristics.

Clay mineralogy and its palaeoclimatic significance in the Luochuan Ioess-palaeosols over -1.3 Ma, Shaanxi, northwestern China

To understand climate changes recorded in the Luochuan loess-palaeosols, Shaanxi province, northwes- tern China, clay mineralogy was studied using X-ray diffraction （XRD）, high-resolution transmission electron microscopy （HRTEM）, and scanning electron microscopy （SEM） methods. XRD results show that clay mineral compositions in the Luochuan loess-palaeosols are dom- inantly illite, with minor chlorite, kaolinite, smectite, and illite-smectite mixed-layer clays （US）. Illite is the most abundant species in the sediments, with a content of 61%- 83%. The content of chlorite ranges from 5%-22%, and the content of kaolinite ranges from 5%-19%. Smectite （or US） occurs discontinuously along the loess profile, with a content of 0-8%. The Kfibler index of illite （IC） ranges from 0.255°-0.491°, and the illite chemical index （ICI） ranges from 0.294-0.394. The CIA values of the loess- palaeosols are 61.9-69.02, and the R3＋/（R3＋ ＋ R2＋ ＋ M＋） values are 0.508-0.589. HRTEM observations show that transformation of illite to illite-smectite has occurred in both the loess and palaeosol, suggesting that the Luochuan loess-palaeosols have experienced a certain degree of chemical weathering. The Luochuan loess-palaeosols have the same clay mineral assemblage along the profile. However, the relative contents of clay mineral species, CIA, ICI, and IC values fluctuate frequently along the profile, and all these parameters display a similar trend. Moreover, climate changes suggested by the clay index are consistent with variations in the deep-sea 6180 records and the magnetic susceptibility value, and thus, climate changes in the Luochuan region have been controlled by global climate change.