Analysis Of Gemological And Petrological Characteristics Of Nanhong Agate From Liangshan

Nanhong Agate is general designation referring to the natural red agates from southwest China in recent years, which is a kind of moist rich agates with red color. Based on the achievements of previous researches, this thesis analyzes the gemological characteristics, chemical compositions, mineral composition, structures and constructions of Nanhong Agate from Meigu county of Liangshan prefecture in Sichuan province, and discusses the causes by means of the conventional gemological test, microscopic observation, X-ray fluorescence spectroscopy, electron probe microanalysis and laser raman spectroscopy.The observation tests show that the conventional gemological characteristics of Nanhong Agate are similar to those of ordinary agates, but there are differences. Nanhong Agate is sub-translucent to translucent with glassy-lipidic luster, forming in fracture development. Its color is light red, red-orange, red-purple and red. Its index of refraction is 1.54, with a relative density from 2.62 to 2.65 and hardness from 6 to 7.The observations under polarizing microscope reveal that Nanhong Agate is cryptocrystalline aggregation. In the center of some sample, crystalline quartz is obsered. The common textures of Nanhong Agate are cryptocrystalline texture, fibrous structure, radiated fibrous structure and granular structure. The analyses by infrared spectroscopy test show that the structure of Nanhong Agate is a transitional structure from aphanitic chalcedony to phanerocrystalline quartz.The data from X-ray fluorescence spectroscopy and electron microprobe indicate that the chemical compositions of Nanhong Agate contain Si and O, in addition to a small amount of Fe, Cr, Mn. Moreover, it is also found that there is a positive correlation between the concentration of red and saturation and the mass franction of Fe. The main mineral component of Nanhong Agate is quartz,and the secondary components are largely hematite which can also count for the red color of Nanhong Agate.The color of Nanhong Agate in Liangshan area can divide into the primary color and the secondary color. After the comprehensive analyses on the basis of Petrography, Mineral chemistry and Rock Geochemistry, it can be discovered that the color of Nanhong Agate from Liangshan is affected by mineral, and meanwhile the distribution of colorgeneous mineral has been influenced by the structure of Nanhong Agate. Hence, a conclusion can be drawn that the color of Nanhong Agate from Liangshan is affected simultaneously by elements, minerals and structure.The lipidic luster of Nanhong Agate is related to its main mineral particles in terms of the shape, size, way of arrangement, degree of tightness and refractive index. When the fibrous quartz is arranged closely in an orientated direction, with more hematite content, the lipidic luster is more distinctive. When the shape of minerals turns from the granular to the columnar and then to the fibrous form, the voids among the particles get smaller and smaller, and Nanhong Agate will be more transparent; otherwise, it will be less transparent. In addition, the transparency also has been affected by the content of hematite. That is, the more hematite content there is, the less transparent Nanhong Agate will be.