Study On Gemmological And Mineralogical Characteristic Of Nanhong Agate From Liangshan,Sichuan Province

Nahong agate from Liangshan, Si Chuan Province, is a volcanic mineral which is divided into Lianhe and Jiukou by locality. Main colors are red with different tonal, white and colorless. In the present, Nahong agate is investigated by Microscope, electron microprobe, XRD and so on to reveal its component, texture, the origin of color and the formation mechanism etc.Nanhong agate is a cryptocrystalline-microcrystalline quartz mineral. Silicon dioxide is main exist as α-Si O2 with Raman spectrum show 465cm-1. The 501cm-1 proved moganite is also existed. Observations under microscope show different textures: fine grains, short fibres, long fibres and macrocrystalline quartzs.The iron oxides are detected by Electron microprobe and Raman shifts 224, 290, 410 and 1318cm-1 prove they are Hematite.Hematite is main mineral induce the color of Nahong agate with two kind forms: 1) as rounded suspend in quartz grains; 2) as cryptocrystalline enter into quartz grains. Former is the origin of color from Lianhe. Color shading depends on the intension and sizes of Hematite. The latter is the origin of color from Jiukou. The content of hematite and color saturation were positively correlated.The formation of cryptocrystalline hematite has two periods: syngenetic and epigenetic.There are three points to distinguish Nahong agate from Linhe to Jiukou: Color, Transparency and Inclusions. Agate from Lianhe has Low degree of saturation and rounded hematite which can be observed by eyes, transparent to translucence. Agate from Jiukou has high degree of saturation and low transparency, and cannot be found rounded hematite.The formation of Nanhong agate from Sichuan has multi-stages: the hydrothermal flowed into the cavity, and form the first layer along the wall. Then begin to form radial chalcedony and long fibre chalcedony until the content of Silicon in hydrothermal declined. Finally, the center became a hole or form macrocrystalline quartzs. If there are later stage hydrothermal entered the center of the first-growth stage, the second-growth stage would begin. Due to the gravity effect, the second stage may form horizontal layers during the growth process.