Structural Characteristic and Cause of Colour of “Zhanguohong” Agate from Xuanhua, Hebei Province

“Zhanguohong” agate from Xuanhua with red and yellow colour is a kind of agate decorated with colourful bands, originating in Dishuiya and Wangqianhu district, Xuanhua, Hebei province. In this paper, “Zhanguohong” agates from Xuanhua were chosen as the research object by using modern testing techniques such as polarizing microscope, environmental scanning electron microscope and Raman spectrometer. The cause of colour and structural characteristics of the samples were studied. According to polarizing microscope, the structure of samples showed long fibrous structure, short fibrous structure, sphaerulitic structure and macrocrystalline quartz. Scanning electron microscope images displayed that the grain size is 0.5—5.0 μm, and different grain sizes present sheet structure of close packing. EDS result demonstrated that the main composition of sample is SiO₂ including Fe, Al, Mg, Na, K, Ca. With the help of Raman spectrometer, the characteristic peaks of hematite could be detected at red stripe and the typical peaks of goethite can be detected at yellow stripe. Besides, heat treatment was conducted at 200, 300, 400 ℃. As a result, The transparency of “Zhangguohong” agate was reduced and its surface became very dry after heat treatment. With the increase of heating temperature, yellow stripes turned red, and above 400 ℃ the yellow colour disappeared, which is caused by lack of water and goethite changing into hematite. In conclusion, the red colour of “Zhangguohong” agate is mainly caused by hematite and the yellow colour is mainly caused by goethite.