Structural Characteristic and Cause of Colour of “Zhanguohong” Agate from Xuanhua, Hebei Province
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摘要: 宣化“战国红”玛瑙是指产于河北省张家口市宣化县塔儿村乡滴水崖、王千户一带的缟玛瑙，主要颜色为红色和黄色。以宣化“战国红”玛瑙原石为研究对象，利用偏光显微镜、环境扫描电子显微镜和拉曼光谱等测试技术对其结构特征进行测试与分析，并结合热处理实验探讨其颜色成因。宣化“战国红”玛瑙在正交偏光显微镜下主要有长纤维状结构、短纤维状结构、球粒状结构和晶质石英等。扫描电子显微镜图像显示晶粒大小约为0.5～5.0 μm，晶粒呈大小不一的片状紧密堆积。能谱仪测试表明宣化“战国红”的主要成分为SiO2，并含有Fe、Al、Mg、Na、K、Ca等元素。结合拉曼光谱测试可知，红色条带可见较强的赤铁矿吸收峰，黄色条带可见较强的针铁矿特征吸收峰。对“战国红”玛瑙在200、300、400 ℃温度下进行热处理，热处理后玛瑙透明度下降，表面干涩，随着加热温度的升高，黄色条带中红色调越来越明显，400 ℃时黄色完全变成红色，这是由针铁矿失水转化为赤铁矿所致。结合拉曼光谱测试与热处理实验可知宣化“战国红”玛瑙红色主要由赤铁矿致色，黄色主要由针铁矿致色。Abstract: “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.
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Keywords:
- “Zhanguohong” agate /
- structure characteristic /
- cause of colour /
- heat treatment /
- Raman
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