阿拉善红玛瑙的的宝石学特征及颜色成因研究

Gemological Characteristics and Color Origin of Alashan Red Agate

本文以内蒙古西北戈壁区的阿拉善红玛瑙为研究对象。通过常规宝石学测试、偏光显微镜观察、红外光谱、紫外—可见光光谱、拉曼光谱以及X射线荧光光谱测试,对14块样品的宝石学特征及其颜色成因进行了研究。结果表明,阿拉善红玛瑙呈橙黄色至深红色,半透明至不透明,玻璃光泽,紫外荧光惰性,摩氏硬度6.42~6.93;主要矿物为石英和斜硅石,次要矿物为赤铁矿和极少量的针铁矿;内部结构包含细粒状结构、纤维状结构以及显晶质结构;XRF结果显示阿拉善红玛瑙内部的主要致色元素是Fe;红色区域的拉曼光谱显示292cm-1和1320cm-1赤铁矿的特征峰,紫外—可见光光谱一阶导数图谱显示红色样品在575nm附近出现特征峰,橙黄色样品在435nm、540nm附近出现特征峰,说明致色矿物为赤铁矿和极少量的针铁矿。致色方式有两种:一种为赤铁矿和针铁矿球粒状集合体致色,包裹体均匀分布在石英颗粒间,大小不一,矿物颗粒越大、分布越密集,玛瑙的红色越浓郁,部分颗粒较大的赤铁矿包裹体密集分布在玛瑙后期生成的内部裂隙中;另一种为隐晶质赤铁矿呈浸染状分布致色。

In this paper, red agate of Alashan from the Northwest desert of Inner Mongolia was selected to be studied. 14 samples were investigated by gemological tests,polarizing microscope observation, infrared spectrum, UV visible spectrum, Raman spectrum and X-ray fluorescence spectrum tests to obtain their gemological characteristics and their coloration. The results show that red agate of Alashan is orange yellow to crimson, translucent to opaque,vitreous luster, UV fluorescence inertia, Mohshardness 6.42~6.93. Agate’s main mineral compositions are quartz and moganite, with hematite and minor amount of goethite. The internal structure includes mainly fine-grained, fibrous and crystalline. The main coloring element is Fe which measured by XRF. Raman spectra of red area show the characteristic peaks of 292 cm-1 and 1320 cm-1 hematite. The first derivative spectra of UV-Vis spectra show that the red sample has a characteristic peak near 575 nm, and the orange sample has a characteristic peak near 435 nm and 540 nm, which indicates that the chromogenic minerals are hematite and goethite. There are two ways to cause color: one is the granular inclusions of hematite and goethite with different sizes evenly distributed among quartz grains. The larger the mineral particles are, the denser the red color of agate is. Some larger hematite inclusions are intensively distributed in the internal fissures formed in the later stage of agate. The other way is cryptocrystalline hematite distributed in disseminated form color.