光谱学与光谱分析, 2020, 40 (8): 2531, 网络出版: 2020-12-03  

北红玛瑙的颜色成因及光谱学特征研究

Color and Genesis of Beihong Agate and Its Spectroscopic Characteristics
作者单位
中国地质大学(北京)珠宝学院, 北京 100083
摘要
在我国东北大、 小兴安岭南段, 广泛产出一种外观以红色为主, 常带有黄色调的半透明玛瑙, 被称为“北红玛瑙”。 通过偏光显微镜观察、 X射线粉晶衍射分析、 拉曼光谱、 紫外-可见光吸收光谱、 岩石化学全分析对北红玛瑙及黄色、 白色对比样品的物相组成、 化学组成和光谱学特征进行了测试分析。 结果显示, 北红玛瑙的主要矿物组成为α-石英, 次要矿物组成为斜硅石和针铁矿, 另外还含有极少量的赤铁矿。 北红玛瑙的红色、 黄色与针铁矿和极少量的赤铁矿有关, 与南红玛瑙主要为赤铁矿致色明显不同。 显微观察时, 针铁矿、 赤铁矿以橙红色点状和浸染状两种形式存在, 其中点状分布的针铁矿、 赤铁矿大小约10 μm, 但不具备明显的单晶晶体形状, 推测是由亚微米级大小的针铁矿及极少量的赤铁矿聚集形成的集合体; 浸染状分布的橙红色针铁矿、 赤铁矿不可见矿物颗粒大小, 推测其颗粒大小和点状分布的致色矿物类似, 均为亚微米级大小, 但与点状分布的致色矿物不同的是, 并未聚集形成显微镜下可见的点状集合体。 整体上外观呈红色的北红玛瑙中针铁矿、 赤铁矿的含量多于黄色对比样品中的致色矿物含量, 致色矿物的含量会直接影响北红玛瑙的颜色色调。 紫外-可见光吸收光谱中, 针铁矿中Fe3+的6A1→4E, 6A1→4E4A1, 2(6A1)→2(4T1)(4G), 6A1→4T2(4G)和极少量赤铁矿中Fe3+的6A1→4E4A1, 6A1→4T2吸收带与针铁矿、 赤铁矿中O2-和Fe3+之间的电荷转移共同作用, 形成了北红玛瑙带黄色调的红色外观。 紫外-可见光吸收光谱的一阶导数谱中, 可见光范围内北红玛瑙的一阶导数谱极小值的位置为555和556 nm, 黄色对比样品为530 nm, 淡黄色对比样品为502 nm, 随着玛瑙红色调的逐渐减少, 可见光范围内一阶导数谱的极小值位置也逐渐减小, 可以据此衡量红色~黄色玛瑙的色调深浅, 这也对石英质玉石的品种鉴定及颜色分级具有重要参考意义。
Abstract
The Beihong agate is commonly known as agate with the translucent and yellowish-red appearance and is widely produced in the southern section of Xing’an Mountains. In this paper, the phase composition, chemical composition and spectroscopic characteristics of Beihong agate and its yellow and white control samples were investigated by polarizing microscope observation, X-ray powder diffraction, Raman spectra, UV-VIS absorption spectra and whole rock chemical analysis. The results show that Beihong agate is composed mainly by α-quartz, with moganite, goethite as the minor mineral composition, a very small amount of hematite can also exist in Beihong agate. The yellowish-red appearance of Beihong agate is related to goethite and a very small amount of hematite, which is different from the coloration mechanism of Nanghong agate by hematite. Based on distribution pattern, goethite and hematite can be divided into scattered and disseminated forms. The size of goethite and hematite with scattered distribution is about 10 μm, but they possess amorphous morphology, it is presumed that the scattered minerals were the aggregates of goethite and hematite crystal at the submicron scale. In addition, the disseminated goethite and hematite exhibit invisible pattern, which are similar to the scattered minerals and goethite and hematite crystal that are all sub-micron in size, but they do not aggregate to form microscopic scattered aggregates. On the whole, the content of goethite and hematite in Beihong agate is higher than that in the yellow control sample, as the content of goethite and hematite increases, the hue of Beihong agate changes from yellowish-red to red. The Beihong agate exhibited 6A1→4E, 6A1→4E4A1, 2(6A1)→2(4T1)(4G), 6A1→4T2(4G) electron transitions of goethite and 6A1→4E4A1, 6A1→4T2 electron transitions of hematite and the charge transfer between O2- and Fe3+, the synergy of electron transition in the act of the crystal field and charge transfer produce significant yellowish-red color. In the first derivative spectra of UV-VIS absorption spectra, the minimum value position of Beihong agate, yellow and light-yellow contrast samples are 555~556, 530 and 502 nm, respectively. As the red tone of agate decreases gradually, the position of the minimum value in the visible range of the first derivative spectrum decreases gradually. It can be used to characterize red to yellow hue of agate, which is also significant for the variety identification and color grade of quartz jades.

鲁智云, 何雪梅, 郭庆丰. 北红玛瑙的颜色成因及光谱学特征研究[J]. 光谱学与光谱分析, 2020, 40(8): 2531. LU Zhi-yun, HE Xue-mei, GUO Qing-feng. Color and Genesis of Beihong Agate and Its Spectroscopic Characteristics[J]. Spectroscopy and Spectral Analysis, 2020, 40(8): 2531.

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