老挝北部黄色图章石的矿物成分及微量元素特征

老挝北部黄色图章石(又称老挝北部黄料)呈明亮的黄色至褐黄色,多见水波纹状内部纹理,与我国图章石中的田黄外观特征具有一定相似性。本文运用X射线粉晶衍射(XRD)、红外光谱(FTIR)、拉曼光谱(LRM)、扫描电子显微镜(SEM)、电子探针(EPMA)、激光剥蚀等离子质谱(LA-ICP-MS)、全铁化学分析等测试方法对其矿物组成、微量元素及产地特征等进行了研究。结果表明,老挝北部黄料的主要组成矿物单一,为无序地开石,杂质矿物包括重晶石、锐钛矿、硬水铝石和赤铁矿等,其中地开石颗粒表面平整,呈板状紧密排列;萝卜纹主要呈枝状、短脉状零星分布在基体中;老挝北部黄料的黄色由Fe~(3+)所致,且颜色深浅与Fe~(3+)含量呈正相关关系,其Cr、Rb、Sr含量低于田黄,Ga含量高于田黄。通过多元统计判别分析法可将老挝北部黄料和田黄进行有效区分,其判别函数对未知样品进行验证的正确率可达92. 9%,且模型的可靠性较高。通过稀土元素配分模式图可以直观区分老挝北部黄料和田黄。

Mineral composition and trace element characteristics of Laos Northern Yellow Stone

Laos Northern Yellow Stone is bright yellow to dark brown in color, and the color distribution is uneven. Short thin turnip grains and wave-like texture are commonly seen. It has certain similarities with the appearance characteristics of Tian Huang in Chinese seal stone. In this paper, X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy (LRM), scanning electron microscopy (SEM), electron probe micro-analysis (EPMA), Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and whole-iron chemical analysis were used to study the mineral composition and trace element characteristics of Laos Northern Yellow Stone. According to the results of X-ray powder diffraction test and FTIR test, Laos Northern Yellow Stone is usually composed of pure dickite, which is disordered dickite. The Raman spectroscopic test shows that other minerals contained in Laos Northern Yellow Stone include barite, anatase, diaspore and hematite, in the form of impure minerals. The micromorphology of Laos Northern Yellow Stone observed by SEM indicates that the dickite in Laos Northern Yellow Stone is arranged in plate shape with a flat and sharp edge as well as a smooth surface. The turnip vein is mainly in the form of dendritic and short veins in the substrate of Laos Northern Yellow Stone. EPMA test and whole-iron chemical analysis show that Fe³⁺ causes the yellow color of Laos Northern Yellow Stone. The iron ion content shows a trend from high to low with the color changing from dark to light. The trace elements content of Laos Northern Yellow Stone was test by LA-ICP-MS. The content of Cr, Rb and Sr in Laos Northern Yellow Stone is lower than that of Tian Huang, and the content of Ga is higher than that of Tian Huang. Discriminant analysis method can effectively distinguish these two groups of similar species from Laos and China respectively. The correctness rate of the unknown sample in Laos Northern Yellow Stone and Tian Huang can reach 92.9% and the reliability of the model is high. According to the rare earth element distribution pattern, it is easy to distinguish Laos Northern Yellow Stone from Tian Huang.