南流江玉的宝石矿物学特征及致色成因

Gemological and Mineralogical Characteristics and Coloration Mechanism of Nanliujiang Jade

摘要: 南流江玉是产自广西合浦南流江流域的特色石英质玉石，主要分为带红色环带的微透明品种和草花状纹理不透明品种，近年来凭借其独特的外观备受收藏界关注。本文采用偏光显微镜、折射仪、偏光显微镜、傅里叶变换红外光谱仪、激光拉曼光谱仪及紫外–可见光光谱仪等测试方法对南流江玉的矿物组成及致色成因开展研究。研究结果表明，南流江玉的矿物为α-石英，有少量赤铁矿和针铁矿，整体呈致密块状构造。为了进一步研究其矿物组成和致色成因，对其微观结构进行观察，两类样品的石英颗粒形态存在显著差异，其中红色环带型样品的石英呈纤维状结构，纤维长度为30~300 μm；草花型样品的石英则呈粒状等粒结构，晶粒大小为10~100 μm。红外光谱显示1184 cm⁻¹、1105 cm⁻¹、796 cm⁻¹、779 cm⁻¹、688 cm⁻¹、533 cm⁻¹、478 cm⁻¹等特征吸收峰与α-石英的标准特征峰高度吻合。激光拉曼光谱分析表明，红色环带型样品在498 cm⁻¹处出现赤铁矿(α-Fe₂O₃)的特征峰，同时在237 cm⁻¹、405 cm⁻¹处检测到针铁矿(α-FeOOH)的特征峰，证实其由赤铁矿与针铁矿共同致色；草花形样品则在238 cm⁻¹、545 cm⁻¹、683 cm⁻¹出现特征峰，其致色矿物以针铁矿与纤铁矿(γ-FeOOH)为主，赤铁矿(α-Fe₂O₃)为次要致色矿物。紫外–可见光光谱显示，所有样品均在380 nm、450 nm处出现Fe³⁺的特征吸收峰，证实了铁离子为南流江玉的核心致色离子。本文系统阐明了南流江玉的宝石矿物学特征与致色机制，其属典型的矿物致色型石英质玉，不同外观类型样品的致色矿物组合及微观结构存在显著分异。

Abstract: Nanliujiang jade is a characteristic siliceous jade produced in the Nanliujiang River basin of Hepu, Guangxi Zhuang Autonomous Region, which is mainly divided into two varieties: a slightly transparent type with red zonal bands and an opaque type with flower-like textures. It has attracted wide attention from the collection circle in recent years due to its unique appearance. In this study, polarizing microscopy, refractometry, Fourier transform infrared spectroscopy (FTIR), laser Raman spectroscopy, and UV-visible spectroscopy were employed to investigate the mineral composition and chromogenic genesis of Nanliujiang jade. The results show that the mineral component of Nanliujiang jade is α-quartz, with minor amounts of hematite and goethite, and the jade exhibits a compact massive structure as a whole. Further microstructural observations for in-depth study of its mineral composition and chromogenic genesis reveal significant differences in the morphological characteristics of quartz grains between the two varieties. Quartz in the red zonal band type presents a fibrous structure with fiber lengths ranging from 30 to 300 μm, while that in the flower-like type shows a granular equigranular structure with grain sizes of 10 to 100 μm. FTIR spectroscopy displays characteristic absorption peaks at 1184 cm⁻¹, 1105 cm⁻¹, 796 cm⁻¹, 779 cm⁻¹, 688 cm⁻¹, 533 cm⁻¹ and 478 cm⁻¹, which are highly consistent with the standard characteristic peaks of α-quartz. Laser Raman spectroscopy analysis indicates that the red zonal band type shows a characteristic peak of hematite (α-Fe₂O₃) at 498 cm⁻¹, and simultaneously detects characteristic peaks of goethite (α-FeOOH) at 237 cm⁻¹ and 405 cm⁻¹, confirming that this variety is jointly colored by hematite and goethite. For the flower-like type, characteristic peaks are observed at 238 cm⁻¹, 545 cm⁻¹ and 683 cm⁻¹; its main chromogenic minerals are goethite and lepidocrocite (γ-FeOOH), with hematite (α-Fe₂O₃) as the secondary chromogenic mineral. UV-visible spectroscopy shows that all samples present characteristic absorption peaks of Fe³⁺ at 380 nm and 450 nm, verifying that iron ions are the core chromogenic ions of Nanliujiang jade. This study systematically clarifies the gemmological and mineralogical characteristics as well as the chromogenic mechanism of Nanliujiang jade. It is confirmed that Nanliujiang jade is a typical mineral-colored siliceous jade, and there are significant differences in the chromogenic mineral assemblages and microstructures among samples with different appearance types.