福建云霄石榴子石宝石矿物学特征

Gem mineralogy characteristics of Yunxiao garnet,Fujian,China

福建云霄是我国重要的宝石级石榴子石产地,然而该区石榴子石的致色机理不清,制约了对其形成机制的理解及后续开发利用。本文选取7件福建云霄橙黄-橙红色石榴子石样品,利用傅立叶红外光谱、紫外-可见光光谱和拉曼光谱分析其谱学特征,使用电子探针及激光剥蚀电感耦合等离子体质谱仪(LA-ICP-MS)分析限定其主量、微量元素组成。结果表明云霄石榴子石主要为锰铝榴石,其颜色主要与二价锰(Mn^(2+))和铁离子(Fe^(2+))对可见光的吸收有关,Mn^(2+)导致其主体呈橙色,少量Fe^(2+)控制其橙红色调,微量Ti^(4+)使其呈褐色调。福建云霄石榴子石样品核部锰含量相对较低而铁、镁含量较高,锰元素含量由核部向边部逐渐升高,且具有重稀土元素富集、轻稀土元素亏损的左倾配分模式和Eu负异常,表明其形成于岩浆结晶作用晚期。

Yunxiao is a significant producing area of gem-quality garnet in China. However, the color origin of garnet in this area remains ambiguous, which restricts the understanding of their formation mechanism and subsequent exploitation. In this paper, 7 samples of orange-yellow-orange-red garnet from Yunxiao, Fujian were selected, and the spectral characteristics of the samples were analyzed by Fourier infrared spectrum, UV-visible spectrum and Raman spectrum, and the characteristics of main and trace elements were defined by electron probe(EPMA) and laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS) analysis. It is concluded that the color of the garnet is predominantly influenced by the absorption of visible light by Mn^(2+) and Fe^(2+). Mn^(2+) dictates the yellow tone of the garnet, while a minor amount of Fe^(2+) governs the orange-red tone. Additionally, a trace amount of Ti^(4+) imparts a brownish appearance to the garnet. The samples exhibited a relatively low manganese content and high iron and magnesium content in their core, a gradual increase in manganese content from the core to the rim. The samples also displayed a left-leaning partition pattern, with an enrichment of heavy rare earth elements and a deficit of light rare earth elements. Europium(Eu) showed a negative anomaly. These findings suggest that the garnets from Yunxiao, Fujian were formed during magmatic processes and are products of late-stage magmatic crystallization.