白-黄色系绿松石“伴生矿”的红外光谱表征及其意义

近期在市场上出现了许多与绿松石相似的天然矿物，市场上俗称绿松石“伴生矿”，被商家作为天然绿松石的特殊品种售卖。这些与绿松石外观极为相似的天然矿物，给鉴定工作带来了一定的困难。为了探索识别的方法，选取市场上常见的白色和黄色品种的天然绿松石伴生矿，通过常规宝石学测试，红外吸收光谱及X射线粉晶衍射对其宝石学特征及矿物组成进行了分析和研究。结果表明：白色系和黄色系天然似绿松石矿物样品均显示不同程度的土状光泽-弱玻璃光泽，均不透明，结构比较疏松。白色系样品折射率约为1.51，相对密度为1.86～2.28；黄色系样品折射率约为1.57～1.60，相对密度为2.32～2.72。白色和黄色天然绿松石伴生矿的组成类型复杂，同色系样品的矿物组成也不尽相同。X射线粉晶衍射测试结果显示：白色系样品的主要矿物为磷铝矾和磷钙铝矾；黄色系样品的主要矿物为钠明矾石。白色和黄色系样品的红外吸收光谱均显示有SO₄/PO₄的基团振动，峰形、峰位区别较大。根据不同样品的主要矿物组成特征，将其红外吸收光谱进行分类，可对其进行快速有效的无损鉴定。

A Study on IR Absorption Spectroscopy and XRD Characteristics of White and Yellow Natural Turquoise Associated Minerals

Recently there have been many natural minerals which are similar to turquoise in the market, commonly known as “turquoise associated minerals”, and are sold for business as a natural turquoise. These natural turquoise associated minerals are similar to natural turquoises in appearance while there are some difficulties in its identification. In this paper, the natural turquoise associated minerals of white and yellow varieties are selected, and their gemological characteristics and mineral composition are analyzed and studied by conventional gemological test, infrared absorption spectroscopy and X-ray powder diffraction. The results show that white and yellow natural turquoise associated minerals show earthy luster-weak glassy luster in different degrees. They are opaque and show relatively loose structure. The refractive index of the white sample is about 1.51 and the relative density is 1.86~2.28. The refractive index of the yellow sample is about 1.57~1.60 and the relative density is 2.32~2.72. The mineral compositions of white and yellow natural turquoise associated minerals are complex, and different even in the same color samples. The test of the spectrum by infrared absorption spectrum cannot effectively indicate the specific mineral composition, while the X-ray powder is the effective means to study the mineral composition of natural turquoise associated minerals. X-ray powder diffraction test results show that the main minerals of the white sample are phosphorus alum and phosphorus calcium alum; and the main mineral of the yellow sample is sodium alunite. The infrared absorption spectra of white and yellow samples show that the groups of SO₄/PO₄ had different vibrations, peak shapes and peak positions. According to the main mineral composition characteristics，the classification of infrared absorption spectrum for different samples can be fast and effective for its non-destructive identification.