The Gemological and Mineral Characteristics of Variscite from Ma'anshan of Anhui Province
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摘要:
近年来在安徽马鞍山地区所在的绿松石矿体附近，一种绿色、半透明的磷铝石相继发现，部分达到宝石级别。本文采用电子探针、X射线粉晶衍射仪、扫描电镜、傅里叶变换红外光谱仪、紫外可见光谱仪等测试技术，对该地区磷铝石的化学成分、矿物成分、微观结构和光谱特征进行对比验证和综合分析，研究其水的赋存形式，进而对磷铝石的呈色机制作了深入探讨。电子探针分析显示该地区磷铝石的化学成分主要以Al、P元素组成，含微量的Fe、V元素。X射线粉晶衍射与红外吸收光谱分析表明主要矿物为磷铝石，基本不含有其他杂质矿物；磷铝石一种水合磷酸盐矿物，含有结晶水以及少量结构水的矿物，且结晶水与结构水多与Al3+（Fe3+）相结合的形式存在。偏光显微镜和扫描电镜观察显示磷铝石整体以鳞片状集合体产出，微观上多以短柱状及板片状堆积，单个晶体显示斜方晶系结晶生长习性。紫外可见吸收光谱中639 nm处吸收谱峰由Fe3+与V3+联合所致，300、423、864 nm处吸收峰由Fe3+所致，说明Fe3+与V3+的共同作用是马鞍山地区磷铝石呈现绿色的主要原因。本研究对于认识该类磷铝石的宝石矿物学性质以及颜色成因具有一定意义。
Abstract:Currently green and translucent variscites have been discovered near the turquoise ore body in the Ma'anshan mine. Some variscites can achieve gem grade. The testing methods of Electron Probe Microanalysis (EPMA), X-ray Powder Diffraction (XRD) analysis, Scanning Electron Microscope (SEM), Fourier Transform Infrared (FTIR) absorption spectroscopy and Ultraviolet-Visible (UV-Vis) absorption spectroscopy were conducted to analyze the chemical composition, mineral structure, microstructure, and spectroscopy features of variscites. According to the study of occurrence status of water, the color mechanism of variscite was further studied. The result of chemical composition by EPMA analysis shows that variscite from Ma'anshan is mainly composed of Al and P with a small amount of Fe and V. The XRD pattern shows variscite is composed of pure variscite mineral. Furthermore, the infrared absorption spectroscopy analysis shows variscite is hydrated phosphate based upon the formula AlPO4·2H2O, which has crystalline water and hydroxyl connected with Al3+(Fe3+). The vibration mode and frequency of H2O, OH- and PO43- determine the infrared spectrum's main feature of variscite samples. The hydroxyl stretching vibrations appear at 3581 cm-1 and the crystalline water stretching vibrations appear at 3382 and 3253 cm-1. Correspondingly, their bending vibrations appear at 910, 842 and 796 cm-1 from the infrared absorption spectroscopy. The phosphate stretching vibrations appear at 1056 cm-1 while the bending vibrations appear at 592 cm-1 with other spilt bands. The observation by SEM shows that the variscite crystals are stacked by short column and short plate as aggregates. One variscite crystal shows the feature of an orthorhombic system. The absorption spectrum at 639 nm in the UV-VIS spectroscopy was caused by Fe3+ and V3+ d-d electronic transition. The absorption spectrum at 300 nm, 423 nm and 864 nm are assigned to Fe3+ d-d electronic transition. Fe3+ and V3+ both cause the green colour of variscite. The modern analytical techniques of FTIR and UV-Vis spectroscopy analysis were used in the study based on the chemical composition and mineral structure analysis, which determined the information of mineral structure, existing form of water and metal ion of variscite from Ma'anshan in order to further study the color-forming mechanism of variscite.
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表 1 马鞍山和国外产磷铝石化学成分
Table 1. Chemical compositions of variscites from Ma'anshan and other countries
样品编号和产地 元素含量(%) Al2O3 P2O5 TFe2O3 Cr2O3 V2O5 K2O CaO SiO2 H2O MV-01 34.21 44.61 1.07 0.00 0.28 0.02 0.03 0.07 - MV-02 30.06 44.90 0.64 0.06 0.31 0.07 0.20 0.03 - BV-01 29.78 42.89 2.02 0.09 0.14 0.12 0.06 0.09 - 美国犹他州* 29.3 44.4 0.05 0.14 0.64 - - - - 法国帕尼西* 29.1 44.7 0.15 0.20 0.40 - - - - 磷铝石的理论值 32.28 44.94 22.78 注：测试单位为国土资源部合肥矿产资源监督检测中心，“-”代表该元素未检测，“*”数据引自Calas等[10]。 
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表 2 磷铝石颜色参数及致色元素含量
Table 2. Color parameters and coloring elements of variscite
样品编号 颜色参数 元素含量(%) 主波长(nm) 彩度 明度 TFe2O3 V2O5 Cr2O3 MV-01(马鞍山) 506.8 16.6 73.8 1.07 0.28 0.00 MV-02(马鞍山) 521.4 23.8 58.0 0.64 0.31 0.06 BV-01(美国产) 513.6 18.5 70.9 2.02 0.14 0.09
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