Yankuang ceshi (Sep 2014)
The Gemological and Mineral Characteristics of Variscite from Ma'anshan of Anhui Province
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.
