宝石级天然玻璃的鉴别特征

Identification Characteristics of Gem-Grade Natural Glass

宝石鉴定中常见的天然玻璃有黑曜岩和玻璃陨石两个品种,它们具有相似的外观特征,在常规鉴定中很难区分。黑曜岩和玻璃陨石形成于不同的地质环境中,不同的地质条件也造成着它们各自的物理化学特征。本文通过对世界各产地的黑曜岩和玻璃陨石的外观特征、显微观察、成分分析以及红外光谱测试讨论这两种天然玻璃的鉴别特征。

There are two kinds of gem-grade natural glass in daily inspection,obsidian and tektite,which have similar appearance.It is difficult to distinguish them.Obsidian and tektite are formed under different geological environments,and possess different physical and chemical characteristics.By using the conventional gemmological tests as well as scanning electron microscope（SEM）,energy dispersive spectrometer（EDS）,and Fourier transform infrared spectrometer（FTIR）,this article contrasted the physical properties and chemical compositions of obsidian and tektite.Both tektite and obsidian are mainly composed of SiO2 and Al2O3.The content of SiO2 in tektite is about 71.3%～79.3%,while SiO2 of obsidian is about 70%.Otherwise,FeOT in obsidian is less than that in tektite,and content of alkali（R2O）is more in obsidian.Tektite is formed under ultra-high temperature and pressure,and it shows highly non-crystallizing and disordering.Stretching vibration mode of[SiO4]appears at 1 200～900cm-1in tektite and obsidian.Comparing with tektite,obsidian is formed under lower temperature and pressure.The absorptions at 750～500cm-1and 1 170cm-1 indicate the existence of quartz and plagioclase.In conclusion,different forming condition of obsidian and tektite caused differences in chemical compositions and infrared absorption characteristics.Therefore,these two kinds of natural glass can be identified in daily gem inspection.