非洲几内亚红宝石的宝石学特征及成分特征

Gemmological Characteristic and Chemical Composition of Ruby from Guinea, Africa

以非洲几内亚近年出产的红宝石为研究对象,借助宝石学常规仪器,结合激光剥蚀等离子质谱仪、紫外-可见光吸收光度计、傅里叶变换红外光谱仪和显微拉曼光谱仪系统探究其宝石学特征、成分特征和谱学特征,并与非洲其他产地红宝石成分特征进行比较。结果表明,几内亚红宝石多数带有紫色调或者褐色调,表面有熔蚀壳,内部包裹体丰富,透明度较差。根据颜色分为紫红色和橘红色两个系列,不同颜色红宝石中Cr、Fe质量分数和Cr/Fe比例有明显差别,均低于优质红宝石。非洲几内亚红宝石中Fe的质量分数为0.05%~0.15%,可以根据Fe质量分数与非洲肯尼亚(<0.0015%)、马达加斯加(0.15%~0.28%)、莫桑比克部分矿区(>0.28%)的红宝石粗略区分,但和坦桑尼亚部分矿区的红宝石(0.030%~0.156%)有重叠。紫红色系列样品随着颜色由浅到深,紫外-可见吸收光谱谱线红移,蓝紫色调增强;橘红色系列样品呈现紫区吸收强度大,绿区吸收强度小。红外吸收光谱中,深紫色样品出现了微弱的3310,3076cm^-1和Ti-OH有关的吸收峰,与深紫色样品中Ti质量分数相对较高相关。拉曼光谱测得几内亚红宝石的包裹体有磷灰石、硬水铝石和金红石。

The rubies originating from Guinea, Africa were studied. The gemmological characteristics of the rubies were tested by conventional gemmological instruments and the composition and spectra characteristics of the ruby sample from Guinea were analysed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), ultraviolet-visible spectroscopy (UV-Vis), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. Besides, composition characteristics of the ruby sample from Guinea were discussed and compared with those from other origins in Africa. The results indicated that the rubies from Guinea with corrosion mantle often show purple or brown hue. A great quantity of inclusions and fissilities cause low transparency of ruby from Guinea. On the basis of colour, ruby samples can be divided into two series: Purplish red and orange. With the colour from pink to dark purple, the content of Cr, Fe and the ratio of Cr/Fe show an obvious difference, which are lower than those of high quality rubies. Moreover, the content of Fe in ruby from Guinea is about 0.05%-0.15%, which can be distinguished from ruby in Kenya (<0.001 5%), Madagascar (0.15%-0.28%) and Mozambique (>0.28%) but overlaps with ruby from Tanzania (0.030%-0.156%). Considering the UV-Vis absorption spectra, the purple-red series have obvious red shift phenomenon with the colour from shallow to deep, while the orange series exhibit high absorption intensity in the purple region and low absorption intensity in the green region. For the appearance of 3 310, 3 076 cm^-1 in FTIR spectra, it must be noted that only the dark red-purple samples that contain relatively high Ti would show the peak. Therefore, the 3 310 cm^-1 peak is assigned for Ti-OH. In addition, apatite, diaspora and rutile as the main crystal inclusions have been determined by Raman spectra.