粉色—紫色缅甸尖晶石热处理前后紫外-可见光谱分析

Analysis of UV-Vis Spectrum on Pink-Purple Spinel from Burma before and after Heat Treatment

选取5颗粉色—紫色缅甸尖晶石原石为实验样品,分别在1 200℃和1 600℃下进行恒温10h的热处理实验。通过分析样品成分及热处理前后紫外-可见光谱,发现粉色—紫色尖晶石样品均为含微量Fe元素的镁铝尖晶石,且在350~600nm范围内出现吸收带的位置基本相同,其致色原因与八面体中Fe^(3+)和四面体中Fe^(2+)的自旋禁阻跃迁及二者间电荷耦合作用有关。对比样品的两次热处理后紫外-可见光谱结果发现,热处理过程促进350~430nm及500~600nm范围内吸收强度增加,前者与高温下尖晶石内部结构无序引起近紫外区O^(2-)→Fe^(2+)、Fe^(3+)离子间电荷转移作用增加有关,后者由进入四面体内部Fe^(3+)同相邻位置内Fe^(2+)共同作用产生。在经1 600℃热处理后,近紫外区吸收边强度再次增加使样品光谱发生明显改变,色调偏红,部分样品因热处理诱发内部裂隙、透明度降低,热处理效果不明显。

In this paper,5pieces of pink-purple spinels from Burma were selected as experimental samples and heated at 1 200℃and 1 600℃for 10 hours respectively.Through laser induced breakdown spectroscopy（LIBS）analysis for the chemical compositions and UV-Vis spectroscopy,it was found that all samples were MgAl2O4 with trace amount of element Fe and the UV-Vis absorption bands were basically the same in the range 350—600nm.The causes for their colours may be related to spin-forbidden transition of Fe^3＋in octahedral site and Fe-（2＋）in tetrahedral site and charge transfer between these pairs.Comparing UV-Vis spectra before and after heat treatment,it was found that the absorption intensity of all samples were enhaced dramatically in 350—430nm and 500—600nm areas.The absorption band at 350—430nm is believed to be caused by increasing intervalence charge transfer between O-（2-）→Fe-（2＋）、Fe^3＋after the disorder structural transition.The absorption band at 500—600nm isbelieved to be caused by Fe^3＋in tetrahedral site effected by Fe-（2＋）in adjacent site.After the heat treatment at temperature of 1 600 ℃,the absorption intensity in near ultraviolet region increased again and led to a clear change in UV-Vis spectrum,which making the apparent colour of samples become redder,however,some samples fail to follow this result due to internal fissures and lower transparency.