云南锡石的热致变色机理初探

Mechanism of Thermochromism of Cassiterite from Yunnan Province,China

锡石作为一种稀有宝石,一度因其强火彩被称为“云南钻石”,国内对其地质特征以及锡石浮选研究较多,目前未见有报道锡石的热致变色现象。本文采用常规宝石学仪器、拉曼光谱仪、激光剥蚀等离子质谱仪(LA-ICP-MS)和紫外-可见光光谱仪等,对近期产自云南的部分锡石进行了测试,对其热致变色现象的机理进行探究。结果表明,云南锡石主要成分为二氧化锡,平均含量高达99.4 wt%,含有微量元素Fe、Ti、Si、Ni、Ta、Nb等。从室温升高至500℃的过程中,锡石晶格膨胀,晶体键力常数减小,带隙值减小,导致锡石原位高温拉曼的特征峰向低频方向偏移,并使得紫外-可见光吸收光谱吸收边缘由380 nm不断红移至450 nm,锡石由近无色变为黄色;从500℃降温至室温过程中,锡石晶格收缩,晶体键力常数增大,带隙值增大,锡石原位高温拉曼的特征峰向高频方向回归,其紫外-可见光吸收光谱吸收边缘又不断蓝移恢复至380 nm,样品颜色恢复为近无色。计算二氧化锡晶体在加热和降温过程中光学带隙值,发现随温度升高,其带隙值E_(g)从3.28 eV降低到2.75 eV。综上,温度变化引起的晶格膨胀和收缩导致晶体的带隙变化是云南锡石出现热致变色的原因。

As a rare gemstone,cassiterite was once called“Yunnan diamond”because of its strong fire.There are many researches on the geological characteristics of cassiterite and its flotation in China,but the thermochromism phenomenon of cassiterite has not yet been reported.In this study,conventional gemmological instruments,Raman spectrometer,laser ablation plasma mass spectrometer(LA-ICP-MS)and UV-Vis spectrometer were used to investigate the mechanism of the thermochromic phenomenon of cassiterite from Yunnan Province,China.The results showed that the main component of cassiterite is tin dioxide,with an average content of up to 99.4 wt%,and it contains trace elements Fe,Ti,Si,Ni,Ta,Nb,et al.In the process of increasing from room temperature to 500℃,the crystal lattice of cassiterite expanded,resulting in the reduction of crystal bond force constant and band gap value,which leaded to a shift of the in situ high-temperature Raman peaks of cassiterite toward lower frequencies,and caused the absorption edge of the UV-Vis absorption spectrum of cassiterite to redshift from 380 nm to 450 nm,cassiterite changed from nearly colorless to yellow.In the process of cooling from 500℃ to room temperature,the crystal lattice shrinked,which increased the crystal bond force constant and band gap value.The in situ high-temperature Raman peaks of cassiterite regressed toward high frequencies,the absorption edge of UV-Vis absorption spectrum continuously blueshifted back to 380 nm,and the sample colour returned to nearly colorless.The optical band gap values of tin dioxide crystal during heating and cooling processes have been calculated.It is found that the bandgap value E_(g) of cassiterite from Yunnan decreased from 3.28 eV to 2.75 eV with the increase of temperature.In conclusion,the change of crystal band gap is due to lattice expansion and contraction caused by temperature change is the reason for the thermochromism of cassiterite from Yunnan.