摘要
变色石榴石是指具有变色效应的石榴石,其矿物成分通常为含有Cr、V等微量元素的镁铝榴石、锰铝榴石或镁铝-锰铝榴石。选取五组不同色调(深绿色、灰蓝色、黑灰色、黄绿色、棕灰色)的变色石榴石,通过常规宝石学仪器、傅里叶变换红外光谱仪、激光剥蚀电感耦合等离子体质谱仪和紫外-可见分光光度计对其进行测试分析,探讨其致色机理及变色成因。结果表明,本文样品是富含Mn^(2+)、Fe^(2+)、V^(3+)、Cr^(3+)的镁铝-锰铝榴石,且具少量的钙铝榴石和铁铝榴石端元成分。紫外-可见吸收光谱和化学成分分析指示Mn^(2+)、Fe^(2+)、V^(3+)、Cr^(3+)过渡金属离子的存在导致了本文中变色石榴石样品的颜色和变色效应:以576 nm为中心的黄区宽缓吸收带与Cr^(3+)d-d轨道自旋允许跃迁4A_(2g)→4T_(2g)和V^(3+)d-d轨道自旋允许跃迁^(3)T_(1g)→^(3)T_(2g)相关;400~460 nm中的422 nm吸收带与Mn^(2+)d-d轨道自旋禁阻跃迁^(6)A_(1g)→4Eg有关,457 nm处吸收峰与Fe^(2+)d-d轨道自旋禁阻跃迁^(5)T_(2g)→^(5)E_(g)相关;当400~460 nm吸收强度相对较大时,Mn^(2+)和Fe^(2+)对颜色的贡献度增加,变色石榴石在日光灯下呈黄色调;当400~460 nm的吸收强度相对较小时,Cr^(3+)和V^(3+)在黄区的宽缓吸收带强度相对较大,对颜色的贡献度增加,变色石榴石在日光灯下呈现蓝绿色调;以576 nm为中心的宽缓吸收带与400~460 nm强而窄的吸收带将可见光分隔成以506 nm为中心的绿区透过窗和660 nm以上的红区透过窗,两个透过窗的透过率相当,不同的样品选择性透过这两个可见光区域,加之日光灯含较多的蓝绿色成分,白炽灯含较多的橙红色成分,样品在不同光源下的颜色产生差异,最终形成变色效应。
Colour-changing garnets are characterized by colour-changing effect.Typically,this type of garnet is composed of pyrope,spessartite,or spessartite with trace elements like chromium(Cr)and vanadium(V).This study investigated five groups of colour-changing garnets exhibiting different tones:dark green,grayish blue,blackish gray,yellowish green,and brownish gray.To understand their colouration mechanisms and the cause of discolouration,our study applies a variety of gemmological techniques,including laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS),ultraviolet visible spectrophotometer(UV-Vis)and Fourier transform infrared spectrometer(FTIR).The results demonstrated that the samples were spessartite rich in Mn^(2+),Fe^(2+),V^(3+),Cr^(3+),with minor components of grossular and almandine garnet.Analysis of UV-Visible absorption spectrum and chemical composition indicated that Mn^(2+),Fe^(2+),V^(3+),and Cr^(3+)caused the colour and discolouration effects in colour-changing garnets.The broad absorption band centered at 576 nm was associated with Cr^(3+)and V^(3+)spin-allowed d-d orbital transitions,specially ^(4)A_(2g)→^(4)T _(2g) and ^(3)T_(1g)→^(3)T _(2g),respectively.Additionally,the 422 nm absorption band in 400-460 nm was assigned to the Mn^(2+)d-d orbital spin-forbidden ^(6)A_(1g)→^(4)E_(g) transition,while the 457 nm absorption peak corresponded to the Fe^(2+)d-d orbital spin-forbidden ^(5)T_(2g)→^(5)E_(g) transition.When the absorption intensity in the 400-460 nm range was high,Mn^(2+)and Fe^(2+)mainly contributed to the colour,resulting in yellow colour under fluorescent lamps.Conversely,with lower absorption intensity,yellow colour dominated by Cr^(3+)and V^(3+)were absorbed,leading to blue-green hues under fluorescent lamps.The intricate interplay of absorption bands,particularly the 576 nm and 400-460 nm bands,resulted in distinct transmittance regions at 506 nm(green)and 660 nm(red)beyond.The observed colour-changing and discolouration effects in colour-changing garnets are a manifestation of the selective transmission through different light regions,influenced by the varying compositions of light sources.
作者
徐娅芬
陶飞宇
刘衔宇
杨天畅
XU Yafen;TAO Feiyu;LIU Xianyu;YANG Tianchang(College of Jewelry,Shanghai Jian Qiao University,Shanghai 201306,China)
基金
上海市晨光计划项目(AASH2106)资助
上海市民办高校“民师计划”项目(2023-00140)。
关键词
变色石榴石
变色效应
谱学特征
变色机理
colour-changing garnet
colour-changing effect
spectral characteristics
colour-changing mechanism
