Cause of Tri-Colour Change Effect of A Rare Garnet
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摘要: 采用常规宝石学测试、电子探针和光纤光谱仪对一颗具有三变色效应（紫色、蓝色、绿色）的石榴石进行化学成分和颜色成因分析。基于电子探针数据计算了该石榴石的晶体化学式，以Grs-Prp-（Alm+Sps）图解投影圈定了其名称为含铁钙锰质镁铝榴石，以V为主导，Mn、Fe、Cr协同引起变色效应。紫外-可见光谱中以450 nm为中心的宽吸收带由Fe和Mn引起，以575 nm左右为中心的宽吸收带则由V和Cr共同引起，可见光区两个强度接近的宽吸收带是样品在白炽灯与日光下颜色不同的直接原因。白炽灯下，样品对黄绿光波段的强烈吸收导致样品呈紫色；弱日光灯下样品对橙光区产生剧烈吸收导致样品呈现蓝色；强日光灯下样品对红光区大范围吸收致使绿光残余占主导，样品呈现绿色。高质量分数V的吸收峰位往中短波段扩展，与中短波段的Mn和Fe的吸收峰形成耦合所致，以508 nm左右为中心的狭窄低透过率区连接两个宽吸收带对该样品的三变色效应有重要作用。Abstract: Standard gemmological equipments, electron probe microanalyzer and fiber optic spectrometer were used to study a alexandrite effect garnet,which presents phenomenon as purple, blue and green under different light resources. The chemical formula of sample was calculated based on EPMA data and the subspecies name is determined as iron calcium-bearing manganic pyrope by Grs-Prp-(Alm+Sps) diagram plotting. The alexandrite effect is dominated by V, with contribution of Mn, Fe and Cr. The wide absorption band with 450 nm as the center is caused by Fe and Mn, while the absorption band at about 575 nm as center is the result of V and Cr. The intensity approach of the two wide absorption band in visible light is the direct cause of the garnet sample's colour change under incandescent and fluorescent light. Strong absorption of yellow green area makes the garnet sample display purple under incandescent light. Strong absorption of orange area makes the garnet sample display blue under weak fluorescent lamp, while wide and strong absorption of red area makes it show green under strong fluorescent light. The thin and low transparency area with 508 nm as the center is caused by absorption of rich V and spreads on mid-short wave area coupling with Mn and Fe, which plays an important role in the rare tri-colour change effect of the garnet sample.
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Keywords:
- garnet /
- tri-colour change effect /
- cause of colour
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