红外吸收光谱特征对石榴石种属及颜色成因的指示——以一颗黄色石榴石为例

FTIR Investigation of Garnet:Indication for Specific Species and Coloration:A Case Study of A Yellow Garnet

采用红外吸收光谱(FTIR)对1颗未知种属的黄色石榴石进行了深入研究,研究结果表明,可通过石榴石特征的A、B、C谱峰区分钙系石榴石和铝系石榴石,继而通过D谱峰确定石榴石的具体种属。黄色石榴石红外吸收光谱的A谱峰、B谱峰、C谱峰分别为962、862、842 cm^-1,说明该样品为钙系石榴石中的钙铝榴石。根据钙铝榴石D谱峰的波数(D=611 cm^-1),可获得Y位中的Al/(Fe+Al)质量分数比值为0.843,与EPMA的分析结果较为接近[Al/(Fe+Al)≈0.813]。Y位中Fe^3+的存在是黄色钙铝榴石致色的主要原因,也正是Y位中Fe 3+对Al^3+的部分类质同像替代,使得黄色钙铝榴石的紫外-可见吸收光谱中出现435 nm和581 nm的吸收峰,以及相对密度和折射率处于钙铝榴石端元和钙铁榴石端元之间。研究进一步证实,红外光谱可作为石榴石种属鉴别及颜色成因判断的快速有效的方法。

A yellow garnet sample of unknown species was investigated by FTIR.The results indicated that garnets of calcium series and aluminum series could be distinguished by the typical A,B and C absorptions,and the specific species was further determined by the D absorption.The A,B,C and D FTIR absorptions of the yellow garnet sample are respectively located at 962,862,842 cm-1 and 611 cm-1,indicating that the yellow garnet sample is grossularite of calcium series.Based on the wave number of D absorption,the Al/(Fe+Al)value 0.843 of Y site was obtained,which is very close to the EPMA results[Al/(Fe+Al)≈0.813].Coloration of the yellow garnet sample is attributed to the occurrence of Fe 3+at Y site.Also,the partial isomorphic replacement of Fe 3+to Al 3+at Y site results in the relative density and the refractive index of the yellow grossularite lying between grossularite end-member and andradite end-member.This investigation further confirms that FTIR can be used as a fast and effective method for the identification of garnet species and their colour genesis.