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

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. Coloration of the yellow garnet sample is attributed to the occurrence of Fe³⁺ at Y site. Also, the partial isomorphic replacement of Fe³⁺ to Al³⁺ 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.