The thermally modulated absorption (TMA) of two spessartine and two almandine garnets show absorption peaks with positive temperature coefficient in the region between 10,000 cm-1 and 25,000 cm-1 at room temperature. The TMA peaks in the 16,000-25,000 cm-1 region are 3d → 3d forbidden transitions of Fe2+, Fe3+ and Mn2+ ions and agree generally with the presently accepted assignment.

In the 10,000-16,000 cm-1 region, The TMA spectra show an unexpected strong band near 12,000 cm-1. This new 12,000 cm-1 band and the known 14,000 cm-1 band are interpreted as intervalence charge transfer (IVCT) from 8-coordinate Fe2+ (VIII) to Fe3+ at octahedral (VI) and tetrahedral (IV) site, respectively. We speculate that the temperature coefficient of IVCT absorption, dα/dT, can be positive or negative depending on whether the initial and final site of IVCT being different, (e.g., garnets) or equivalent (e.g., micas and vivianite) type(s), respectively.

we propose a minor modification on the 3d → 3d forbidden assignment of Fe2+ (VIII). We also introduce a field-independent level 4A1(G) 4E(G) of Fe3+(IV) in spessartine and almandine at 22,300 cm-1. which coincides with that in the closely related ugrandite garnets.

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