Thermally Modulated Absorption of Fe²⁺, Fe³⁺, and Mn²⁺ in Spessartine and Almandine Garnets Available to Purchase

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.