Room-temperature FTIR spectra of ten chemically and structurally well-characterized synthetic spinel-hercynite solid solution single crystals containing between 4 and 98 mol% hercynite component show that absorption in the range 2500-7000 cm - 1 is linearly correlated (R 2 = 0.995) with I V Fe 2 + concentration. The integral molar absorption coefficient for this absorption equals 1.49.10 8 cm/mol. In addition, the structure of the absorption envelope in this spectral region remains unchanged throughout the entire composition range. These two findings are consistent with assignments to a spin-allowed electronic d-d transition ( 5 E → 5 T 2 ) in I V Fe 2 + , split by the dynamic Jahn-Teller effect, and demonstrate that next-nearest interactions in Fe clusters have little or no effect on this absorption feature at ambient conditions.