Temperature-dependent absorption investigations between 10 and 823 K reveal photon-assisted hopping (PAH) of small polarons in biotites as the dominating excitation in the near-infrared spectral range. These data show a transfer of oscillator strength of the PAH of small polarons to Fe2+–Fe3+ intervalence-charge transfer and Fe2+ d–d transitions with decreasing temperature. The loss in oscillator strength of PAH is related to a pinning of the polarons due to the decreasing number in lattice vibrations in thermal equilibrium.

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