Optical absorption and diffuse reflectance spectra of several Fe2O3 and FeOOH polymorphs (hematite, maghemite, goethite, lepidocrocite) in the near-infrared to near-ultraviolet spectral regions (2000–200 nm) are presented. The spectra consist primarily of Fe3+ ligand field and ligand to metal charge-transfer transitions. Band assignments were made using ligand field theory and the results of recent SCF-Xα-SW molecular orbital calculations of (FeO6)9− coordination polyhedra. Values of the ligand field parameters 10Dq, B and C were found to be 14000–15900, 550–610, and 3400–3500 cm−1, respectively. The lowest energy ligand-to-metal charge-transfer transition is near 40,000 cm−1 in agreement with the molecular orbital calculations. The Fe3+ ligand field transitions are strongly intensified by magnetic coupling of adjacent Fe3+ cations. Several additional spectral bands are assigned to “double exciton” processes involving the simultaneous excitation of two Fe3+ cations which are magnetically coupled. These results show that the red to yellow colors of these minerals are not due to the LMCT transitions but result from pair excitations and Fe3+ ligand field transitions which have been intensified by magnetic coupling of adjacent Fe3+ cations.

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