Optical (diffuse-reflectance) spectra are used to characterize the crystal chemistry of Mn in pegmatite-associated montmorillonite. The optical spectra indicate that Mn is in the 3+ oxidation state and show Mn3+ ligand-field transitions near 10400, 18800, 20600, and 22400 cm−1. As indicated from the number and energies of the ligand-field absorption bands, the Mn3+ coordination site has undergone a tetragonal distortion associated with the Jahn-Teller effect. Further distortion has eliminated even the tetragonal symmetry. Assuming that the Mn site is approximately tetragonal, however, the ligand-field theory parameters 10Dq, Ds, and Dt are estimated to be 18800, 1934, and 533 cm−1. The large Jahn-Teller distortion of the Mn3+ coordination site may explain the limited solubility of Mn3+ cations in montmorillonite and the absence of a manganian (Mn3+) smectite end-member phase.

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