Gordon Smith, Ulf Haalenius, Hans Annersten, Lothar Ackermann; Optical and Mossbauer spectra of manganese-bearing phlogopites; Fe (super 3+) IV-Mn (super pi L) VI pair absorption as the origin of reverse pleochroism. American Mineralogist ; 68 (7-8): 759–768. doi:
Samples containing Mn (super 2+) VI and Fe (super 3+) VI are normal pleochroic, while samples containing Mn (super 2+) VI and Fe (super 3+) IV are reverse pleochroic. Chemical and optical differences are reflected by pronounced variations in the shape and band content of the optical absorption spectra. Intense absorption bands are assigned to Fe (super 3+) IV-Mn (super 2+) VI pair transitions. Inverse temperature dependence on sample cooling.--Modified journal abstract.