Polarized optical absorption spectra of single crystals of the two dimorphic Mn 2 AsO 4 (OH)-minerals eveite and sarkinite show that minor concentrations of Mn (super 3+) at the octahedral site determine the colour and cause the distinct pleochroism in green and yellow of eveite. In the sarkinite spectra only absorption bands due to spin-forbidden d-d transitions in Mn (super 2+) at six-and five-coordinated sites are observed, which produce a weak flesh-red mineral colour and a very faint pleochroism.

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