A vibrational spectroscopic study of hydrated Fe3+ hydroxyl-sulfates; polymorphic minerals butlerite and parabutlerite

Raman and infrared spectra of two polymorphous minerals with the chemical formula Fe3+(SO4)(OH)center dot 2H(2)O, monoclinic butlerite and orthorhombic parabutlerite, are studied and the spectra assigned. Observed bands are attributed to the (SO4)(2-) stretching and bending vibrations, hydrogen bonded water molecules, stretching and bending vibrations of hydroxyl ions, water librational modes, Fe-O and Fe-OH stretching vibrations, Fe-OH bending vibrations and lattice vibrations. The O-H center dot center dot center dot O hydrogen bond lengths in the structures of both minerals are calculated from the wavenumbers of the stretching vibrations. One symmetrically distinct (SO4)(2-) unit in the structure of butlerite and two symmetrically distinct (SO4)(2-) units in the structure of parabutlerite are inferred from the Raman and infrared spectra. This conclusion agrees with the published crystal structures of both mineral phases. (C) 2011 Elsevier B.V. All rights reserved.