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

摘要

Raman and infrared spectra of two polymorphous minerals with the chemical formula Fe~(3+)(SO_4)(OH)·2H_2O, monoclinic butlerite and orthorhombic parabutlerite, are studied and the spectra assigned. Observed bands are attributed to the (SO_4)~(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?O hydrogen bond lengths in the structures of both minerals are calculated from the wavenumbers of the stretching vibrations. One symmetrically distinct (SO _4)~(2-) unit in the structure of butlerite and two symmetrically distinct (SO_4)~(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.