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)·2H2O, 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⋯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.

Graphical abstractRaman and infrared spectra of two polymorphous minerals with the chemical formula Fe3+(SO4)(OH)·2H2O, monoclinic butlerite and orthorhombic parabutlerite, are studied and the spectra assigned. 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.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Parabutlerite and butlerite both have a chemical formula Fe3+(SO4)(OH)·2H2O. ► Fe3+ hydroxyl-sulfates butlerite and parabutlerite and bands are assigned. ► One symmetrically distinct (SO4)2− unit is in the structure of butlerite. ► Two symmetrically distinct (SO4)2− units is in the structure of parabutlerite. ► O–H⋯O hydrogen bond lengths are between 2.87 and 2.69 Å.