Micro-Raman studies of hydrous ferrous sulfates and jarosites

Ferrous sulfates of various hydration states (FeSO4·xH2O; x = 7, 4, 1) and jarosites (MFe3(SO4)2(OH)6; M = Na or K) were synthesized and studied by micro-Raman spectroscopy between 295 and 8 K. Spectral analyses of the sulfate and water/hydroxyl vibrational modes are presented. Fingerprint regions attributed to the symmetric (ν1) and antisymmetric (ν3) stretching vibrations of the sulfate group are found to vary with the degree of hydration in hydrous ferrous sulfate. In jarosites, the Raman shift of the OH stretching mode is related to the type of alkali metal present between the tetrahedral and octahedral layers. The Raman technique can thus unambiguously identify ferrous sulfate of various hydration states and jarosites bearing different alkali metal ions.