Raman Spectroscopic Investigation of Ferrous Sulfate Hydrates

Introduction: There is a growing interest in the study of sulfate minerals on the surface of Mars, supported by the discovery of jarosite at Meridiani Planum [1-3]. There is also the possibility of finding hydrous sulfate minerals in the permafrost, located at the polar regions. Sulfate minerals often precipitate in a variety of hydration states in the presence of liquid water. The spectra of suspected sulfate minerals obtained by reflectance spectroscopy have broad spectral features [4, 5], making the identification of mixtures of hydrous sulfates difficult. We therefore carry out an investigation of various forms of ferrous sulfate (FeSO4) hydrates by Raman spectroscopy. Our objective is to unambiguously distinguish between the hepta-, tetra-, and monohydrates of FeSO4. We also employ isotope substitution and low temperature techniques to help understand the unique spectral features of these hydrates in relation to changing molecular structures. Low temperature studies are particularly important for planetary missions as the Raman spectra of hydrous minerals often differ substantially in comparison to those acquired at ambient temperature. Experimental: Reagent grade FeSO4.7H2O (melanterite) was used for the preparation of FeSO4.4H2O (rozenite) and FeSO4.H2O (szomolnokite) by heating in air and recrystallization in dilute sulfuric acid, respectively. The 488-nm radiation from an Ar-ion laser was used for excitation. Raman spectra were collected using a Spex Triplemate spectrometer equipped with a CCD detector. The spectra of these hydrates were acquired at ambient conditions and at reduced temperatures (down to 8 K) in a helium cryostat. Deuterated analogs were prepared by recrystallization of the monohydrate in D2O. Further details regarding to sample preparation and instrumentation can be found elsewhere [6]. Results and Discussion: The Raman spectra of the hepta-, tetra-, and monohydrates of FeSO4 are shown in Fig. 1. The symmetric stretching vibrational (ν1) mode of the sulfate (SO4) ion gives the most intense and narrow Raman line. Its wavenumber increases (976 to 1018 cm) with lowering water content, and therefore can be used as a fingerprint for identifying FeSO4 of different hydration states (Fig. 2). Similar trend has been reported for the hydrates of MgSO4 [7], and the increase in wavenumber is believed to be caused by weakening H-bonding. The anti-symmetric stretching vibrational mode of the SO4 ion splits into three Raman lines (1050 to 1200 cm), indicating the SO4 tetrahedron ions in these hydrates are at sites of lower symmetry. The magnitude of crystal-field splitting strengthens with increasing geometric distortion of the SO4 ion. Raman lines from the lattice vibrational modes differ considerably among these hydrates, reflecting large variations in the structural hierarchy with Fig. 1. Micro-Raman spectra of FeSO4.nH2O (n = 7, 4, 1) at 298 K: lattice and SO4 modes (left), H2O bending modes (middle) and H2O stretching modes (right). 150 350 550 750 95