CONVERSION OF CRYSTALLINE MgSO4XH2O TO THE HYDRATED AMORPHOUS

Introduction: Recent orbital and landed missions, OMEGA on Mar Express and two Mars Exploration rovers [1-5] have found Mg-sulfates on Mars. In order to further advance our knowledge on the hydrologic history of Mars and to evaluate its potential for hosting life, in situ identification of hydration states of magnesium sulfates, as well as the Ca-, Fe-, hydrated sulfates, will be crucial for future landing missions. Because of the large daily temperature variation at the surface of Mars [>100K] and the full-scale swing in relative humidity (RH) of ~ 0% to ~ 100% occurring during these temperature excursions, it is anticipated that the hydration state and degree of crystallinity of Mg-sulfates may change during the Martian diurnal cycle [2]. During the rapid dehydration processes, a magnesium sulfate with high degree of hydration such as hexahydrite and epsomite may transform into less hydrated, amorphous phases. Because the rehydration of these amorphous phases may be extremely sluggish at the cold night temperatures of Mars when the RH is higher, it has been suggested that non-crystalline magnesium sulfates holding more water than kieserite would be the most commonly seen sulfate phase on the surface of Mars [6]. In addition to the Raman study of eight hydrated crystalline magnesium sulfates that we report elsewhere [7], we report here our use of Raman, XRD, and reflectance NIR spectroscopy to study the conversion of crystalline, hydrated magnesium sulfates into an amorphous, hydrated phase. We also compare the NIR spectra of this amorphous, hydrated phase with NIR spectra obtained by the OMEGA instrument on Mars Express – data that were obtained from the dark etched terrain at Meridiani Planum where the Opportunity rover landed. Experiments and results: Amorphous, hydrated magnesium sulfate was prepared from finely powdered, crystalline epsomite and hexahydrite. The samples were placed in a vacuum desiccator and evacuated to ~ 650 millitorr at 21 ̊C ±1 ̊C for 14 days. The weight-loss measurements showed that 5 moles of water were lost per mol epsomite and 4 moles of water were lost per mole of hexahydrite. The XRD powder pattern of crystalline epsomite shown in Fig 1a is converted to the amorphous phase (Fig. 1b) where all crystalline diffraction lines associated with the orthorhombic structure of epsomite have disappeared. The powder diffraction now shows only a broad featureless hump at a 2θ angle centered at ~ 29 ̊. Although this dehydration product has a featureless XRD pattern, the Raman spectrum of this material (Fig.1c) contains a prominent, broad water band centered at 3460 cm and a shoulder at ~ 3290 cm. Furthermore the SO4 ν1 fundamental band lies at ~1030 ∆cm (Fig. 2a), having a much broader peak width than any crystalline hydrated or anhydrous Mg-sulfate (Fig. 2a) [7]. Unlike XRD, the translational symmetry in the structure is not a necessary condition to produce Raman and infrared spectral peaks. Gases, liquids, and glasses all have characteristic vibrational spectra. Nevertheless, disordered solids such as glasses, polymers, and disordered crystalline lattices, arising during rapid changes in chemical composition or shocks due to sudden changes in pressure and/or temperature, will have broadened peak width. The crystalline and amorphous forms of a chemically identical species can generally be distinguished. A disordering of the lattice of epsomite or hexahydrite due to rapid vacuum dehydration would be reflected in a broadening of the Raman bands of the product such was observed in this study. The broadened Raman bands in the XRD-amorphous, hydrated magnesium sulfate that we have produced are thus indicative of a highly disordered, non-crystalline hydrated magnesium sulfate. The presence of a water band in the Raman spectrum of this amorphous phase (Fig. 1c) plus the position of its fundamental SO4 ν1 band at ~1030 ∆cm (Fig. 2b) -lying between the 1047 ∆cm band crystalline kieserite (MgSO4H2O) and the 1000 ∆cm band of crystalline starkeyite (MgSO44H2O) -supports the weight-loss evidence which shows only two moles of water in this amorphous magnesium sulfate phase. a. XRD pattern of Epsomite