Infrared Spectroscopy of Amorphous Sulfate Phases

Overview: Stability experiments indicate that amorphous sulfate phases may be important constituents of martian surface materials [1-2]; there is also considerable spectral evidence for non-crystalline sulfate phases on Europa [3-4]. However, the thermal infrared (TIR) and near-infrared (NIR) spectral characteristics of these phases are largely unknown. X-ray amorphous Mg-and Fe-sulfate phases were synthesized and TIR bidirectional reflectance spectra were collected from the synthetic samples. Compared to their well-crystalline counterparts, the amorphous phases exhibit significant broadening and shifting of both fundamental internal vibration modes as well as Mg-O and Fe-O vibrational bands. Further TIR and NIR spectral characterizations of these materials will be directly applicable to analyses of Background: Recent experimental investigations of the stability of Mg-sulfates on Mars indicate that an amorphous Mg-sulfate phase forms through rapid dehydration of hex-ahydrite or epsomite at low temperatures (<~30°C) and low relative humidity—conditions similar to those found in Martian equatorial regions during the day [1-2]. The synthesized phases were found to be metastable under these conditions, with sluggish rates of conversion to more crystalline forms. On the basis of these findings, Chipera and Vaniman [2] suggest that amorphous sulfate should be a common sulfate phase on the martian surface. Poorly crystalline sulfates have been identified as bypro-ducts of basalt weathering under acidic conditions [5-6]. Recent modeling of rock chemical compositions observed by the Mars Exploration Rovers suggest that acidic conditions have dominated the geologic history of Mars. Because of the probable acidic conditions and prevalence of basaltic starting materials, it has been proposed that salt crusts on Mars may include amorphous sulfate [5]. NIR spectra of dark, non-ice materials on Europa indicate the presence of a hydrated sulfate phase [7]. However, close inspection of the spectral detail indicates that these materials lack the spectral structure that is typical of crystalline sulfates [3-4]. Alternate candidate materials may be flash-frozen brines, forming disordered, hydrated MgSO 4 [3-4] or sulfuric acid hydrates [8]. A systematic study of Raman spectral changes with hydra-tion state in MgSO 4 phases, both crystalline and non-crystalline, demonstrated that amorphous Mg-sulfate is spectrally unique from crystalline forms [9]. In addition, McCord et al. [3] show that disordered Mg sulfates formed through flash-freezing of brines exhibit differences in NIR spectra from their well-crystalline counterparts. In these examples, however, systematic examination of spectral changes with increasing crystallinity or varying major cation composition has not been carried out. Furthermore, the NIR and TIR reflectance properties …