Paper
Visible and Infrared Spectral Characteristics and Morphology of Amorphous Iron Sulfates
Published 2014 · E. Sklute, H., B. Jensen
0
Citations
0
Influential Citations
Abstract
Introduction: The discovery of sulfates on Mars has led to the hypothesis that ancient Mars experienced acidic fluid conditions near the surface [1-2]. Oxidative weathering of sulfides and volcanic activity would have led to sulfate-rich fluids. These solutions would have precipitated sulfates as they become saturated with respect to a given phase [3]. For iron sulfates, ironand sulfur-rich fluids tend to precipitate fine grained, crystalline phases at Earth’s surface temperatures and pressures [4]. Cloutis et al. (2008) showed that, once formed, many sulfates are stable under simulated Martian conditions [5], however, Xu et al. (2009) showed that amorphous phases form when iron sulfate fluids are subjected to low relative humidities [6], as one would expect on Mars under certain conditions [7]. The recent discovery by MSL that 27% of the soil at Rocknest in Gale Crater is x-ray amorphous material containing both iron and sulfur [8], raises new questions regarding the presence of amorphous sulfates on Mars. Yet the spectral properties, physical characteristics, morphologies, and formation pathways for amorphous iron sulfates are not well understood. This is critical for recognizing these phases in existing landed and remote data sets. This abstract describes the spectral and morphological characteristics of amorphous iron sulfate phases and describes implications for MSL observations. A companion abstract [Jensen et al.] describes multiple formation pathways for amorphous sulfate phases and discusses possible path-dependent differences that can help constrain formation pathways. Methods: For this work, ferric iron sulfates were synthesized from two starting materials: first using unaltered Acros Organics 97% Fe(SO4)3•5H2O, identified by XRD to be the monoclinic phase lausenite (Fe(III)(SO4)3•6H2O); second by heating the starting material for 2 h at 350 oC to form the anhydrous trigonal phase mikasaite (Fe(III)(SO4)3) [6]. Both powders were placed in 92% RH at RT, using DI water as a humidity buffer. Once hydrated, the materials were dehydrated via vacuum (3x10 -2 mbar) for 2 weeks to simulate the rapid loss of water that crystalline phases would likely experience once precipitated on the Martian surface. These samples will be referred to as Lamorphous and M-amorphous to reflect their starting materials of lausenite and mikasaite, respectively. Amorphous ferrous sulfate was prepared by vacuum dehydrating melanterite (FeSO4•7H2O) for 3 days. The materials were confirmed amorphous by XRD. Synchrotron X-ray total scattering data were also collected at the Advanced Photon Source (Argonne National Lab) for pair distribution function (PDF) analysis [9]. Once created, the ferric sulfates were kept at low RH (less than 11%) except during spectral analyses (< 20min). Lack of crystallinity was checked by XRD after each analysis. VNIR spectra were collected on an ASD Fieldspec3 Max UV-VIS-NIR bidirectional spectrometer (referenced to Spectralon, average of 300 scans). When laboratory humidity exceeded 20%, VNIR spectra were collected in an N2 filled glove bag. Spectral emissivity measurements were taken in a dry air purged chamber and only collected when laboratory humidity was less than 15%. To avoid phase changes that could occur upon heating, the samples were cooled to 30° below the detector temperature (rather than heated above detector temperature) to achieve adequate signal to noise for spectral measurement [10]. Thermogravimetric analysis (TGA) was used to determine sample water contents, and SEM was used to image sample morphologies. Results and Discussion: When processed in a low humidity environment after synthesis, amorphous ferric sulfates grind from an amber into a fine powder that is X-ray amorphous (Figure 1). Ferrous sulfates transform into a silky gray powder and maintain the shape of the crystals from which they formed(Figure 1 inset).
Amorphous iron sulfates, when processed in a low humidity environment, transform into a silky gray powder with X-ray amorphous properties, potentially indicating their presence on Mars under certain conditions.
Full text analysis coming soon...