Morphological, structural, and spectral characteristics of amorphous iron sulfates

Abstract

Current or past brine hydrologic activity on Mars may provide suitable conditions for the formation of amorphous ferric sulfates. Once formed, these phases would likely be stable under current Martian conditions, particularly at low-to mid-latitudes. Therefore, we consider amorphous iron sulfates (AIS) as possible components of Martian surface materials. Laboratory AIS were created through multiple synthesis routes and characterized with total X-ray scattering, thermogravimetric analysis, scanning electron microscopy, visible/near-infrared (VNIR), thermal infrared (TIR), and Mössbauer techniques. We synthesized amorphous ferric sulfates (Fe(III) 2 (SO 4 ) 3 ·~6-8H 2 O) from sulfate-saturated fluids via vacuum dehydration or exposure to low relative humidity (<11%). Amorphous ferrous sulfate (Fe(II)SO 4 ·~1H 2 O) was synthesized via vacuum dehydration of melanterite. All AIS lack structural order beyond 11 Å. The short-range (<5 Å) structural characteristics of amorphous ferric sulfates resemble all crystalline reference compounds; structural characteristics for the amorphous ferrous sulfate are similar to but distinct from both rozenite and szomolnokite. VNIR and TIR spectral data for all AIS display broad, muted features consistent with structural disorder and are spectrally distinct from all crystalline sulfates considered for comparison. Mössbauer spectra are also distinct from crystalline phase spectra available for comparison. AIS should be distinguishable from crystalline sulfates based on the position of their Fe-related absorptions in the visible range and their spectral characteristics in the TIR. In the NIR, bands associated with hydration at~1.4 and 1.9 μm are significantly broadened, which greatly reduces their detectability in soil mixtures. AIS may contribute to the amorphous fraction of soils measured by the Curiosity rover.

Amorphous salts formed from rapid dehydration of multicomponent chloride and ferric sulfate brines: Implications for Mars

“…For these initial experiments, samples were rapidly dehydrated at 21°C in both vacuum and in 11% RH to force non-equilibrium precipitation conditions (Ling and Wang, 2010;Sklute et al, 2015;Wang et al, 2012;Xu et al, 2009;Xu and Parise, 2012). Though these low RH values are consistent with daytime RH values observed on the Martian surface (Harri et al, 2014;Savijarvi, 1995), and Martian surface temperatures can exceed 20°C at the equator (Spanovich et al, 2006), this combination of RH-T values represents a narrow range of current Martian conditions.…”

“…Each solution was made using ultrapure Fe 2 (SO 4 ) 3 that had first been heated at 350 °C for 2 h to transform it from the bottled phase, which does not occur in nature, to the mineral mikasaite (Xu et al, 2009). Because Sklute et al (2015) showed that amorphous phases formed through the deliquescence and dehydration of different starting materials show subtle spectral variations, the phase found in nature was considered the more appropriate starting material. Table 1.…”

“…Supporting measurements from the APXS and SAM instruments have constrained the bulk chemistry (including S and Cl) and hydrated nature of the amorphous fraction (Leshin et al, 2013;McAdam et al, 2014;Dehouck et al, 2015;Morris et al, 2015a); however, in general, the phase(s) that comprise this fraction are poorly constrained. A variety or combination of phases are plausible, including volcanic glasses, allophane (Morris et al, 2013), hisingerite (Milliken and Bish, 2014), amorphous sulfate (Sklute et al, 2015;Dehouck et al, 2014), chemisorbed sulfate on nanophase materials (McAdam et al, 2014;Rampe et al, 2016), or other nanophases (e.g., Dehouck et al, 2016). Important steps toward constraining the composition(s) of the constituents of the amorphous fraction include determining the possible formation mechanisms of amorphous materials, their stability under Martian conditions (e.g., Dehouck et al, 2016), and their spectral and X-ray diffraction characteristics.…”

“…Generally, there are two broad pathways through which amorphous salts have been shown to form: (1) rapid dehydration of crystalline hydrates or (2) direct precipitation from brines, under non-equilibrium conditions. Higher crystalline hydrates (> 6H 2 O per sulfate) of Mg-or Fe(II)-sulfate exposed to vacuum will form an amorphous phase through rapid dehydration (Vaniman et al, 2004;Wang et al, 2006;Chipera and Vaniman, 2007;Wang et al, 2009Wang et al, , 2011Wang et al, , 2012Wang et al, , 2013Sklute et al, 2015). However, the rate of dehydration and amorphization from a crystalline starting material is temperature dependent (Wang et al, 2009.…”

“…For example, amorphous ferric sulfate solids were produced from rapid dehydration of ferric sulfate brines, either by decreasing RH or decreasing atmospheric pressure (Xu et al, 2009;Xu and Parise, 2012;Sklute et al, 2015). Rapid precipitation has also been accomplished through cryoprecipitation, wherein liquids become quickly supersaturated through a rapid decrease in temperature (Toner et al, 2014;Morris et al, 2015b).…”