Structural and Spectral Characteristics of Amorphous Iron Sulfates

Abstract:

Substantial evidence points to the existence of hydrated sulfate phases on the Martian surface^1-3. In addition, the discovery of recurring slope lineae could point to an active brine hydrologic cycle on the surface^4,5. The rapid dehydration of both hydrated sulfates and sulfate-rich brines can lead to the formation of amorphous sulfates. Evidence suggests that the Rocknest soil target and the Sheepbed mudstone interrogated by the Mars Science Laboratory at Gale crater contain ~30 wt.% XRD amorphous material that is rich in both sulfur and iron⁶. These factors have led us to consider hydrated amorphous iron sulfates as possible components in Martian surface materials. Amorphous iron sulfates were created through multiple synthesis routes, and then characterized with total x-ray scattering, TGA, SEM, visible/near-infrared (VNIR), thermal infrared (TIR), and Mössbauer techniques. We synthesized amorphous ferric sulfates (Fe(III)₂(SO₄)₃•~5-8H₂O) from sulfate-saturated fluids via two pathways: vacuum dehydration and exposure to low relative humidity (<11%) using a LiCl buffer. Amorphous ferrous sulfate (Fe(II)SO₄•~1H₂O) was synthesized via vacuum dehydration of melanterite (Fe(II) SO₄•7H₂O). We find that both the ferric and ferrous sulfates synthesized from these methods lack long-range (>10Å) order, and thus are truly amorphous. VNIR and TIR spectral data for the amorphous sulfates 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 all crystalline phase spectra available for comparison. The amorphous sulfates should be distinguishable based on the position of their Fe-related absorptions in the visible range and their spectral characteristics in the TIR. In the NIR, which is the spectral range that has primarily been used to detect sulfates on Mars, the bands associated with hydration at ~1.4 and 1.9 µm are significantly weakened, which greatly reduces their detectability in soil mixtures.

References: 1) Milliken et. al (2009) GRL, 36. 2)Roach et al. (2009) JGR-planet, 114. 3) Wang et al. (2013) Icarus, 226, 980-991. 4) McEwen et al. (2011) Science, 333. 5) Chevrier and Altheide (2008) GRL, 35. 6)Morris et al. (2013) LPSC 44, #1653.