Mössbauer mineralogy of rock, soil, and dust at Meridiani Planum, Mars: Opportunity's journey across sulfate‐rich outcrop, basaltic sand and dust, and hematite lag deposits

Abstract

The Mössbauer (MB) spectrometer on Opportunity measured the Fe oxidation state, identified Fe‐bearing phases, and measured relative abundances of Fe among those phases at Meridiani Planum, Mars. Eight Fe‐bearing phases were identified: jarosite (K,Na,H3O)(Fe,Al)(OH)6(SO4)2, hematite, olivine, pyroxene, magnetite, nanophase ferric oxides (npOx), an unassigned ferric phase, and metallic Fe (kamacite). Burns Formation outcrop rocks consist of hematite‐rich spherules dispersed throughout S‐rich rock that has nearly constant proportions of Fe3+ from jarosite, hematite, and npOx (29%, 36%, and 20% of total Fe). The high oxidation state of the S‐rich rock (Fe3+/FeT ∼ 0.9) implies that S is present as the sulfate anion. Jarosite is mineralogical evidence for aqueous processes under acid‐sulfate conditions because it has structural hydroxide and sulfate and it forms at low pH. Hematite‐rich spherules, eroded from the outcrop, and their fragments are concentrated as hematite‐rich soils (lag deposits) on ripple crests (up to 68% of total Fe from hematite). Olivine, pyroxene, and magnetite are primarily associated with basaltic soils and are present as thin and locally discontinuous cover over outcrop rocks, commonly forming aeolian bedforms. Basaltic soils are more reduced (Fe3+/FeT ∼ 0.2–0.4), with the fine‐grained and bright aeolian deposits being the most oxidized. Average proportions of total Fe from olivine, pyroxene, npOx, magnetite, and hematite are ∼33%, 38%, 18%, 6%, and 4%, respectively. The MB parameters of outcrop npOx and basaltic‐soil npOx are different, but it is not possible to infer mineralogical information beyond octahedrally coordinated Fe3+. Basaltic soils at Meridiani Planum and Gusev crater have similar Fe‐mineralogical compositions.

Origin of iron oxide spherules in the banded iron formation of the Bababudan Group, Dharwar Craton, Southern India

“…Iron oxide spherules (-Fe 2 O 3 ), coexisting with jarosite, were detected on the Martian surface at Meridiani Planum (Christensen, 2000;Christensen, 2001;Klingelhöfer, 2004;Morris, 2006) and were interpreted as being of abiotic origin, as a result of the breakdown of jarosite through groundwater reaction during oxidizing diagenesis (Chan, 2004;McLennan, 2005), or due to oxidation of primary pyrite by hydrothermal, sulphur-rich solutions (Golden, 2008;Zolotov and Shock, 2005). However, in terrestrial Fe-oxide grains and spherules, organic matter can be present (McConchie, 1987;Klein and Beukes, 1992;Yoshioka et al, 2001;Orberger et al, 2006;Pinti et al, 2007) and thus the Fe-oxide spherules on Mars may represent an important target in the search of biomarkers.…”

Single-crystal X-ray diffraction study of synthetic sodium–hydronium jarosite

“…The most flexible areas in the jarosite structure are localized at AO 12 edges that are not shared with neighbouring FeO 6 octahedra. Importantly, for the application of 1 3 to K-Na-H 3 O jarosite solid solutions with possible substitution of Al 3+ for Fe 3+ (Morris et al 2006). Reflectance VIS-NIR spectra from the Mars Reconnaissance Orbiter of deposits south of Ius/Melas Chasma were assigned to nonstoichiometric H 3 O-bearing, Fe-deficient jarosite (Milliken et al 2013).…”

Ordered ferrimagnetic form of ferrihydrite reveals links among structure, composition, and magnetism

“…Being antiferromagnetic at ambient temperature (15,18) and with only a weak ferrimagneticlike component, ferrihydrite is normally not considered in the interpretation of magnetic enhancement in soils on Earth (19) or Mars (20), nor is it considered useful in the tailoring of functional ferrimagnetic nanomaterials, in contrast with the ferrimagnets magnetite (Fe 3 O 4 ) and maghemite (γ-Fe 2 O 3 ) (21,22). However, as will be shown below, ferrihydrite aged at different temperatures in the presence of selected anions undergoes a significant magnetic enhancement corresponding to the formation of an intermediate phase preceding its transformation into hematite (α-Fe 2 O 3 ) (11,(23)(24)(25), and this phase may play an important role in the magnetic enhancement of aerobic soils.…”