Widespread detections of phyllosilicates in Noachian terrains on Mars imply a history of near-surface fluid-rock interaction. Ferrous trioctahedral smectites are thermodynamically predicted products of basalt weathering on early Mars, but to date only Fe 3+ -bearing dioctahedral smectites have been identified from orbital observations. In general, the physicochemical properties of ferrous smectites are poorly studied because they are susceptible to air oxidation. In this study, eight Fe 2+ -bearing smectites were synthesized from Fe 2+ -Mg-Al silicate gels at 200°C under anoxic conditions. Samples were characterized by inductively coupled plasma optical emission spectrometry, powder X-ray diffraction, Fe K-edge X-ray absorption spectroscopy (XAS), Mössbauer spectroscopy, and visible/near-infrared (VNIR) reflectance spectroscopy. The range of redox states was Fe respectively. The spectra for ferrous saponites are distinct from those for dioctahedral ferric smectites, permitting their differentiation from orbital observations. X-ray diffraction patterns for synthetic high-Mg ferrosaponite and high-Mg ferrian saponite are both consistent with the Sheepbed saponite detected by the chemistry and mineralogy (CheMin) instrument at Gale Crater, Mars, suggesting that anoxic basalt alteration was a viable pathway for clay mineral formation on early Mars.

Surface conditions on early Mars were likely anoxic, similar to early Earth, but the timing of the evolution to oxic conditions characteristic of contemporary Mars is unresolved. Ferrous trioctahedral smectites are the thermodynamically predicted products of anoxic basalt weathering, but orbital analyses of Noachian‐aged terrains find primarily Fe3+‐bearing clay minerals. Rover‐based detection of Fe2+‐bearing trioctahedral smectites at Gale Crater suggests that ferrous smectites are the unoxidized progenitors of orbitally detected ferric smectites. To assess this pathway, we conducted ambient‐temperature oxidative alteration experiments on four synthetic ferrous smectites having molar Fe/(Mg + Fe) from 1.00 to 0.33. Smectite suspension in air‐saturated solutions produced incomplete oxidation (24–38% Fe3+/ΣFe). Additional smectite oxidation occurred upon reexposure to air‐saturated solutions after anoxic hydrothermal recrystallization, which accelerated cation and charge redistribution in the octahedral sheet. Oxidation was accompanied by contraction of the octahedral sheet (d(060) decreased from 1.53–1.56 Å to 1.52 Å), consistent with a shift toward dioctahedral structure. Ferrous smectite oxidation by aqueous hydrogen peroxide solutions resulted in nearly complete Fe2+ oxidation but also led to partial Fe3+ ejection from the structure, producing nanoparticulate hematite. Reflectance spectra of oxidized smectites were characterized by (Fe3+,Mg)2‐OH bands at 2.28–2.30 μm, consistent with oxidative formation of dioctahedral nontronite. Accordingly, ferrous smectites are plausible precursors to observed ferric smectites on Mars, and their presence in late‐Noachian sedimentary units suggests that anoxic conditions may have persisted on Mars beyond the Noachian.

[1] High-silica materials have been observed on Mars, both from orbit by the CRISM spectrometer and in situ by the Spirit rover at Gusev Crater. These observations potentially imply a wet, geologically active Martian surface. To understand silica formation on Mars, it is useful to study analogous terrestrial silica deposits. We studied silica coatings that occur on the 1974 Kilauea flow in the Ka'u Desert, Hawaii. These coatings are typically composed of two layers: a ∼10 mm layer of amorphous silica, capped by a ∼1 mm layer of Fe-Ti oxide. The oxide coating is composed of ∼100 nm spherules, suggesting formation by chemical deposition. Raman spectroscopy indicates altered silica glass as the dominant phase in the silica coating and anatase and rutile as dominant phases in the Fe-Ti coating; jarosite also occurs within the coatings. Oxygen isotopic contents of the coatings were determined by secondary ion mass spectrometry (Cameca 7f and NanoSIMS). The measured values, d18 O Fe-Ti = 14.6 ± 2.1‰, and d 18 O silica = 12.1 ± 2.2‰ (relative to SMOW), are enriched in 18 O relative to the basalt substrate. The observations presented are consistent with a residual formation mechanism for the silica coating. Acid-sulfate solutions leached away divalent and trivalent cations, leaving a silicaenriched layer behind. Micrometer-scale dissolution and reprecipitation may have also occurred within the coatings. Chemical similarities between the Hawaiian samples and the high-silica deposits at Gusev suggest that the Martian deposits are the product of extended periods of similar acid-sulfate leaching.

[1] Airborne Visible/Near-Infrared Imaging Spectrometer (AVIRIS) data acquired over the Ka'u Desert are atmospherically corrected to ground reflectance and used to identify the mineralogic components of relatively young basaltic materials, including 250-700 and 200-400 year old lava flows, 1971 and 1974 flows, ash deposits, and solfatara incrustations. To provide context, a geologic surface units map is constructed, verified with field observations, and supported by laboratory analyses. AVIRIS spectral endmembers are identified in the visible (0.4 to 1.2 mm) and short wave infrared (2.0 to 2.5 mm) wavelength ranges. Nearly all the spectral variability is controlled by the presence of ferrous and ferric iron in such minerals as pyroxene, olivine, hematite, goethite, and poorly crystalline iron oxides or glass. A broad, nearly ubiquitous absorption feature centered at 2.25 mm is attributed to opaline (amorphous, hydrated) silica and is found to correlate spatially with mapped geologic surface units. Laboratory analyses show the silica to be consistently present as a deposited phase, including incrustations downwind from solfatara vents, cementing agent for ash duricrusts, and thin coatings on the youngest lava flow surfaces. A second, Ti-rich upper coating on young flows also influences spectral behavior. This study demonstrates that secondary silica is mobile in the Ka'u Desert on a variety of time scales and spatial domains. The investigation from remote, field, and laboratory perspectives also mimics exploration of Mars using orbital and landed missions, with important implications for spectral characterization of coated basalts and formation of opaline silica in arid, acidic alteration environments.

Natural and synthetic hydrous amorphous silicas were investigated with single-pulse 29 Si magic angle spinning (MAS) NMR and with vibrational spectroscopic methods. Samples included a volcanically derived silica coating on young basalt from Kilauea, Hawaii, as well as hyalite (opal-AN), silica sinters, and synthetic silica gels and silicic acid. Pulse delays of up to an hour were employed for silica samples with slow spin lattice relaxation rates, and nearly fully relaxed spectra (90-100%) were demonstrably achieved for all samples. 29 Si NMR spectra consisted of two broad, overlapping peaks at -111 and -102 ppm and a smaller peak at -92 ppm, corresponding to Q 4 , Q 3 , and Q 2 sites, respectively. The Hawaiian silica coating and silicic acid samples displayed high Q 3 and Q 2 contents; in particular, the structural Si-OH content of the coating was unusually high for a natural silica (5.4 ± 0.4 wt% H 2 O). Saturation-recovery spectra of the Hawaiian silica with increasing delay times were consistent with "stretched exponential" relaxation behavior and three-dimensional distribution of paramagnetic centers. Attenuated total reflectance infrared (ATR-IR) and Raman spectra of the silica powders indicated fully amorphous structures, and displayed hydrous (SiO 3 OH) and anhydrous silicate vibrational bands in positions consistent with previous work. Raman spectra of some samples indicated modest grain to grain heterogeneity. Inferred Si-OH contents from ATR-IR band ratios were strongly correlated with hydroxyl contents calculated from NMR spectra. The high Si-OH content of the Hawaiian silica coating suggests it is diagenetically immature and has not been exposed to elevated temperatures.