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Abstract:Opportunity has investigated in detail rocks on the rim of the Noachian age Endeavour Crater, where orbital spectral reflectance signatures indicate the presence of Fe +3 -rich smectites.The signatures are associated with fine-grained, layered rocks containing spherules of diagenetic or impact origin. The layered rocks are overlain by breccias and both units are cut by calcium sulfate veins precipitated from fluids that circulated after the Endeavour impact. Compositional data for fractures in the layered rocks suggest formation of Al-rich smectites by aqueous leaching. Evidence is thus preserved for water-rock interactions before and after the impact, with aqueous environments of slightly acidic to circum-neutral pH that would have been more favorable for prebiotic chemistry and microorganisms than those recorded by younger sulfaterich rocks at Meridiani Planum. Main Text:Introduction.

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Preparation of Nickel-substituted Goethite and Hematite Ni(II)-substituted goethite and hematite samples used for kinetic release experiments were prepared using modified previously reported methods (Schwertmann and Cornell, 2000). Ni(II)-goethite was synthesized by slowly adding 125 mL of a solution containing 0.98 M ferric nitrate and 0.02 M Ni(II) chloride to 225 mL of 5 M NaOH. The slurry was then diluted to 1 L and placed in an oven at 70 °C for 5 days. Ni(II)-hematite was prepared by slowly adding 600 mL of a solution containing 0.198 M ferric nitrate and 0.002 M Ni(II) chloride to 360 mL of 1 M NaOH. The pH of this slurry was then adjusted to 8.5 by dropwise addition of 1.0 or 0.1 M NaOH. The suspension was then placed in an oven set to 98 °C for 11 days. Once removed from the oven and cooled to room temperature, both materials were treated with 0.25 M HCl for 2 hr using a solid to solution ratio of 1:100. This acid treatment was employed to remove residual adsorbed cations and amorphous Ni-containing iron oxides. The materials were then washed with DI water by centrifugation until a pH of >5 was achieved. The solids were resuspened into 250 mL of DI water and stored as a suspension until further use. An aliquot was removed and oven dried at 70 °C to obtain powder that was used for sample characterization. Mineral surface area (Table DR1) was determined by collecting nitrogen B.E.T. adsorption isotherms at 77 K using a Quantachrome instruments Autosorb-1. X-ray diffraction (Rigaku Geigerflex D-MAX/A diffractometer using Cu-Kα radiation) confirmed that the Ni-substituted Fe oxides consisted of goethite or hematite and contained no other crystalline impurities. Ni content (Table DR1) was determined by inductively-coupled plasma optical emission spectrometry (ICP-OES, Perkin-Elmer Optima 7300DV) after digestion of the solid in a 20% HNO 3 :5% HCl (trace metal grade, Fisher Scientific) mixture. The digested sample liquid was diluted as necessary before analysis. Fe(II)-catalyzed Nickel Incorporation into Goethite and Hematite All experiments were conducted under anoxic conditions in an anaerobic chamber (Coy Laboratory Products, Inc.) containing a 3% H 2 /97% N 2 atmosphere with a Pd catalyst to eliminate residual O 2. Doubly deionized water (18.2 MΩ cm) was sparged with ultra-high purity N 2 gas prior to being brought into the anaerobic chamber. Trace oxygen levels in the chamber atmosphere were further lowered by a secondary oxygen trap; the chamber atmosphere was bubbled in sequence through a 15% pyrogallol/ 50% KOH solution and deionized water. The KOH solution also served to remove trace amounts of CO 2 from the chamber. The filtered gas stream was used to re-sparge and further deoxygenate the deionized water and any solution not prepared in the chamber. Dissolved oxygen content of the deionized water was estimated colorimetrically (CHEMetrics test kit K-7511) and all analyses were below the detection limit of 1 μg/L. Fe(II) stock solutions were prepared from deoxygenated deionized water and reagentgrade ...

Electron transfer and atom exchange (ETAE) between aqueous Fe(II) and Fe(III) oxides induces surface growth and dissolution that affects trace element fate and transport. We have recently demonstrated Ni(II) cycling through goethite and hematite (adsorbed Ni incorporates into the mineral structure and preincorporated Ni releases to solution) during Fe(II)-Fe(III) ETAE. However, the chemical parameters affecting net trace element release remain unknown. Here, we examine the chemical controls on Ni(II) and Zn(II) release from Ni- and Zn-substituted goethite and hematite during reaction with Fe(II). Release follows a rate law consistent with surface reaction limited mineral dissolution and suggests that release occurs near sites of Fe(III) reductive dissolution during Fe(II)-Fe(III) ETAE. Metal substituent type affects reactivity; Zn release is more pronounced from hematite than goethite, whereas the opposite trend occurs for Ni. Buildup of Ni or Zn in solution inhibits further release but this resumes upon fluid exchange, suggesting that sustained release is possible under flow conditions. Mineral and aqueous Fe(II) concentrations as well as pH strongly affect sorbed Fe(II) concentrations, which directly control the reaction rates and final metal concentrations. Our results demonstrate that structurally incorporated trace elements are mobilized from iron oxides into fluids without abiotic or microbial net iron reduction. Such release may affect micronutrient availability, contaminant transport, and the distribution of redox-inactive trace elements in natural and engineered systems.

Understanding the interaction of water with metal oxide surfaces is important to a diverse array of fields and is essential to the interpretation of surface charging and ion adsorption behavior. High-resolution specular X-ray reflectivity was used to determine the termination of and water adsorption on the alpha-Al2O3 (012)-aqueous solution interface. Interference features in the reflectivity data were used to identify the likely termination, consisting of a full Al2O3 layer plus an additional oxygen layer that completes the coordination shell of the upper aluminum site. This was further investigated through a model-independent inversion of the data using an error correction algorithm, which also revealed that there are two sites of adsorbed water above the surface. Characteristics of these two water sites were quantified through a model-dependent structural refinement, which also revealed additional layering in the interfacial water that gradually decays toward disordered bulk water away from the surface. Although the termination observed in this study differs from that assumed in past studies of surface charging, the density of key surface functional groups is unchanged, and thus, predictions of surface charging behavior are unchanged. As the pH(pzc) of this surface has yet to be modeled accurately, a full 3-dimensional surface structural analysis based on the termination observed in this study is needed so that the effects of surface functional group bond length changes on the pK(a) values can be incorporated. Consideration of the termination and sites of water adsorption suggest that singly coordinated oxygen groups will be the primary sites of ion adsorption on this surface.