Synthesis of akaganeite in the presence of sulfate: Implications for akaganeite formation in Yellowknife Bay, Gale Crater, Mars

Abstract

Introduction: Akaganeite (β-FeOOH) is an Fe(III) (hydr)oxide with a tunnel structure usually occupied by chloride. Akaganeite has been recently discovered in a mudstone on the surface of Mars by the Chemistry and Mineralogy (CheMin) and Sample Analysis at Mars (SAM) instruments onboard the Mars Science Laboratory (MSL) Curiosity Rover in Gale crater [1, 2]. Akaganeite was detected together with sulfate minerals [anhydrite (CaSO4) and basanite (2CaSO4·2H2O)] in the drilled Cumberland and John Clein mudstone samples at Yellowknife Bay [2]. Discovery of akaganeite and sulfates in the same samples suggests that sulfate ions could be present in aqueous solution during akaganeite formation. However, mechanism and aqueous environmental conditions of akaganeite formation (e.g., pH and range of sulfate concentration) in Yellowknife Bay remain unknown.The objective of our work was to perform synthesis of akaganeite without or with sulfate addition at variable pHs in order to constrain formation conditions of akaganeite in Yellowknife Bay, Gale crater on Mars.

Effect of Solution pH and Chloride Concentration on Akaganeite Precipitation: Implications for Akaganeite Formation on Mars

“…Solution pH was monitored with an AccupHast pH electrode and Orion Star A329 multiparameter meter. Solutions remained clear until pH ~2, and a brown precipitate, likely 2‐line ferrihydrite (Peretyazhko et al, ), formed at higher pH values. All prepared samples (initial pH 1.6, 4, 6, and 8) were heated in PYREX™ beakers covered with watch glasses in the oven at 90 °C for 24 hr.…”

“…Subsamples were also mixed with corundum to quantify the amounts of precipitated Fe(III) (hydr)oxides by Rietveld refinement and internal corundum standard method (Bish & Howard, ; Bish & Post, ). The amorphous phase abundance was calculated as the difference between the total content of all phases (100%) and crystalline phases (Peretyazhko et al, ). Starting structure parameters for akaganeite were taken from (Post et al, ).…”

“…Midinfrared diffuse reflectance spectra were measured with a Thermo Nicolet Nexus 670 FT‐IR spectrometer (1.67–16.7 μm, 600–6,000 cm −1 ) configured with a Pike integrating sphere that was purged with dry N 2 gas derived from liquid N 2 (Peretyazhko et al, ). The powder samples were placed into sample cups and stored in a glove box purged with N 2 (H 2 O ~100 ppm v ; RH < 0.5%) for 138 hr before analysis.…”

“…Akaganeite formation at higher pH values has been scarcely investigated. Peretyazhko et al () demonstrated formation of akaganeite though hydrolysis of FeCl 3 at initial solution pH of 2, while no akaganeite was detected after hydrolysis reaction at initial pH 4. Formation of akaganeite was reported at pH 4–6 through oxidation of Fe(II) chloride aqueous solution (Refait & Génin, ; Remazeilles & Refait, ) and pH 8 and 10 through Fe(III) chloride hydrolysis (Chitrakar et al, ; Deliyanni et al, ).…”

Large sulfur isotope fractionations in Martian sediments at Gale crater

“…Akaganeite, a chlorine-and iron-bearing hydroxide mineral that can form as an alteration product of pyrrhotite, is interpreted as a possible cementing agent in the hollow nodules that are more prevalent at CB than JK 14,28 . Structural equivalents of akaganeite that contain sulfate anions in place of chloride also exist 73 . Potential isotopic effects and reaction intermediates associated with the formation akaganeite as a result of pyrrhotite alteration, with or without structural sulfate, are presently unknown.…”