Reaction of akaganeite with Mars-relevant anions - ACS Earth and

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Reaction of akaganeite with Mars-relevant anions Tanya S. Peretyazhko, Michelle J Pan, Douglas W Ming, Elizabeth B Rampe, Richard V. Morris, and David G Agresti ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.8b00173 • Publication Date (Web): 18 Jan 2019 Downloaded from http://pubs.acs.org on January 21, 2019

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ACS Earth and Space Chemistry

Reaction of akaganeite with Mars-relevant anions Tanya S. Peretyazhko1*+, Michelle J. Pan2+, Douglas W. Ming3, Elizabeth B. Rampe3, Richard V. Morris3, David G. Agresti4

1Jacobs,

NASA Johnson Space Center, Houston, TX, USA

2Franklin 3NASA

and Marshall College, Lancaster, PA, USA Johnson Space Center, Houston, TX, USA

4University

*Corresponding

of Alabama at Birmingham, AL, USA

author: Tanya S. Peretyazhko, Jacobs, NASA Johnson Space Center,

Houston, TX 77058 ([email protected]) +equal

contribution

Keywords: iron oxide, anion exchange, Gale crater, Robert Sharp crater, alteration In preparation for ACS Earth and Space Chemistry

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ACS Earth and Space Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Akaganeite is an Fe(III) (hydr)oxide with a tunnel structure typically occupied by chloride. The mineral can undergo anion-exchange reactions in aqueous solution, resulting in incorporation of other anions together with Cl- into the tunnels. Identification of anions present in akaganeite tunnels may permit characterization of solution compositions in which akaganeite precipitated and/or existed. Akaganeite has been reported in several locations on Mars, including Yellowknife Bay in Gale crater. However, the nature of the tunnel anions has not been investigated. In order to constrain the nature of the tunnel anions in martian akaganeite, synthetic akaganeite (72 mg/g total Cl- content) was reacted with Mars-relevant anions (F-, OH- and SO42-). Release of Cl- into solution was monitored with ion chromatography. Anion-reacted akaganeite was characterized with instruments analogous to instruments onboard robotic space crafts including X-ray diffraction (XRD), Mössbauer spectroscopy, thermal and evolved gas analysis (TA/EGA), and visible and near infrared reflectance spectroscopy (VNIR). The results demonstrated that 17-71% of Cl- was released from akaganeite during 96h incubation in water and anion-bearing solutions. A combination of XRD and TA/EGA, similar to the instruments onboard the Mars Science Laboratory rover Curiosity, can distinguish between unreacted akaganeite and akaganeite reacted with SO42-, F- and OH- based on diffraction peak positions and evolved HCl, HF and SO2. VNIR analogous to the instrument on the Mars Reconnaissance Orbiter is only sensitive to Cl- replacement by OH- but not by SO42- and F- as evident from the changes in the shape and position of OH combination band. Mössbauer measurements at room temperature, with instruments similar to those onboard the Mars Exploration Rovers, did not distinguish among different tunnel anion compositions. The laboratory data are collectively consistent that akaganeite detected in Yellowknife Bay contains only Cl- in tunnels and likely

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formed and/or was in contact with Cl-bearing solutions during late stage diagenesis or aqueous alteration.

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Introduction Akaganeite (β-FeOOH) is a chloride-containing Fe(III) (hydr)oxide with a hollandite-type tunnel structure formed by corner-sharing of neighboring double chains of Fe(III) octahedra1-3. Chloride anions (Cl-) occupy the tunnels and stabilize the akaganeite structure4. The chloride is mobile and can be replaced by other anions, including fluoride (F-), bromide (Br-) and hydroxide (OH-) by anion-exchange reactions5. Sulfate (SO42-) has also been shown to be incorporated into the tunnels as in the Fe(III) phase schwertmannite6-7. The presence of other anions in the tunnels depends on solution composition in which akaganeite precipitated and/or existed3, 5. Terrestrial akaganeite usually forms through Fe(III) hydrolysis under acidic Cl-rich conditions and only Clcontaining akaganeite has been identified in natural environments8-12. Akaganeite occurrence on Mars was first proposed for Gusev crater and Meridiani Planum. The mineral could be a component of nanophase Fe(III) oxide (npOx), based on Mössbauer observations by the Mars Exploration Rovers (MER) and a correlation between iron and chlorine contents13-16. Minor amounts of akaganeite (up to 1.7 wt%) have been then directly identified 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 ancient (~3.5 Ga) lacustrine mudstones at Yellowknife Bay in Gale crater17-18. The mineral was also detected from orbit by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) onboard the Mars Reconnaissance Orbiter (MRO) in Robert Sharp crater and Antoniadi basin19. In contrast to terrestrial akaganeite, formation mechanisms and tunnel composition of martian akaganeite are still unclear. Comparison of martian observations with experimental data on akaganeite formation at different values of pH and dissolved Cl- concentrations reveals that akaganeite on Mars could form under different environmental conditions. Akaganeite in

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Yellowknife Bay likely precipitated under moderately saline, acidic to alkaline conditions while akaganeite in Robert Sharp crater may have formed under saline acidic conditions20. Fluoridebearing rocks and sulfate-containing veins were detected in Yellowknife Bay by the Curiosity rover21-22, indicating that F- and SO42- could be present in aqueous solutions in contact with akaganeite-containing sediments. In addition, mineralogical observations suggest that the aquatic systems in Yellowknife Bay likely experienced neutral pH conditions22. Based on previous experimental studies, such conditions could affect akaganeite tunnel composition by replacing chloride with fluoride, sulfate and hydroxide ions5-6. Identification of the anions in akaganeite tunnels is, therefore, important for interpretation of akaganeite data on Mars and for placing constraints on ancient aqueous environment conditions on Mars. The structure and thermal properties of akaganeite are sensitive to changes in tunnel composition caused by incorporation of other anions5, 23. However, it remains unknown if Clcontaining akaganeite can be distinguished from akaganeite in which Cl- in the tunnels is partially replaced by other anions using instruments onboard orbital and landed missions. The objective of this study was to determine if akaganeite with different tunnel compositions can be differentiated with laboratory mission-like instruments by reacting synthetic akaganeite with Mars-relevant anions (F-, OH- and SO42-). The initial and anion-reacted akaganeites were characterized by X-ray diffraction, thermal and evolved gas analysis, visible and near infrared reflectance spectroscopy, and Mössbauer spectroscopy which provide data analogous to those obtained from the MSL CheMin and SAM, the MRO CRISM and MER MIMOS-II instruments, respectively.

Materials and Methods 5



Akaganeite synthesis Akaganeite (hereafter denoted as Cl-akaganeite, Table 1) was synthesized by forced hydrolysis of a solution containing Fe(III) and chloride with a molar Fe/Cl ratio equal to 0.5. Iron(III) perchlorate and sodium chloride salts were chosen for the synthesis in order to simulate potential martian salts present at the time of akaganeite formation. Perchlorate is reported as one of the oxychlorine phases identified by SAM while halite (NaCl) has been tentatively identified by CheMin in the Yellowknife Bay mudstone17, 24. The presence of perchlorate in the initial solution does not affect anion tunnel composition because the perchlorate anion is not incorporated into tunnels25. For Cl-akaganeite synthesis, 7.08 g Fe(ClO4)3 (Sigma Aldrich) and 2.40 g NaCl (Fisher Scientific) were dissolved in 200 mL ultrapure deionized water (MilliQ, Millipore), then heated in an oven at 90 °C for 24h20. After hydrolysis, the precipitates were washed three times with ultrapure water by centrifugation and freeze-dried prior to characterization and anion exchange experiments. Synthesized Cl-akaganeite was also acid digested to determine the total chloride content. A 30 mg sample of Cl-akaganeite was digested in 20 mL of ultrapure 5 M HNO3 (Fisher Scientific) on a hot plate for 1h and then diluted to 100 mL with ultrapure water. Total Cl- content was measured by ion chromatography. Incubation experiments The incubation experiments were carried out with a procedure developed by Cai et al5 for preparation of akaganeite with different anions in the tunnels. In order to change the anion composition in akaganeite tunnels, synthesized Cl-akaganeite was incubated in aqueous solutions of NaOH (Sigma Aldrich), NaF (Fisher Scientific) and Na2SO4 (Acros Organics). Cl-akaganeite was also incubated in a solution containing a mixture of NaF and Na2SO4 to obtain a sample with three different anions in the tunnels. For single anion experiments, 200 mg of Cl-akaganeite was

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mixed with 50 mL of 0.1 M NaF, Na2SO4 or NaOH solution in 125-mL serum bottles. For two anion experiments, 200 mg Cl-akaganeite was added to 50 mL solutions containing a mixture of 0.05 M NaF with 0.05 M Na2SO4. Added anion concentrations were larger than SO42-, F- or OHconcentrations reported in natural waters (e.g., seawater: 28 mM SO42- and 0.07 mM F-; river water: 0.1 mM SO42- and 0.01 mM F-, pH 6.5-8.5)26-27 but allowed synthesis of akaganeite with different tunnel composition. Control experiments with no anion addition were also performed. Four replicates were prepared for each treatment. The suspensions were incubated in an oven at 55 °C, removed from the oven, and cooled to room temperature (RT) at selected time points (0, 24, 48, 72, and 96h). For dissolved Cl- analysis, 5 mL of the akaganeite suspension was collected into a 10 mL syringe and filtered through a 0.2 µm syringe filter (PVDF, Fisherbrand). At the end of the 96h incubation, solids were collected by centrifugation, rinsed with ultrapure water and freeze-dried for characterization. Sample nomenclature used hereafter is summarized in Table 1. The sample name corresponds to the anion with which the initial Cl-akaganeite was reacted (e.g., F-akaganeite corresponds to Cl-akaganeite reacted with F- and H2O-akaganeite to Cl-akaganeite incubated in ultrapure water without anion addition). It should be noted that variable amounts of Cl- were still in the tunnels in all reacted akaganeite samples (See “Dissolved Cl- and pH” section and Table 1). Characterization Chloride was measured by ion chromatography using a Dionex ICS-2000 instrument. The instrument is equipped with a Dionex IonPac AS18 column (4 x 250 mm), Dionex EGC III KOH eluent and a suppressed conductivity detector Dionex AERS 500 with a 20 µm injection volume. Measurements of solution pH were performed using a Thermo Scientific Orion Star Series pH Meter.

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X-ray diffraction (XRD) patterns were recorded using a Panalytical X’Pert Pro instrument with Co Kα radiation. Powder samples were placed on zero background slides and analyzed at 45 kV and 40 mA with a 0.02 2 step size and 1 min per step counting rate. The instrument was operated under ambient conditions and calibrated with a novaculite standard (Gemdugout, State College, PA). Rietveld refinement was carried out using MDI Jade software (Materials Date Incorporated, Livermore, California) with initial structure parameters for akaganeite taken from Post et al.28. Background patterns were fit by a polynomial and peaks were modelled by a Pearson-IV profile function. Visible and near-infrared reflectance (VNIR) spectra (0.35-2.5 µm) were collected with an Analytical Spectral Devices FieldSpec3 fiber-optic based spectrometer. The instrument was operated with a reflectance probe attachment (MugLite) operating in absolute reflectance mode. Continuum removed spectra were obtained using ENVI software (Harris Corporation). Prior to VNIR analysis, all samples were stored in N2-purged glove box (H2O ~100 ppmv measured by Vaisala MI70 humidity meter with DMP74A probe) for 114 h to mimic desiccating conditions on martian surface29. Measurements were performed at RT and ambient laboratory humidity immediately after removing the samples from the glove box. Mössbauer analyses were performed at RT with MIMOS-II instruments (SPESI, Inc. 30). Velocity calibration was carried out with the MERView software31 using the MIMOS-II differential signal and the spectrum for metallic Fe foil collected at RT. Mössbauer parameters at RT [center shift () with respect to metallic Fe foil, quadrupole splitting (EQ), full width at halfmaximum intensity (FWHM), and subspectral areas (A)] were obtained with a least-squares fitting procedure with MERFit software32. Uncertainties for , EQ and FWHM were ± 0.02 mm/s and ± 2% for A.

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Thermal analysis, including thermal gravimetry, differential scanning calorimetry and evolved gas analysis (TA/EGA), were conducted using a Labsys Evo Simultaneous Thermal Analysis instrument (Setaram Instrumentation, KEP technologies) connected to a quadrupole mass spectrometer (Thermostar GSD 320, Pfeiffer Vacuum Incorporated). The instrument has been configured to run under conditions similar to the SAM instrument on Curiosity (ramp rate 35 ºC/min, 25 mbar furnace pressure, 0.8 sscm flow rate, helium carrier gas)18, 33 enabling comparison of the data we collect to analysis of samples on Mars. Akaganeite samples were heated to 1000 ºC at a ramp rate of 35 ºC/min and 30 mbar furnace pressure with a flow rate of 10 sscm for helium carrier gas. Analyses performed in this study allow to understand thermal behavior of akaganeite with different tunnel composition and can be used as a guide in interpreting SAM data for natural samples analyzed on Mars. However, natural samples are composed of multiple mineral phases that have the potential to affect the evolved gas behavior of individual thermally decomposing phases.

Results and Discussion

Dissolved Cl- and pH The initial Cl-akaganeite was Cl-rich, containing about 72 mg/g Cl- (Table 1) that was within the range of previously reported Cl- content in akaganeite (1-100 mg/g)3, 12, 34. Variable amounts of Cl- were released in the incubation experiments with or without anion additions (Figure 1, Table 1). The Cl- release initiated immediately after adding akaganeite to all solutions (0h, Figure 1) and then Cl- continued to increase, reaching steady state within 24h for H2O-, F-, SO4and OH-akaganeite samples. Dissolved Cl- in F/SO4-akaganeite continued to increase from 0 to

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24 h, remained unchanged from 24 to 72h, and then increased again (Figure 1). Overall, the amount of chloride released by the end of experiment increased in the order of H2O-akaganeite (17.1 ± 0.6 %) < SO4-akaganeite (42.0 ± 0.7 %) < F-akaganeite (48.5 ± 0.6 %) < F/SO4akaganeite (69.4 ± 6.2 %) ≈ OH-akaganeite (71.8 ± 0.6 %). Presence of larger amounts of dissolved Cl- in F-, SO4-, OH- and F/SO4-akaganeite than in H2O-akaganeite indicated replacement of Cl- with F-, OH- and SO42- in the tunnels5. Dissolution of akaganeite was not expected to contribute to Cl- release, because akaganeite solubility is low at pH range of exchange experiments (pH96h 3 -12.5, Table 1)35. In addition to anion uptake, water incorporation in the tunnels might also occur during incubation36-37. The presence of three anions (Cl-, F- and SO42-) and H2O in the tunnels could likely destabilize the tunnel structure resulting in different dissolution behavior of F/SO4-akaganeite with respect to other samples (Figure 1). The changes in pH over time reflected the release of Cl- and might also indicate incorporation of other anions into akaganeite tunnels. Presence of Cl- in akaganeite tunnels requires a proton cosorption in order to balance the negative chloride charge2, 23. As a result, Clrelease from the tunnels causes a decrease in solution pH while Cl- incorporation leads to pH increase38. Although it has not been previously shown experimentally, incorporation of other anions in akaganeite tunnels might also affect pH in a way similar to Cl-. Initial pH values in the akaganeite suspensions (pH0h) were lower than pH values in the respective akaganeite-free solutions resulting from release of Cl- observed in the samples at the beginning of experiment, except in the presence of OH- (Figure 1, Table 2). During incubation, pH in H2O- and SO4akaganeite samples decreased with time and increased in the F-akaganeite (Table 2). The decrease in pH observed in H2O-, and SO4-akaganeite is explained by the release of H+ accompanying Cl- leaching from akaganeite tunnels38. The pH drop in the SO4-akaganeite might

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also indicate that H+ and Cl- releases were not compensated by H+ adsorption due to uptake of SO42- into tunnels. Increase in pH in the F-akaganeite was likely because H+ adsorption during Fuptake was larger than H+ release due to Cl- leaching from tunnels. The pH0h of the F/SO4akaganeite sample was also lower than the pH of corresponding akaganeite-free solution and remained unchanged during 96h incubation (Table 2). The pH of the OH-akaganeite suspension was around 12.51 ± 0.10 throughout the experiment and close to the pH of 0.1M NaOH (12.56 ± 0.07, Table 2), because of the excess of hydroxide ions relative to the released protons from the akaganeite structure. X-ray diffraction X-ray diffraction analysis of initial and reacted akaganeite revealed the sole presence of crystalline akaganeite, with no traces of other oxides (Figure 2). However, incubation with anion additions led to shifts in diffraction peaks (Figure 2) caused by changes in unit cell size with respect to Cl-akaganeite (Table 3). Unit-cell parameters of H2O-akaganeite were close to Cl-akaganeite with only a 0.023 ± 0.001 Å decrease in a direction and 0.009 ± 0.001 Å increase in c direction with respect to the initial Cl-akaganeite (Table 3). Reaction of Cl-akaganeite with NaOH led to more substantial changes in unit-cell parameters. The a and c dimensions decreased by 0.067 ± 0.001 and 0.084 ± 0.002 Å, respectively, in the OH-akaganeite (Table 3). Changes in unit-cell parameters of OHakaganeite correspond to structural alterations induced by release of substantial Cl- amounts23 (72% Cl- released after 96h incubation, Figure 1). Chloride exchange with OH- in the tunnels likely led to inward movement of iron atoms and hydroxyl groups in the tunnels causing unit-cell contraction39. Water incorporation in tunnels36-37 probably did not affect akaganeite unit cell size

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in both samples as evident from small structural changes in the H2O-akaganeite (17% Cl- release, Figure 1). Changes in unit-cell parameters of Cl-akaganeite reacted with anions were caused by Clrelease and anion incorporation into akaganeite tunnels. Unit cell parameters of F-akaganeite were similar to the OH-akaganeite (Table 3) although only 48% Cl- was leached from the sample (Figure 1). The observed decrease in unit cell parameters in the F-akaganeite can be explained by smaller ionic radius of F- compared to Cl- (1.33 Å F-, 1.81 Å Cl- 40). The cell dimensions of Fakaganeite prepared in this work through replacement of Cl- with F- were larger than the unit cell parameters previously reported for akaganeite synthesized through hydrolysis of Fe(III) fluoride salts and, therefore, containing only F- in the tunnels (a: 10.435-10.447 Å, b: 3.023-3.028 Å and c: 10.416-10.445 Å41-42). The smaller size of akaganeite with only F- in the tunnels results from partial replacement of OH- with F- in Fe octahedra during akaganeite formation through hydrolysis41-43. Such structural alteration would be highly improbable during Cl-akaganeite reaction with F- in solution. Unit-cell parameters of SO4-akaganeite slightly decreased in a and c dimensions with respect to the unreacted Cl-akaganeite (Table 3). Sulfate has been previously proposed to incorporate into akaganeite tunnels together with chloride6, 44. Release of Cl- from tunnels and formation of sulfate bidentate complexes by sharing oxygen atoms with Fe(III) in tunnels likely caused the decrease in cell parameters3, 44. Reaction of Cl-akaganeite with F/SO4containing solution also led to decrease in unit-cell parameters with respect to Cl-akaganeite (Table 3). The refined cell size was close to parameters reported for F- and SO4-akaganeite samples (Table 3) suggesting incorporation of both F- and SO42- into tunnels. Mössbauer spectroscopy

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Spectra of initial and anion-reacted akaganeite were fit by two Fe(III) doublets in octahedral coordination (Table S1). The fitted center shift () and quadrupole splitting (EQ) were the same within error for all samples ( ~0.037 mm/s; EQ ~ 0.55 mm/s and 0.97 mm/s; Table S1) and similar to the values reported previously for akaganeite (0.36-0.38 mm/s δ; 0.53-0.54 and 0.950.96 mm/s ΔEQ) at RT41, 43. Similarity of Mössbauer parameters for all samples indicates that Clrelease from the tunnels and incorporation of other anions did not cause substantial changes in Fe(III) local environments detectable in RT Mössbauer analysis. Visible and near-infrared spectroscopy Four main spectral features in the visible and near-infrared spectral range were 1) OH combination stretching and bending vibration bands centered at ~2.46 µm, (νOH + δOH: 3410 cm-1 + 650 cm-1 = 4060 cm-1 or 2.46 µm) 2) H2O stretching and bending combination band at ~2 µm, (νH2O + δH2O = 3473 + 1523 = 4996 cm-1 or 2 µm 3) overtone of OH stretching vibrations centered at ~1.45 µm (2νH2O = 2x(νH2O + 85.6 cm-1)) and 4) Fe3+ crystal field adsorption bands near 0.39, 0.49, 0.61 and 0.92 µm3, 45-46 (Figures 3 and S3). VNIR analyses revealed that release of 72% Cl- observed in the OH-akaganeite led to changes in the band position, shape and intensity with respect to Cl-akaganeite. The broad H2O combination band in Cl-akaganeite at 1.97 µm had contributions from interaction of H2O molecules with Cl- in tunnels, isolated H2O molecules in tunnels, and H2O on akaganeite surface36, 46. The band center shifted to a shorter wavelength in OH-akaganeite (1.93 µm, Figure 3b). Decrease in Cl- content alters hydrogen bonding environments in tunnels and has been shown to cause a shift of OH stretching vibrations to shorter wavelenghts47. We hypothesize that changes in hydrogen bonding might also induce a shift to shorter wavelength in the H2O combination band. Cl-akaganeite had a sharp asymmetric OH band at 2.46 µm with several

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shoulders in the 2.25-2.42 µm range. The band was assigned to combination stretching and outof-plane bending vibrations of hydrogen-bonded OH groups with Cl- in tunnels46. The shoulders were assigned to in-plane bending and stretching of isolated OH at 2.41 µm, out-of-plane bending and stretching of isolated OH at 2.33 µm and a combination of stretching and in-plane bending vibrations of hydrogen-bonded OH groups with H2O molecules at 2.25 µm23, 46. Release of Cl- in H2O-akaganeite led to weakening of the 2.46 µm band and increase in intensity of the 2.41 µm shoulder (Figure 3b). Further decrease in Cl- content in OH-akaganeite caused the transformation of the 2.46 µm band into a weak shoulder (Figure 3b). An opposite trend was observed for a spectral feature at 2.41 µm, which became a dominant band in the OH-akaganeite (Figure 3b), indicating increased abundance of isolated OH groups after removal of 72% Clfrom akaganeite tunnels. The shape and position of VNIR bands of akaganeite reacted with one or two anions were similar to Cl-akaganeite. All anion-reacted akaganeite had a well-resolved 2.46 µm band and shoulders in the 2.25-2.42 µm range (Figure 3c). A shoulder at 2.41 µm was more intense in the anion-reacted akaganeite than in Cl-akaganeite (Figure 3c) likely due to release of 42-69% Cl(Figure 1). The results demonstrate that VNIR can differentiate between Cl- and OH-akaganeite. However, the technique cannot distinguish between anion-reacted akaganeite (F-, SO4- and F/SO4-akaganeite) and Cl-akaganeite. Thermal and evolved gas analysis All samples had similar thermal behaviors but evolved gases varied depending on tunnel anion composition. The weight loss began at ~50 ºC in all samples and continued until 580-830 ºC (Figure S1). The initial loss of 2-3 wt% at temperatures 150 ºC was due to dehydroxylation and release of anions from collapsing akaganeite tunnels3, 5, 34.

All samples had an endothermic peak in the temperature range from 90 to 290 ºC (Figure S1) due to loss of adsorbed water and anions, dehydroxylation, and formation of hematite with low crystallinity3, 48. Exothermic peaks observed between 350 and 850 ºC were attributed to structural rearrangements, increase in crystallite size, and anion release accompanying hematite recrystallization3, 48-49. The highest temperature exothermic peak for akaganeite reacted with a single anion followed the order: Cl-akaganeite (504 ± 1 ºC) < H2O-akaganeite (767 ± 11 ºC) < SO4- akaganeite (801 ± 1 ºC) < F-akaganeite (821 ± 0 ºC) < OH-akaganeite (844 ± 1 ºC, Figure S1). The trend was consistent with the order of chloride release during akaganeite incubation with pure water and single anion solutions (Figure 1) and was likely caused by increased interaction of anions with OH groups in tunnels. For instance, Cai et al.5 reported higher decomposition temperature for F-akaganeite than for Cl-akaganeite and ascribed the difference to different electronegativity of halide anions. Fluoride has higher electronegativity and stronger hydrogen bonding interactions than Cl- in the tunnels, resulting in a higher temperature for hematite crystallization. The higher temperature for OH-akaganeite than for Cl-akaganeite could be due to retarded HCl release resulting from an elevated abundance of H2O molecules in tunnels5. The last exothermic peak for SO4-akaganeite was at higher temperature than the exothermic peaks reported for sulfate-containing akaganeite prepared by hydrolysis of FeCl3 in the presence of sulfate ions (550 ºC)50 and schwertmannite (530-580 ºC)51-52. The highest temperature exothermic peak for akaganeite F/SO4-akaganeite was expected to be close to that of the OH-akaganeite sample as similar amounts of Cl- were released in both samples (Figure 1).

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However, F/SO4-akaganeite (580 ± 3 ºC) had the lowest decomposition temperature after Clakaganeite. We hypothesize that the presence of three different anions (Cl-, SO42- and F-) and water in tunnels destabilizes the structure resulting in lower decomposition temperatures. Release of H2O occurred in the temperature range of 50 - 700 ºC, and the H2O peak (230-250 ºC) corresponded to the endothermic peaks of dehydroxylation (Figures 4 and S1). Variations in H2O peak temperature between the samples were not sensitive to the Cl- content in akaganeite. A small HCl release was detected at low temperatures between 50 and 300 ºC in all samples (Figure 4) and likely originated from surface bound chloride34. Loss of HCl mainly occurred at higher temperature (300 to 800 ºC) and the shape and peak position of evolving HCl varied substantially between the samples (Figure 4). Cl-akaganeite and H2O-reacted akaganeite had overlapping HCl peaks around 370, 420, and 500 ºC as a result of different Cl- environments (Figure 4a and b). Chloride was likely present in thermally collapsed tunnels within akaganeite, and/or adsorbed on the surface of the forming hematite2, 49, 53. The highest temperature of HCl release from akaganeite reacted with the single anion was lower than the temperature of the last exothermic peak but followed similar order: Cl-akaganeite ≈ H2O-akaganeite (500 ºC) < SO4akaganeite (610 ºC) < F-akaganeite (620 ºC) < OH-akaganeite (750 ºC, Figure 4). The results indicate that HCl release preceded final hematite recrystallization. In contrast, the last HCl peak in the F/SO4-akaganeite occurred at higher temperature (603 ºC, Figure 4) than the exothermic peak (580 ± 3 ºC, Figure S1). In addition to evolving HCl, HF releases occurred in F- and F/SO4-akaganeite samples (Figure 4d and f). The low temperature HF peak was centered at ~250 ºC and coincided with HCl release originating from adsorbed chloride. This observation might indicate that part of fluoride was surface bound. Fluoride has been shown to adsorb in a pH range from acidic to

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slightly alkaline on Fe(III) (hydr)oxides including granular ferric hydroxide, goethite, hematite and ferrihydrite54-57. The second HF loss was observed in the range from 400 to 800 ºC and coincided with HCl escape from akaganeite tunnels. This high temperature HF release supports that F- was incorporated into akaganeite tunnels. SO4- and F/SO4-akaganeite samples showed SO2 release resulting from thermal decomposition of sulfate to sulfur dioxide58-59 at the same temperature range as HCl (Figure 4c and f). The samples had some SO2 between 50 and 300 ºC; however, SO2 release was lower than HCl and HF in this temperature range. In addition, SO4- and F/SO4-akaganeite samples evolved SO2 in the temperature range from 400 to 800 ºC similar to schwertmannite (≥ 590 ºC)60-61. Comparison with evolving HCl and HF indicates that little sulfate was adsorbed onto the akaganeite surface and sulfate was mainly incorporated into the tunnels. Identification of anion-reacted akaganeite on Mars XRD and TA/EGA characterization of anion-reacted akaganeite in our experiments can be used to qualitatively search for varying anion compositions in the tunnels of akaganeite analyzed by the CheMin and SAM instruments on board the Curiosity rover on Mars. These analyses used together can distinguish between Cl-rich akaganeite, similar to Cl-akaganeite, and anion-reacted akaganeite. However, separate application of either of these techniques may not be sufficient to identify anions compositions. X-ray diffraction shows that replacement of Cl- with SO42- and Fcauses a shift of diffraction peaks to lower d-spacing and shrinking of the akaganeite unit cell (Table 3). A similar effect is also observed in the OH-akaganeite (Table 3), which makes it difficult to determine tunnel composition of akaganeite on Mars using CheMin data alone. Positive identification of F- and SO42- in the tunnels is possible using SAM characterization. Release of HF and/or SO2 gases at the same temperature as HCl can indicate that F- and/or SO42-

17



are incorporated in tunnels (Figure 4). Presence of akaganeite with low Cl- content and no other anions in tunnels, similar to OH-akaganeite, can be inferred from smaller akaganeite unit-cell parameters combined with the lack of evolved HF and SO2 at the same temperature range as HCl. As an example, we consider Cumberland drilled sample analyzed by CheMin and SAM instruments in ~3.5 Ga lacustrine mudstone in Yellowknife Bay, Gale crater17. Mineralogical analysis by CheMin identified detrital silicates (forsterite, plagioclase, pigeonite, augite, orthopyroxene), smectite, Fe (hydr)oxides (magnetite, akaganeite, hematite), Ca sulfates (anhydrite, basanite), Fe(II) sulfide (pyrrhotite) and X-ray amorphous material17. The amount of akaganeite detected in Cumberland was ~1.7 wt%17. Fluoride-bearing rocks were also detected in Yellowknife Bay by Chemical Camera (ChemCam) instrument21. Presence of sulfate- and fluoride-containing materials together with akaganeite might indicate that F- and SO42- could be present in aqueous solutions in contact with akaganeite-containing sediments and incorporate into the tunnels. SAM analysis of the Cumberland sample showed minor SO2 release (0.16 ±0.08 wt%) from 250 to 800 ºC (Figure 5a) that was assigned to decomposition of pyrrhotite and sulfate phases24. The release of HCl (0.014 ± 0.006 wt%) was also observed at 250 - 800 ºC temperature interval (Figure 5a) and was attributed to decomposition of oxychlorine species18, 24. Evolved HF was not detected in the Cumberland sample by SAM analysis. Comparison of laboratory and martian observations revealed that HCl and SO2 from SO4-akaganeite were evolved at similar temperature range (300-800 ºC, Figure 4c). The results suggest that akaganeite decomposition might contribute to the release of both HCl and SO2 in Cumberland. Akaganeite detected in Cumberland has an 001 diffraction peak at 7.48 Å (Figure S2) closer to the d-spacing of Cl-akaganeite (7.47 Å) than to the d-spacing of SO4-akaganeite (7.40 Å, Table 3). The results,

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therefore, indicate that akaganeite in Yellowknife Bay is close in composition to Cl-akaganeite and no other anions are present in the tunnels. These observations also suggest that akaganeite in Yellowknife Bay was in contact with Cl-containing solutions depleted in sulfate and fluoride. If akaganeite in Yellowknife Bay is close in composition to Cl-akaganeite than thermal decomposition of the sample with 1.7 wt% akaganeite would lead to release of ~0.1wt% HCl. The result indicates that akaganeite decomposition could have a contribution to the observed evolved HCl (Figure 5a). Larger calculated than observed HCl release in Cumberland might imply that released Cl- was trapped in the sample, for instance as chloride salts. Our laboratory results indicate that VNIR remote sensing can only distinguish between Cland OH-akaganeite and, therefore, it is only sensitive to Cl- replacement by OH- but not by Fand SO42- (Figure 3c). As a result, the presence of anions other than Cl- in akaganeite tunnels cannot be determined from VNIR data alone (Figure 3c). Akaganeite has been detected by remote sensing in several locations on Mars, including Robert Sharp crater19. Akaganeite detected in Robert Sharp crater has a sharp asymmetric OH combination band at 2.45 µm19 similar to Cl-akaganeite but different than OH-reacted akaganeite (Figure 5b). This comparison indicates that Cl-rich akaganeite is present in Robert Sharp crater, although occurrence of other anions in akaganeite tunnels cannot be ruled out. Additional data on chemical composition and mineralogy of the sediment deposits where akaganeite is detected on Mars would be necessary to make inferences about tunnel composition using VNIR instruments in Mars orbit or on the surface. Akaganeite, if present in Gusev crater and Meridiani Planum, is likely superparamagnetic or paramagnetic, because no magnetic ordering occurred in Mössbauer spectra at martian range of temperatures (190-290K)13-15. Akaganeite can be a part of a nanophase Fe(III) oxide assigned to

19



mineralogically non-specific Fe(III) doublet. Mössbauer analysis at RT demonstrated that the technique is not sensitive to the tunnel anion composition. Assuming magnetic splitting does not occur at martian temperatures, variation in Cl- content or presence of other anions in the tunnels cannot be determined in martian akaganeite. Conclusions Our results demonstrated that the akaganeite tunnel composition is sensitive to chemical properties of the aqueous solution in contact with the mineral. Depending on the aqueous conditions, variable amounts of Cl- were released from tunnels of Cl-akaganeite in the presence of SO42-, F- and OH-. Characterization of anion-reacted akaganeite showed that combined analysis by XRD and TA/EGA could distinguish between Cl-akaganeite and anion-reacted akaganeite. As a result, instruments analogous to CheMin and SAM onboard the Curiosity rover can be used to identify chemical composition of the akaganeite tunnels. VNIR remote sensing can differentiate between Cl- and OH-akaganeite but not sensitive to Cl- exchange with F- and SO42-. Room temperature Mössbauer spectroscopy cannot distinguish between akaganeites with different tunnel compositions. Comparison of our experimental data and martian results indicate that the akaganeite detected by CheMin in the lacustrine mudstone at Yellowknife Bay is Cl-rich, suggesting that the last solutions present in Yellowknife Bay were chloride-bearing and poor in sulfate and fluoride. Supporting information Table with fitted Mössbauer parameters and additional figures (TG and DSC data, XRD of Cumberland drilled sample and VNIR spectra). Acknowledgments

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We would like to thank Joanna Hogancamp for help with TA/EGA analysis. We are grateful to Dr John Carter for providing us with CRISM data for akaganeite in Robert Sharp crater and Dr Brad Sutter for providing us with SO2 and HCl SAM data for Cumberland. Michelle Pan acknowledges a Summer Intern Scholarship of Lunar and Planetary Institute. We thank three anonymous reviewers for valuable suggestions and comments that helped to improve the quality of the manuscript. We thank the Associate Editor Dr. Herbst for handling the manuscript. This work was supported by NASA Solar System Workings grant #15-SSW15_2-0074. References 1. Post, J. E.; Buchwald, V. F., Crystal structure refinement of akaganeite. Am. Mineral. 1991, 76, 272-277. 2. Stahl, K.; Nielsen, K.; Jiang, J.; Lebech, B.; Hanson, J. C.; Norby, P.; van Lanschot, J., On the akaganéite crystal structure, phase transformations and possible role in post-excavational corrosion of iron artifacts. Corros. Sci. 2003, 45, 2563-2575. 3. Cornell, R.; Schwertmann, U., The iron oxides: structure, properties, reactions, occurrence and uses. VCH: 2003. 4. Schwertmann, U.; Cornell, R. M., Iron oxides in the laboratory: preparation and characterization. Wiley-VCH: 2000. 5. Cai, J.; Liu, J.; Gao, Z.; Navrotsky, A.; Suib, S. L., Synthesis and anion exchange of tunnel structure akaganeite. Chem. Mater. 2001, 13, 4595-4602. 6. Peretyazhko, T. S.; Fox, A.; Sutter, B.; Niles, P. B.; Adams, M.; Morris, R. V.; Ming, D. W., Synthesis of akaganeite in the presence of sulfate: Implications for akaganeite formation in Yellowknife Bay, Gale Crater, Mars. Geochim. Cosmochim. Acta 2016, 188, 284-296.

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7. Bigham, J.; Schwertmann, U.; Carlson, L.; Murad, E., A poorly crystallized oxyhydroxysulfate of iron formed by bacterial oxidation of Fe (II) in acid mine waters. Geochim. Cosmochim. Acta 1990, 54, 2743-2758. 8. Mackay, A., -ferric oxyhydroxide-akaganéite. Mineral. Mag. 1962, 33, 270-280. 9. Bibi, I.; Singh, B.; Silvester, E., Akaganeite (-FeOOH) precipitation in inland acid sulfate soils of south-western New South Wales (NSW), Australia. Geochim. Cosmochim. Acta 2011, 75, 6429-6438. 10. Buchwald, V. F.; Clarke, R. S., Corosion of Fe-Ni alloys by Cl-containing akaganeite (betaFeOOH)-the Antartic mineral case Am. Mineral. 1989, 74, 656-667. 11. Pye, K., An occurance of akaganeite in recent oxidized carbonate concretions, Norfolk, England Mineral. Mag. 1988, 52, 125-126. 12. Johnston, J., Jarosite and akaganéite from White Island volcano, New Zealand: an X-ray and Mössbauer study. Geochim. Cosmochim. Acta 1977, 41, 539-544. 13. Morris, R. V.; Klingelhoefer, G.; Bernhardt, B.; Schröder, C.; Rodionov, D. S.; De Souza, P. A.; Yen, A. S.; Gellert, R.; Evlanov, E. N.; Foh, J., Mineralogy at Gusev Crater from the Mössbauer spectrometer on the Spirit Rover. Science 2004, 305, 833-836. 14. Morris, R. V.; Klingelhoefer, G.; Schröder, C.; Rodionov, D. S.; Yen, A. S.; Ming, D. W.; De Souza, P. A.; Fleischer, I.; Wdowiak, T.; Gellert, R., Mössbauer mineralogy of rock, soil, and dust at Gusev crater, Mars: Spirit's journey through weakly altered olivine basalt on the plains and pervasively altered basalt in the Columbia Hills. J. Geophys. Res. Planets 2006, 111, E02S13. 15. Morris, R. V.; Klingelhoefer, G.; Schröder, C.; Rodionov, D. S.; Yen, A. S.; Ming, D. W.; De Souza, P. A.; Wdowiak, T.; Fleischer, I.; Gellert, R., Mössbauer mineralogy of rock, soil, and

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22. Grotzinger, J. P.; Sumner, D. Y.; Kah, L. C.; Stack, K.; Gupta, S.; Edgar, L.; Rubin, D.; Lewis, K.; Schieber, J.; Mangold, N., A habitable fluvio-lacustrine environment at Yellowknife Bay, Gale Crater, Mars. Science 2014, 343, 1242777. 23. Song, X.; Boily, J.-F., Variable hydrogen bond strength in akaganéite. J. Phys. Chem. 2012, 116, 2303-2312. 24. Sutter, B.; Mcadam, A. C.; Mahaffy, P. R.; Ming, D. W.; Edgett, K. S.; Rampe, E. B.; Eigenbrode, J. L.; Franz, H. B.; Freissinet, C.; Grotzinger, J. P., Evolved gas analyses of sedimentary rocks and eolian sediment in gale crater, mars: results of the curiosity Rover's Sample Analysis at Mars (SAM) instrument from Yellowknife Bay to the Namib Dune. J. Geophys. Res. Planets 2017, 122, 2574-2609. 25. Paterson, R.; Rahman, H., The ion exchange properties of crystalline inorganic oxidehydroxides. Part II. Exclusion of perchlorate from βFeOOH by an ion sieve mechanism. J. Colloid Interface Sci. 1984, 97, 423-427. 26. Hem, J. D. Study and interpretation of the chemical characteristics of natural water; US Government Printing Office: 1959. 27. Stumm, W.; Morgan, J. J., Aquatic chemistry: chemical equilibria and rates in natural waters. John Wiley & Sons: 1996. 28. Post, J. E.; Heaney, P. J.; Von Dreele, R. B.; Hanson, J. C., Neutron and temperatureresolved synchrotron X-ray powder diffraction study of akaganéite. Am. Mineral. 2003, 88, 782788. 29. Morris, R. V.; Graff, T.; Achilles, C.; Agresti, D.; Ming, D.; Golden, D., Visible and near-IR reflectance spectra of Mars analogue materials under arid conditions for interpretation of Martian surface mineralogy. LPSC abstract #2757 2011.

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30. Klingelhoefer, G.; Morris, R. V.; Bernhardt, B.; Rodionov, D.; De Souza, P.; Squyres, S.; Foh, J.; Kankeleit, E.; Bonnes, U.; Gellert, R., Athena MIMOS II Mössbauer spectrometer investigation. J. Geophys. Res. Planets 2003, 108, 8067. 31. Agresti, D. G.; Dyar, M. D.; Schaefer, M. W., Velocity scales for Mars Mössbauer data. Hyperfine Interact. 2006, 170, 67-74. 32. Agresti, D. G.; Gerakines, P. A., Simultaneous fitting of Mars Mössbauer data. Hyperfine Interact. 2009, 188, 113-120. 33. Archer, P. D.; Franz, H. B.; Sutter, B.; Arevalo, R. D.; Coll, P.; Eigenbrode, J. L.; Glavin, D. P.; Jones, J. J.; Leshin, L. A.; Mahaffy, P. R., Abundances and implications of volatile‐bearing species from evolved gas analysis of the Rocknest aeolian deposit, Gale Crater, Mars. J. Geophys. Res. Planets 2014, 119, 237-254. 34. Chambaere, D. G.; Degrave, E., A study of the non-stoichiometrical halogen and water content of -FeOOH Phys. Status Solidi A Appl. Res. 1984, 83, 93-102. 35. Deliyanni, E.; Bakoyannakis, D.; Zouboulis, A.; Matis, K., Sorption of As (V) ions by akaganeite-type nanocrystals. Chemosphere 2003, 50, 155-163. 36. Song, X.; Boily, J.-F., Water vapor diffusion into a nanostructured iron oxyhydroxide. Inorg. Chem. 2013, 52, 7107-7113. 37. Mazeina, L.; Deore, S.; Navrotsky, A., Energetics of bulk and nano-akaganeite, β-FeOOH: Enthalpy of formation, surface enthalpy, and enthalpy of water adsorption. Chem. Mater. 2006, 18, 1830-1838. 38. Kozin, P. A.; Boily, J.-F., Proton binding and ion exchange at the akaganeite/water interface. J. Phys. Chem. 2013, 117, 6409-6419.

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39. Reguer, S.; Mirambet, F.; Dooryhee, E.; Hodeau, J.-L.; Dillmann, P.; Lagarde, P., Structural evidence for the desalination of akaganeite in the preservation of iron archaeological objects, using synchrotron X-ray powder diffraction and absorption spectroscopy. Corros. Sci. 2009, 51, 2795-2802. 40. Weast, R. C., Handbook of chemistry and physics CRC Press Inc: 1981. 41. Klimas, V.; Mažeika, K.; Jasulaitienė, V.; Jagminas, A., Formation, morphology and composition of F−-and Cl−-stabilized iron β-oxyhydroxides. J. Fluorine Chem. 2015, 170, 1-9. 42. Demourgues, A.; Wattiaux, A., Investigation of Fe-based oxyhydroxy-fluoride with hollandite-type structure. J. Fluorine Chem. 2011, 132, 690-697. 43. Ohyabu, M.; Ujihara, Y., Study of the chemical states of chlorine and fluorine in akaganétte. J. Inorg. Nucl. Chem. 1981, 43, 3125-3129. 44. Chitrakar, R.; Tezuka, S.; Sonoda, A.; Sakane, K.; Hirotsu, T., Bromate ion-exchange properties of crystalline akaganéite. Ind. Eng. Chem. Res. 2008, 48, 2107-2112. 45. Sherman, D. M.; Burns, R. G.; Burns, V. M., Spectral characteristics of the iron oxides with application to the Martian bright region mineralogy. J. Geophys. Res. Solid Earth 1982, 87, 10169-10180. 46. Bishop, J. L.; Murad, E.; Dyar, M., Akaganéite and schwertmannite: Spectral properties and geochemical implications of their possible presence on Mars. Am. Mineral. 2015, 100, 738-746. 47. Song, X.; Boily, J.-F., Surface hydroxyl identity and reactivity in akaganéite. J.Phys. Chem. 2011, 115, 17036-17045. 48. Morales, J.; Tirado, J.; Macias, M., Changes in crystallite size and microstrains of hematite derived from the thermal decomposition of synthetic akaganéite. J. Solid State Chem. 1984, 53, 303-312.

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49. Goni-Elizalde, S.; García-Clavel, M. E., Thermal behaviour in air of iron oxyhydroxides obtained from the method of homogeneous precipitation: Part II. Akaganeite sample. Thermochim. Acta 1988, 129, 325-334. 50. Musić, S.; Krehula, S.; Popović, S., Thermal decomposition of β-FeOOH. Mater. Lett. 2004, 58, 444-448. 51. Bigham, J.; Carlson, L.; Murad, E., Schwertmannite, a new iron oxyhydroxysulphate from Pyhäsalmi, Finland, and other localities. Mineral. Mag. 1994, 58, 641-648. 52. Parafiniuk, J.; Siuda, R., Schwertmannite precipitated from acid mine drainage in the Western Sudetes (SW Poland) and its arsenate sorption capacity. Geol. Q. 2010, 50, 475-486. 53. Klimas, V.; Mažeika, K.; Pakštas, V.; Spudulis, E.; Jagminas, A., Peculiarities of heatinginduced transformations in Fe (III) β-oxyhydroxides. J. Fluorine Chem. 2015, 173, 55-62. 54. Kumar, E.; Bhatnagar, A.; Ji, M.; Jung, W.; Lee, S.-H.; Kim, S.-J.; Lee, G.; Song, H.; Choi, J.-Y.; Yang, J.-S., Defluoridation from aqueous solutions by granular ferric hydroxide (GFH). Water Res. 2009, 43, 490-498. 55. Hiemstra, T.; Van Riemsdijk, W., Fluoride adsorption on goethite in relation to different types of surface sites. J. Colloid Interface Sci. 2000, 225, 94-104. 56. Mohapatra, M.; Rout, K.; Singh, P.; Anand, S.; Layek, S.; Verma, H.; Mishra, B., Fluoride adsorption studies on mixed-phase nano iron oxides prepared by surfactant mediationprecipitation technique. J. Hazard. Mater. 2011, 186, 1751-1757. 57. Shimizu, K.; Shchukarev, A.; Kozin, P. A.; Boily, J.-F., X-ray photoelectron spectroscopy of fast-frozen hematite colloids in aqueous solutions. 5. Halide Ion (F–, Cl–, Br–, I–) Adsorption. Langmuir 2013, 29, 2623-2630.

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58. Scheidema, M. N.; Taskinen, P., Decomposition thermodynamics of magnesium sulfate. Ind. Eng. Chem. Res. 2011, 50, 9550-9556. 59. Kim, T.-H.; Gong, G.-T.; Lee, B. G.; Lee, K.-Y.; Jeon, H.-Y.; Shin, C.-H.; Kim, H.; Jung, K.-D., Catalytic decomposition of sulfur trioxide on the binary metal oxide catalysts of Fe/Al and Fe/Ti. Appl. Cat. A 2006, 305, 39-45. 60. Henderson, S.; Sullivan, L. In Low temperature transformation of schwertmannite to hematite with associated CO2, SO and SO2 evolution, Proceedings of 19th world congress of soil science, soil solutions for a changing world, 2010; pp 72-75. 61. Lauer Jr, H.; Ming, D. W.; Golden, D.; Lin, I.-C.; Morris, R.; Boynton, W., Thermal and evolved gas analyses at reduced pressures: a mineral database for the Thermal Evolved Gas Analyzer (TEGA). In LPSC, 2000.

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Table 1. Sample nomenclature and Cl- content in the initial akaganeite (Cl-akaganeite) and akaganeite incubated in water and anioncontaining solution for 96h. Sample name

Description

Cl- content in akaganeite1, mg/g

Cl-akaganeite

Akaganeite synthesized from Fe(ClO4)3 in the presence of NaCl

71.6 ± 4.5

H2O-akaganeite

Cl-akaganeite incubated in water (control)

59.8 ± 4.6

F-akaganeite

Cl-akaganeite incubated in NaF solution

36.8 ± 4.5

SO4-akaganeite

Cl-akaganeite incubated in Na2SO4 solution

41.5 ± 4.5

OH-akaganeite

Cl-akaganeite incubated in NaOH solution

20.2 ± 4.5

F/SO4-akaganeite

Cl-akaganeite incubated in solution containing both NaF and Na2SO4

21.9 ± 4.4

1 Cl-

content in the reacted akaganeite was calculated as a difference between Cl- content in Cl-akaganeite and dissolved Cl- measured in the reacted samples at 96h (Figure 1). The results indicate that Cl- was not completely removed from the tunnels at the end of incubation experiment in all reacted samples.

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Table 2. pH of solutions with and without akaganeite addition. With akaganeite

Without akaganeite

Sample

pH0h1

pH96h1

Sample

pH

H2O-akaganeite

3.26 ± 0.03

3.01 ± 0.02

H2O

4.82 ± 0.11

SO4-akaganeite

3.54 ± 0.03

3.19 ± 0.01

0.1 M Na2SO4

4.31 ± 0.26

F-akaganeite

6.16 ± 0.06

6.60 ± 0.04

0.1 M NaF

7.47 ± 0.50

OH-akaganeite

12.47 ± 0.06 12.51 ± 0.01

0.1 M NaOH

12.56 ± 0.07

F/SO4-akaganeite

5.22 ± 0.04

0.05 M Na2SO4/ 0.05 M NaF

5.69 ± 0.04

5.21 ± 0.03

1pH

at the beginning (pH0h) and at the end (pH96h) of akaganeite incubation in water or anioncontaining solutions.

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Table 3. Rietveld refined cell parameters of Cl-akaganeite before and after incubation with water and anion-containing solutions and 110 peak position. Sample

a, Å

b, Å

c, Å

110 position, Å

Cl-akaganeite

10.548 ± 0.001

3.030 ± 0.001

10.529 ± 0.001

7.47

H2O-akaganeite

10.525 ± 0.001

3.031 ± 0.001

10.538 ± 0.001

7.45

SO4-akaganeite

10.537 ± 0.001

3.030 ± 0.002

10.510 ± 0.001

7.40

F-akaganeite

10.481 ± 0.002

3.032 ± 0.002

10.495 ± 0.002

7.35

OH-akaganeite

10.481 ± 0.002

3.029 ± 0.002

10.445 ± 0.002

7.36

F/SO4-akaganeite

10.512 ± 0.001

3.031 ± 0.001

10.485 ± 0.001

7.37

31



60

40

-

-

60

Released Cl , mg/g

80

Released Cl , %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 37

40

H2O-akaganeite SO4-akaganeite

20

F-akaganeite OH-akaganeite F/SO4-akaganeite

20

0

0 0

20

40

60

80

100

Time, hrs

Figure 1. Time-dependent release of chloride from Cl-akaganeite during incubation in water and anion-containing solutions.

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Figure 2. Powder X-ray diffraction patterns of Cl-akaganeite before and after incubation with water and anion-containing solutions. Dotted lines show the position of the 110 (7.47 Å), 200 (5.28 Å) and 310 (3.33 Å) diffraction peaks in Cl-akaganeite.

33



Figure 3. (a) Normal and (b)-(c) continuum removed VNIR reflectance spectra of Cl-akaganeite before and after incubation with water and anion-containing solutions. OH combination overtones (1.4 µm), H2O combination (2 µm), OH combination (2.46 µm) and Fe3+ adsorption bands (0.39, 0.49, 0.61 and 0.92 µm) are marked with dotted lines. 2.41 and 2.46 µm positions are marked below the OH combination band in (b). Spectra are offset for clarity and wavenumbers are shown in (a).

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Figure 4. Evolved gases (H2O m/z18, HF m/z20, HCl m/z36 and SO2 m/z64, m/z- mass to charge ratio) for (a) Cl-akaganeite, (b) H2O-akaganeite, (c) SO4-akaganeite, (d) F-akaganeite, (e) OHakaganeite and (f) F/SO4-akaganeite. 35



Figure 5. (a) Evolved HCl (m/z 36) and SO2 (m/z 64) detected by SAM in Cumberland drilled sample in Yellowknife Bay, Gale crater. (b) Continuum removed CRISM spectrum of akaganeite at Robert Sharp crater plotted together with continuum removed VNIR reflectance spectra for OH- and Cl-akaganeite.

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Reaction of akaganeite with Mars-relevant anions - ACS Earth and

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