1 2 3 4 5 Effect of solution pH and chloride concentration on akaganeite precipitation:
6 Implications for akaganeite formation on Mars
7 8 9 10 T.S. Peretyazhko1, D.V. Ming2, E.B. Rampe2, R.V. Morris2, D.G. Agresti3 11 12 13 14 15 1 Jacobs, NASA Johnson Space Center, Houston, TX 77058 16 17 2 NASA Johnson Space Center, Houston, TX 77058 18 19 3University of Alabama at Birmingham, Birmingham, AL 35294 20 21 22 23 Corresponding author: T. S. Peretyazhko, Jacobs, NASA Johnson Space Center, 24 Houston, TX 77058 ([email protected]) 25 26 27 28 29 In preparation for 30 31 Journal of Geophysical Research: Planets 32 33 34 Key points: 35 Akaganeite formation on Mars is controlled by solution pH and chloride concentration 36 Akaganeite formed in acidic to alkaline solutions of moderate salinity in Yellowknife 37 Bay 38 Akaganeite formed in acidic environments in Robert Sharp crater 39 40
1
41 Abstract 42 43 Akaganeite, a Cl-bearing Fe(III) (hydr)oxide, has been reported in several locations on Mars,
44 including Yellowknife Bay in Gale crater and Robert Sharp crater, but formation conditions
45 remain unknown. We investigated akaganeite precipitation as a function of solution pH and
46 chloride concentration. Akaganeite was synthesized through hydrolysis of Fe(III) perchlorate in
47 the presence of 0.02, 0.05 and 0.1 M chloride and at initial solution pH of 1.6, 4, 6 and 8 at 90
48 °C. Mineralogy of the precipitated Fe(III) (hydr)oxides was characterized by X-ray diffraction
49 (XRD), infrared spectroscopy and Mössbauer spectroscopy and total chloride and perchlorate
50 contents were determined by ion chromatography. XRD revealed that akaganeite formed alone or
51 in mixtures with ferrihydrite, hematite and/or goethite at initial pH 1.6 with 0.02, 0.05 and 0.1 M
52 Cl- and at initial pH from 4 to 8 with 0.05 and 0.1 M Cl-. Mössbauer parameters for akaganeite
53 and ferrihydrite were similar to nanophase Fe(III) oxide reported on Mars. Infrared analysis
54 revealed that akaganeite bands at ~2 and ~2.45 µm were sensitive to amount of akaganeite, total
55 chloride content and presence of other Fe(III) (hydr)oxides. The obtained results indicate that
56 akaganeite in Yellowknife Bay likely formed under acidic to alkaline (1.6 < pH < 8) oxidizing
57 conditions in solutions containing > 0.05 M Cl- by the dissolution of basaltic minerals.
58 Akaganeite in Robert Sharp crater may have formed in acidic pH < 4 solutions with ~ 0.1 M Cl-
59 through oxidative dissolution of Fe(II) sulfides and/or oxidizing dissolution of basalts in
60 localized acidic solutions.
61
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62 1. Introduction 63 64 Akaganeite (β-FeOOH) is Fe(III) (hydr)oxide with a tunnel structure stabilized by the
65 presence of chloride [Cornell and Schwertmann, 2003]. Akaganeite has been proposed as a
66 possible Fe(III) (hydr)oxide on Mars as a part of nanophase Fe(III) oxide on the basis of
67 Mössbauer data [Morris et al., 2004; Morris et al., 2006a]. The hypothesis is supported by the
68 observation that Cl/Fe ratio of 0.14 for nanophase Fe(III) oxide in basaltic soils [Morris et al.,
69 2008] was close to the Cl/Fe ratio reported for akaganeite (0.11-0.17, e.g., [Bibi et al., 2011;
70 Song and Boily, 2012; Stahl et al., 2003; Xiong et al., 2008]). Presence of akaganeite has only
71 been recently confirmed by instruments onboard Mars robotic spacecraft [Carter et al., 2015;
72 Vaniman et al., 2014]. The mineral was first detected as a minor phase in the drilled Cumberland
73 and John Klein mudstone samples at Yellowknife Bay, Gale crater by the Chemistry and
74 Mineralogy (CheMin) instrument onboard the Mars Science Laboratory (MSL) Curiosity Rover
75 [Vaniman et al., 2014]. Akaganeite was also reported in Robert Sharp crater and Antoniadi basin
76 from observations by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on
77 board the Mars Reconnaissance Orbiter (MRO) [Carter et al., 2015]. A variety of contrasting
78 environmental conditions have been proposed for the deposits where akaganeite occurs on Mars.
79 Morphological, mineralogical and chemical observations indicated that an ancient lake-like body
80 of near-neutral, low salinity water likely existed in Yellowknife Bay [Grotzinger et al., 2014]. In
81 contrast, evaporitic lagoon-like environments with mildly acidic pH 3-6 conditions and elevated
82 chloride (Cl-) concentrations have been hypothesized to exist in Robert Sharp crater and
83 Antoniadi basin [Carter et al., 2015]. Existence of such diverse environments implies akaganeite
84 precipitation under a wide range of solution pH and Cl- concentrations. However, limited
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85 experimental evidence is available to support akaganeite formation on Mars under substantially
86 different environmental conditions.
87 Solution pH and Cl- concentration are the dominant factors controlling akaganeite formation
88 in terrestrial environments. Natural akaganeite usually forms through oxidation of Fe(II) and
89 metallic Fe to Fe(III) followed by Fe(III) hydrolysis to akaganeite under chloride-rich acidic
90 conditions in hot brines, marine environments, oxidized sulfide-rich sediments, carbonate
91 concretions, volcanic rocks, and meteorites [Bibi et al., 2011; Buchwald and Clarke, 1989;
92 Johnston, 1977; Mackay, 1962; Morris et al., 2000; Pye, 1988]. Synthetic akaganeite forms
93 through forced hydrolysis of 0.1-2 M Fe(III) chloride solutions at room temperature or at
94 elevated temperatures (40-120 °C) under acidic (initial pH < 2) conditions [Zhao et al., 2012].
95 Akaganeite formation at higher pH values has been scarcely investigated. Peretyazhko et al.
96 [2016] demonstrated formation of akaganeite though hydrolysis of FeCl3 at initial solution pH of
97 2, while no akaganeite was detected after hydrolysis reaction at initial pH 4. Formation of
98 akaganeite was reported at pH 4 - 6 through oxidation of Fe(II) chloride aqueous solution [Refait
99 and Génin, 1997; Remazeilles and Refait, 2007] and pH 8 and 10 through Fe(III) chloride
100 hydrolysis [Chitrakar et al., 2006; Deliyanni et al., 2001]. However, a systematic study of a
101 combined effect of pH and Cl- concentration on akaganeite formation has not been undertaken.
102 The objective of this work was to investigate formation of akaganeite at variable pH and
103 dissolved Cl- concentrations and to characterise ancient aqueous environments in Gale and
104 Robert Sharp craters where akaganeite is found.
105
106 2. Materials and methods 107 108 2.1. Synthesis procedure
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109 Iron(III) (hydr)oxides were synthesized by thermal hydrolysis of solution containing both
110 Fe(ClO4)3 (Aldrich, Cl <0.005% ) and NaCl (Fisher, >99% purity). Perchlorate and chloride salts
111 were used on the basis of tentative evidence for their existence on the martian surface.
112 Perchlorate is inferred from Yellowknife Bay mudstone that contains oxychlorine phases in
113 association with akaganeite [Ming et al., 2014]. Chloride salts are likely present in Gale crater
114 [Sutter et al., 2017] and halite (NaCl) has been tentatively identified by the CheMin at
115 Yellowknife Bay [Vaniman et al., 2014] and definitively identified higher in the stratigraphic
116 section in the Quela sample in the Murray formation [Achilles et al., 2017].
117 All solutions were prepared in ultrapure deionized water (Milli-Q, Millipore). Syntheses
118 were performed at different initial chloride concentrations and initial pH values. In the first set of
119 samples, variable amounts of NaCl were added to 0.1 M Fe(ClO4)3 solution to obtain initial
120 chloride concentrations of 0.1, 0.05 and 0.02 M. The values of pH were not adjusted in these
121 samples and averaged ~1.6. The second set of samples was adjusted to pH ~4, the third to pH ~6
122 and the fourth to pH ~8 by slowly adding 0.1 M NaOH (~0.5-0.7 ml/min) to stirred solutions.
123 Solution pH was monitored with an AccupHast pH electrode and Orion Star A329
124 multiparameter meter. Solution remained clear until pH ~2, and a brown precipitate, likely 2-line
125 ferrihydrite [Peretyazhko et al., 2016], formed at higher pH values. Solution of Fe(ClO4)3 and
126 NaCl with unadjusted pH 1.6 and suspensions containing brown precipitates with pH adjusted to
127 4, 6 and 8 were then heated in PYREXTM beakers covered with watch glasses in the oven at 90
128 °C for 24h, cooled to room temperature and pH was measured again. The final precipitates were
129 washed 3 times with Milli-Q water by centrifugation and then freeze-dried prior to mineralogical
130 and compositional characterization. The sample names used hereafter correspond to initial values
131 of solution pH and chloride concentration; for example, ‘‘pH0 1.6/Cl 0.1M” corresponds to a
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132 sample prepared from a solution having initial pH (pH0) of 1.6 and a 0.1 M chloride
133 concentration.
134 2.2. Characterization
135 X-ray diffraction (XRD) patterns were recorded using a Panalytical X’Pert Pro with Co Kα
136 radiation. Samples were analysed at 45 kV and 40 mA with a 0.02 2 step size and 1 min per
137 step counting rate. Programmable divergent and antiscatter slits were set to 1/4 with manual
138 1/2 antiscatter slit. The instrument was operated under ambient conditions and calibrated with
139 novaculite (quartz) standard (Gemdugout, State College, PA). A standard powder mount method
140 was applied for mineralogy analyses. Subsamples were also mixed with corundum to quantify
141 the amounts of precipitated Fe(III) (hydr)oxides by Rietveld refinement and internal corundum
142 standard method [Bish and Howard, 1988; Bish and Post, 1993]. In order to quantify the amount
143 of X-ray amorphous material, the weights of all crystalline phases were first estimated with
144 Rietveld refinement at fixed corundum amount (20 wt%) and then the amounts of all crystalline
145 phases were corrected using the known corundum abundance. Finally, the content of amorphous
146 phase was calculated as a difference between the total content of all phases (100%) and
147 crystalline phases. Crystallite size was calculated based on XRD line broadening analysis and
148 lanthanum hexaboride (NIST 660a LaB6) standard was used to determine instrument line
149 broadening. Ferrihydrite identification was verified by comparing XRD of precipitated Fe(III)
150 (hydr)oxide and pure 2-line ferrihydrite synthesized by grinding Fe(NO3)3·9H2O and NH4HCO3
151 [Smith et al., 2012]. Reitveld refinements, instrumental line broadening, and crystallite size
152 analyses were performed with MDI Jade software package (Materials Date Incorporated,
153 Livermore, California).
154
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155 Mid-infrared diffuse reflectance spectra were measured with a Thermo Nicolet Nexus 670
156 FT-IR spectrometer (1.67-16.7 µm, 600-6000 cm-1) configured with a Pike integrating sphere
157 that was purged with dry N2 gas derived from liquid N2. Prior to analysis, the powder samples
158 were loaded into sample cups and stored in a N2-purged glove box at room temperature (H2O ~
159 100 ppmv; RH <0.5%) for at least 138h to mimic desiccating conditions on the martian surface
160 [Morris et al., 2011]. Spectral measurements were made at room temperature (RT) under
161 ambient atmospheric pressures. Visible and near-infrared reflectance (VNIR) spectra (0.35-2.5
162 µm) were measured with an Analytical Spectral Devices FieldSpec3 fiber-optic based
163 spectrometer with a reflectance probe attachment (internal light source) operating in absolute
164 reflectance mode using a 99% Spectralon reflectance standard. Analyses were performed in a dry
165 N2-purged glove box and samples were stored for 102h in the glove box prior to analysis to
166 achieve desiccating conditions. Continuum removed spectra were obtained using ENVI software.
167 Mössbauer analyses were performed at RT under ambient conditions using MIMOS-II
168 instruments that are the laboratory equivalent of the instruments onboard the Mars Exploration
169 Rovers (SPESI, Inc., [Klingelhoefer et al., 2003]). Velocity calibration was carried out with the
170 program MERView [Agresti et al., 2006] using the MIMOS-II differential signal and the
171 spectrum for metallic Fe foil acquired at RT. Mössbauer parameters [center shift () with respect
172 to metallic Fe foil at RT, quadrupole splitting (EQ), magnetic hyperfine field strength (Bhf), and
173 subspectral areas (A)] were obtained with a least-squares fitting procedure using MERFit
174 [Agresti and Gerakines, 2009]. Reported values for A were converted to % Fe using the f-factor
175 ratio, fFe(III)/f Fe(II) = 1.21 [Morris et al., 1995]. Uncertainties for , EQ and full width at half
176 maximum (FWHM) were ± 0.02 mm/s, ± 0.8 T for Bhf and ± 2% for A. Selected pure akaganeite
177 and akaganeite-free samples were analysed by Mössbauer spectroscopy.
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178 For analyses of total chloride and perchlorate associated with Fe(III) (hydr)oxides,
179 precipitated Fe(III) (hydr)oxides were digested in nitric acid (ultra-pure HNO3, Fisher).
180 Triplicate analyses were done using a 10 mg samples that were mixed with 5 ml 2.5 M HNO3,
181 boiled on a heating plate for 20-30 min until completely dissolved, and then diluted with Milli-Q
182 water to 20 ml. Total chloride was measured by ion chromatography using Dionex ICS-2000
183 instrument equipped with Dionex IonPac AS20 column (4 x 250 mm), Dionex EGC III KOH
184 eluent and a suppressed conductivity detector Dionex AERS 500 4 mm with a 20 µm injection
185 volume.
186 3. Results 187 188 3.1. pH before and after hydrolysis reaction 189 190 Precipitation of Fe(III) (hydr)oxides was accompanied by changes in pH (Table 1). Solution
191 pH decreased to 0.95-1.07, 2.54-2.71, and 4.24-4.45 in the samples with pH0 1.6, 4 and 6,
192 respectively, after 24h heating at 90 °C as a result of Fe(III) hydrolysis and precipitation of
193 Fe(III) (hydr)oxides. Precipitation of Fe(III) (hydr)oxides also occurred for pH0 8 samples, but
194 pH slightly increased to 8.3 at the end of reaction except the pH0 8/Cl 0.05M sample (Table 1 ),
195 presumably from inadvertent addition of excess NaOH during initial pH adjustment to pH0 8.
196 3.2. X-ray diffraction
197 Akaganeite formation was affected by initial pH and chloride concentration in solution. X-
198 ray diffraction analysis of the samples synthesized under the most acidic pH conditions revealed
199 precipitation of well-crystalline akaganeite alone in pH0 1.6/Cl 0.1M, coprecipitation with
200 hematite in pH0 1.6/Cl 0.05M and coprecipitation with hematite, goethite and amorphous phase
201 in pH0 1.6/Cl 0.02M (Figure 1a, Table 2). Refined unit-cell parameters for crystalline akaganeite
202 were consistent with Cl-containing akaganeite (Table S1). Akaganeite precipitation also occurred
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- 203 at pH0 4, 6 and 8 and dissolved Cl concentration of 0.05 and 0.1 M, except it was completely
204 supressed at 0.02 M Cl- (Figure 1b-d, Table 2). Akaganeite peaks were less intense and broader
205 than in the pH0 1.6 samples due to decrease in akaganeite amount with pH increase (Table 2) and
206 lower crystallite size of the forming akaganeite (Table S2). Variable amounts of goethite and/or
207 hematite precipitated together with akaganeite at pH0 ≥ 4 (Figure 1b- d, Table 2). Similar to pH0
208 1.6/Cl 0.02M, all pH0≥4 samples contained a substantial amount of X-ray amorphous material as
209 determined by Rietveld refinement (40-93 wt%, Table 2). The X-ray amorphous material is
210 assigned to 2-line ferrihydrite as evidenced by the two broad peaks between 30 and 50° 2θ and
211 between 70 and 80° 2θ in all samples (Figure S1). The phase is hereafter denoted as
212 “ferrihydrite”.
213 214 3.3. Total chloride and perchlorate contents in Fe(III) (hydr)oxides
- 215 Pure akaganeite formed in the pH0 1.6/Cl 0.1M sample had the highest Cl content of ~6wt%
216 with respect to other akaganeite-containing samples (Figure 2a). The exact amount of chloride
217 associated with akaganeite precipitated in the mixture with hematite, goethite and/or ferrihydrite
218 could not be derived from total chloride measurements alone. Analysis of akaganeite-free pH0
- 219 4/Cl 0.02M, pH0 6/Cl 0.02M and pH0 8/Cl 0.02M samples showed that little Cl (≤ 0.3 wt%) was
220 associated with Fe(III) (hydr)oxides (Figure 2a, Table 2) in agreement with our previous
221 observations [Peretyazhko et al., 2016]. We, therefore, assume that chloride detected in
222 akaganeite mixtures was mainly associated with akaganeite tunnels. Minor amount of Cl- could
223 also be adsorbed onto akaganeite surfaces [Chambaere and Degrave, 1984].
- 224 Increase in solution pH0 and initial dissolved Cl concentration had opposing effects on total
- - 225 Cl content in akaganeite. As pH0 raised from 1.6 to 8, total Cl content decreased for each initial
226 dissolved Cl- concentration (Figure 2a). In contrast, an increase in initial dissolved Cl-
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- 227 concentration from 0.02 to 0.1 M led to increase in total Cl content at each value of pH0 (Figure
228 2a). Chloride was below the detection limit (<6 µg/l or <0.001 wt%) in the pH0 8/Cl 0.05M
229 samples which contained 17 wt% akaganeite based on XRD analysis (Figure 2a, Table 2). The
230 result might indicate that Cl was replaced by H2O in the tunnels [Mazeina et al., 2006] resulting
231 in formation of akaganeite with only traces of Cl.
- 232 Variable amounts of perchlorate were present at all pH0 values as a result of ClO4
233 complexation with Fe(III) (hydr)oxide surface sites. Perchlorate was likely not incorporated into
234 akaganeite tunnels because it has been shown to exchange for chloride on akaganeite surface but
235 not in tunnels [Paterson and Rahman, 1984]. The total perchlorate content was below 0.3wt% in
- - 236 all pH0 1.6 samples (Figure 2b) which had the largest total Cl content (Figure 2a). Low ClO4
- - 237 content might indicate that Cl outcompeted ClO4 for the surface sites as chloride is a stronger
238 adsorbing anion than perchlorate [Bourikas et al., 2001; Rietra et al., 2000]. Samples synthesized
- 239 at pH0 4 and 6 contained 2-6 wt% ClO4 (Figure 2b) due to perchlorate outer-sphere
240 complexation on Fe(III) (hydr)oxide surfaces [Harrison and Berkheiser, 1982]. Similar to pH0
241 1.6 samples, perchlorate content was ≤0.3wt% in all pH0 8 samples. The values of pH at the end
242 of hydrolysis of pH0 8 samples (pH24h 6.9-8.3, Table 1) were close to or higher than the point of
243 zero net charge for Fe (hydr)oxides (pH > 7, [Cornell and Schwertmann, 2003; Sposito, 2008]).
244 As a result, the surface became uncharged or negatively charged leading to a decrease in
245 perchlorate adsorption.
246 3.4.Infrared reflectance spectroscopy
247 The main spectral features in the visible and near infrared spectrum range were multiple Fe-
248 OH and H2O combination bands centered at 2.30-2.45 and 1.92-1.99 µm, respectively, and an
249 overtone of Fe-OH and H2O stretching vibrations centered at ~1.45 µm (Figures 3 and S2,
10
250 Tables S3-S6). The band in the 1.92-1.99 µm range was assigned to a combination of H2O
251 stretching and bending vibrations in Fe(III) (hydr)oxides (Tables S3-S6). The band was sensitive
252 to chloride content in akaganeite-dominated samples [Bishop et al., 2015; Peretyazhko et al.,
253 2016; Song and Boily, 2012]. The band position has been previously shown to correlate linearly
254 with the total chloride content in Fe(III) (hydr)oxide mixtures containing >60wt% akaganeite
255 under condition that all chloride was associated with akaganeite [Peretyazhko et al., 2016].
256 Akaganeite was the major phase only in two samples synthesized in this work (100 wt%
257 akaganeite in pH0 1.6/Cl 0.1M and 94 wt% akaganeite in pH0 1.6/Cl 0.05M, Table 2). The
258 correlation remained linear when values for these two samples were included in Figure S3. The
259 remaining samples were excluded because presence of other Fe(III) (hydr)oxides affected the
260 band position (Tables S3-S6).
261 The combination band at ~2.45 µm in akaganeite-containing samples was assigned to
262 combination stretching and out-of-plane bending vibrations of Fe-OH hydrogen bonded with
263 chloride in tunnels [Bishop et al., 2015]. The VNIR analyses demonstrated that the band
264 intensity and shape were sensitive to akaganeite amount in the sample, variations in total
265 chloride content and presence of other Fe(III) (hydr)oxides (Figures 3 and S2). Akaganeite-
266 dominated samples (pH0 1.6/Cl 0.1M and pH0 1.6/Cl 0.05M) had a sharp asymmetric band at
267 ~2.45 µm with several shoulders in the 2.25-2.42 µm range (Tables S3-S6, Figures 3 and S2).
268 The observed shoulders were assigned to in-plane bending vibrations of isolated OH at 2.42 µm,
269 out-of-plane bending vibration of isolated Fe-OH groups at 2.33 µm and a combination of
270 stretching and bending vibrations of hydrogen-bonded Fe-OH groups with H2O molecules
271 [Bishop et al., 2015; Song and Boily, 2012]. The ~2.45 µm band was less intense and
272 transformed into a weak shoulder of the band centred at 2.40 µm in the samples containing 14 -
11
273 31wt% akaganeite with the exception of the pH0 8/Cl 0.05 M sample (Figure 3a). The band was
274 not detected in this Cl-free sample containing 17wt% akaganeite (Figure 3a). The akaganeite
275 amount in pH0 8/Cl 0.05 M sample was the same as in the pH0 1.6/Cl 0.02 M in which 2.45 µm
276 band was well resolved (Figure 3a). The results indicate that total Cl content in akaganetite
277 affects intensity of Fe-OH combination band. The 2.40 µm band was assigned to combination
278 vibrations of Fe-OH groups in ferrihydrite and it was also observed in the akaganeite-free
279 samples containing 52-93 wt% ferrihydrite and in synthetic 2-line ferrihydrite (Figure 3b, Table
280 2).
281 The combination bands at ~2.00 and ~2.45 µm, taken together and as represented by samples
282 pH0 1.6/Cl 0.1M and pH0 1.6/Cl 0.05M are considered to be spectrally diagnostic for akaganeite
283 and were previously used to identify akaganeite on Mars from CRISM data [Carter et al., 2015].
284 Our results indicate that akaganeite band positions, shapes and relative intensities are sensitive to
285 chloride content, akaganeite abundance and presence of other Fe(III) (hydr)oxides. Akaganeite
286 with low chloride content and/or akaganeite present as a non-dominant phase in mixture with one
287 or more Fe(III) (hydr)oxides (e.g., ferrihydrite, goethite and hematite) could be not recognized
288 by orbital remote sensing.
289 Mid-infrared analysis demonstrated that all Fe(III) (hydr)oxides had Fe-OH bending
290 vibrations in the 11-12 µm range and overlapping bands of Fe-OH and H2O stretching centred
291 near 2.8 µm (Figures 4 and S4, Tables S3-S6). In addition, some akaganeite-containing samples
292 had a sharp band at ~8 µm that has not been previously reported. A band of similar position
293 (~8.3 µm) has been observed in synthetic akaganeite [Glotch and Kraft, 2008]. Although the
294 band was not assigned by the authors, it could be explained by a combination of two Fe-OH
295 bending vibrations (823 + 380 = 1203 cm-1 or 8.31 µm) reported by Glotch and Kraft [2008]. All
12
296 Fe(III) (hydr)oxides had a band near ~6.1 µm attributed to fundamental bending vibration of
297 H2O molecules adsorbed on particle surface [Bishop et al., 2015; Rochester and Topham, 1979;
298 Tejedor-Tejedor and Anderson, 1986]. In addition, perchlorate-containing samples had a broad
299 band near 8.5-9.1 µm. A band in this range has been assigned to perchlorate ν3 vibration in salts
300 (8.5-9.5 µm in KClO4 and 8.6-9.7 in Cu(ClO4)2·2H2O, [Harrison and Berkheiser, 1982]) and
301 adsorbed on oxide surfaces (9 µm for Cr(III) (hydr)oxide and Ti(IV) oxide [Lefèvre, 2004]) and
302 9.2 µm for ferrihydrite [Harrison and Berkheiser, 1982]). Because residual perchlorate salts
303 were not detected by XRD, the 8.5-9.1 µm band was assigned to ν3 vibration of adsorbed
304 perchlorate. The band corresponds to distorted Td geometry of outer-sphere perchlorate
305 complexes formed on Fe(III) (hydr)oxide surfaces [Harrison and Berkheiser, 1982; Lefèvre,
306 2004].
307 3.5.Mössbauer spectroscopy
308 Mössbauer spectra were obtained for pure akaganeite pH0 1.6/Cl 0.1M and akaganeite-free
309 pH0 6/Cl 0.02M samples (Figure 5, Table 3). Both spectra were fitted by two doublets of
310 mineralogically non-specific Fe(III) in octahedral coordination. Based on XRD analysis,
311 doublets were assigned to akaganeite and ferrihydrite in the pH0 1.6/Cl 0.1M and pH0 6/Cl
312 0.02M, respectively. Doublet parameters (Table 3) were similar to Fe(III) doublet parameters (:
313 0.33-0.40 mm/s, EQ: 0.72-1.16 mm/s) reported in sediments in Gusev crater and Meridiani
314 Planum on Mars [Morris et al., 2004; Morris et al., 2006a; Morris et al., 2006b; Morris et al.,
315 2008]. The Fe(III) doublet observed on Mars was assigned to nanophase Fe(III) oxide, a generic
316 name for a composite of octahedrally-coordinated Fe(III) phases containing various Fe(III)
317 (hydr)oxides such as superparamagnetic forms of hematite and goethite, lepidocrocite,
318 akaganeite, schwertmannite, hydronium jarosite and ferrihydrite [Morris et al., 2004]. In addition
13
319 to Fe(III) doublets, sample pH0 6/Cl 0.02M had two sextets assigned to goethite and hematite
320 (Table 3). Because all phases in the sample are Fe-bearing, the spectral contributions of Fe(III) in
321 hematite, goethite and ferrihydrite in pH0 6/Cl 0.02M were in agreement with results of Rietveld
322 refinement (Tables 2 and 3).
323 4. Discussion 324 325 326 Our results demonstrated that akaganeite formation is influenced by initial pH and solution
327 chloride concentration. Akaganeite precipitated as a single phase or in mixture with ferrihydrite,
- 328 hematite and/or goethite at pH0 between 1.6 and 8 and in the presence of 0.02-0.1 M Cl .
329 Formation conditions determined in this work together with the results of akaganeite
330 characterization by mission-like instrumentation (XRD, Mössbauer and VNIR) can help to
331 constrain ancient aqueous conditions in areas where akaganeite has been detected on Mars.
332 Below, we discuss potential akaganeite formation environments in Yellowknife Bay and Robert
333 Sharp crater.
334 4.1. Akaganeite formation environments in Yellowknife Bay
335 The aquatic environments in Yellowknife Bay likely had neutral pH, low salinity and variable
336 redox conditions [Grotzinger et al., 2014]. Existence of such aqueous conditions are supported
337 by mineralogical observations of magnetite and lack of phases usually formed under acidic
338 conditions (e.g., jarosite) [Grotzinger et al., 2014; McLennan et al., 2014; Vaniman et al., 2014].
339 Small amounts of akaganeite of around 1.1 and 1.7 wt% were detected by the CheMin XRD
340 instrument in the drilled John Klein and Cumberland mudstone samples, respectively, in
341 Yellowknife Bay [Vaniman et al., 2014]. In addition to akaganeite, other Fe(III) (hydr)oxides are
342 likely present in mudstone. Hematite at the detection limit of CheMin (0.6-0.7 wt%) was found
343 in both John Klein and Cumberland. Both drill samples contained up to 31 wt% of X-ray
14
344 amorphous material enriched in Fe, S, Cl and P [Vaniman et al., 2014]. Iron enrichment in the
345 amorphous phase might indicate the presence of nanophase Fe(III) oxide including ferrihydrite
346 [Dehouck et al., 2014]. The phase could be similar to Fe(III)-bearing alteration products detected
347 in Gusev crater and Meridiani Planum [Morris et al., 2004; Morris et al., 2006a; Morris et al.,
348 2006b; Morris et al., 2008]. In summary, akaganeite in Yellowknife Bay mudstone is likely
349 present together with traces of hematite and potentially ferrihydrite as a component of X-ray
350 amorphous material. Therefore, synthetic samples that have akaganeite, ferrihydrite and hematite
351 could be considered as a qualitative analogue of Fe(III) (hydr)oxide assemblage in Yellowknife
352 Bay mudstone.
353 We did not synthesize exactly the same mixture as in Yellowknife Bay because the samples
354 that had akaganeite, hematite and ferrihydrite also contained some goethite (Table 2). The closest
355 qualitative analogues were the pH0 4/Cl 0.1M and pH0 6/Cl 0.1M samples which contained
356 akaganeite and ferrihydrite but no hematite (Table 2). The time and mechanism of hematite
357 formation in Yellowknife Bay mudstone is unknown. However, it might be possible that
358 hematite was not present in the initially precipitated Fe(III) (hydr)oxide mixture but formed later,
359 for instance, through transformation of ferrihydrite. Near-neutral aqueous conditions proposed
360 for Yellowknife Bay could be favourable for ferrihydrite preferential transformation into
361 hematite [Cornell and Schwertmann, 2003]. In addition, air-dried ferrihydrite can also transform
362 into hematite if adsorbed water content is sufficiently high (e.g., 100-150 g H2O/kg, [Cornell and
363 Schwertmann, 2003]).
364 We hypothesize that akaganeite and ferrihydrite precipitated first in Yellowknife Bay
365 followed by incomplete crystallization of ferrihydrite to hematite. This puts constraints on
366 aqueous conditions prevailing at the time of akaganeite formation in Gale crater. Akaganeite and
15
367 ferrihydrite could form together through solution Fe(III) hydrolysis at the initial pH 1.6 < pH0 <
368 8 and dissolved Cl- concentration > 0.05 M (Table 2). Iron(III) hydrolysis under other conditions
369 would lead to formation Fe(III) (hydr)oxide assemblage different than the proposed initial Fe(III)
370 (hydr)oxide mixture in Yellowknife Bay by the presence of goethite or lack of ferrihydrite (Table
371 2). Such pH0 conditions cover a wide pH range from acidic to mildly alkaline but include the
372 neutral aquatic conditions proposed for Yellowknife Bay [Grotzinger et al., 2014]. The dissolved
373 Cl- of 0.05 M corresponds to ~3g/l NaCl consistent with moderately saline environments (2-5 g/l,
374 [Hillel, 2000]).
375 4.2.Akaganeite formation environments in Robert Sharp crater
376 Akaganeite was detected by CRISM in Robert Sharp crater based on the presence of Fe-OH
377 and H2O combination bands at ~2 and ~2.45 µm (Figure 6, [Carter et al., 2015]). Comparison of
378 spectral features revealed that the pure akaganeite pH0 1.6/Cl 0.1M sample, akaganeite-
379 dominated pH0 1.6/Cl 0.05M sample and akaganeite from Robert Sharp crater had similar
380 asymmetric shape of the ~2.45 µm band (Figure 6). The shape of this band in other akaganeite-
381 bearing samples did not match CRISM data (Figure S5). The H2O combination band in the pH0
382 1.6 samples was similar in shape but was deeper with respect to CRISM measurements due to
383 the presence of adsorbed water on akaganeite surface. Based on the linear correlation observed
- 384 between H2O combination band position and total Cl content in akaganeite (Figure S3b), we
385 calculated that akaganeite in Robert Sharp crater contained ~6.7 wt% Cl. The obtained value was
386 close to the total Cl- content in the pure-akaganeite synthesized in the presence of 0.1 M Cl- (6.1
387 ± 0.3 wt% Cl in pH0 1.6/Cl 0.1M, Figure 2a).
388 Carter et al. [2015] proposed, on the basis of the CRISM akaganeite detection, that Robert
389 Sharp crater has experienced mildly acidic pH, high salinity and oxidizing conditions. Our
16
390 experimental results support this model and constrain acidic conditions to pH<4 and dissolved
391 chloride concentration to ~ 0.1 M (~ 3 g/l NaCl) which corresponds to moderately saline
392 environments (2-5 g/l, [Hillel, 2000]). The asymmetric shape of Fe-OH combination band also
393 indicates that akaganeite is a pure or dominating phase in the sites analysed in Robert Sharp
394 crater.
395 5. Conclusions 396 397 Formation of akaganeite through Fe(III) forced hydrolysis was investigated in the range of
398 initial pH0 from 1.6 to 8 and in the presence of three different dissolved chloride concentrations
399 (0.02, 0.05 and 0.1 M). The results reveal that akaganeite formed at all studied Cl concentrations
- 400 (0.02-0.1 M) at pH0 1.6 and at 1.6 < pH0 ≤ 8 and Cl > 0.02 M. Depending on initial solution pH
401 and chloride concentration, akaganeite precipitated as a single phase or in mixture with
402 amorphous Fe(III) oxide (mainly 2-line ferrihydrite), hematite and/or goethite.
403 We used the experimental results to place constrains on ancient aqueous conditions in
404 Yellowknife Bay in Gale crater and Robert Sharp crater on Mars where akaganeite has been
405 detected. Mineralogical observations in Yellowknife Bay revealed that akaganeite was present
406 together with hematite and Fe-rich amorphous phase that likely contained ferrihydrite. Based on
407 our experimental data, we can constrain the aqueous environmental conditions for akaganeite
- 408 formation in Yellowknife Bay to pH 1.6 < pH0 < 8 solutions with dissolved Cl concentration >
409 0.05 M. A potential mechanism of akaganeite formation is basalt dissolution under initially
410 acidic oxidizing conditions. Under such conditions, basalt neutralization leads to pH increase up
411 to alkaline and release of Fe(III) [Peretyazhko et al., 2018]. Further hydrolysis of released Fe(III)
412 under moderately saline conditions (>3 g/l NaCl) results in formation of mineral assemblages of
413 akaganeite and ferrihydrite, and further transformation of ferrihydrite to hematite can lead to
17
414 formation of Fe(III) (hydr)oxide mixture presently observed in Yellowknife Bay. Akaganeite in
415 Robert Sharp crater likely formed under acidic pH < 4 conditions and in the presence of ~ 0.1 M
416 dissolved Cl-. A potential mechanism of akaganeite formation is oxidative dissolution of Fe(II)
417 sulfides under localized Cl-rich environments followed by hydrolysis of Fe(III) and precipitation
418 of akaganeite. The proposed mechanism is observed in terrestrial lagoon-like environments
419 where exposure to air of sulfide-rich sediments in a partially dried lagoon resulted in
420 precipitation of akaganeite [Bibi et al., 2011]. Alternatively, oxidizing dissolution of basalts in
421 localized acidic solutions created from magmatic outgassing of HCl might create favourable
422 conditions for akaganeite formation in Robert Sharp crater.
423 Acknowledgments 424 425 We are grateful to Dr John Carter for providing us with CRISM data for akaganeite in Robert
426 Sharp crater. This work was supported by NASA Solar System Workings grant #15-SSW15_2-
427 0074. XRD, Mössbauer, Infrared and total chloride/perchlorate data are reported in Supplemental
428 Information.
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23
562 563 564
24
565 Table 1. Values of solution pH before (pH0) and after 24h (pH24h) heating at 90 ºC.
Sample pH0 pH24h Sample pH0 pH24h Sample pH0 pH24h Sample pH0 pH24h
pH0 1.6/Cl 0.1M 1.60 0.98 pH0 4/Cl 0.1M 4.00 2.71 pH0 6/Cl 0.1M 6.00 4.45 pH0 8/Cl 0.1M 8.10 8.27
pH0 1.6/Cl 0.05M 1.60 0.95 pH0 4/Cl 0.05M 4.01 2.54 pH0 6/Cl 0.05M 6.00 4.24 pH0 8/Cl 0.05M 7.99 6.9
pH0 1.6/Cl 0.02M 1.59 1.07 pH0 4/Cl 0.02M 4.03 2.68 pH0 6/Cl 0.02M 6.03 4.39 pH0 8/Cl 0.02M 8.05 8.3 566
25
567 Table 2. Quantities of Fe(III) (hydr)oxides identified by Rietveld refinement. Sample Fe(III) (hydr)oxide, wt% Akaganeite Goethite Hematite Ferrihydrite
pH0 1.6/Cl 0.1M 100 0 0 0
pH0 1.6/Cl 0.05M 94 0 6 0
pH0 1.6/Cl 0.02M 17 7 36 40
pH0 4/Cl 0.1M 31 0 0 69
pH0 4/Cl 0.05M 18 3 0 79
pH0 4/Cl 0.02M 0 6 1 93
pH0 6/Cl 0.1M 27 0 0 73
pH0 6/Cl 0.05M 14 6 0 80
pH0 6/Cl 0.02M 0 13 3 84
pH0 8/Cl 0.1M 28 5 5 62
pH0 8/Cl 0.05M 17 7 36 40
pH0 8/Cl 0.02M 0 17 31 52 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590
26
591 Table 3. Mössbauer parameters of pure akaganeite and akaganeite-free samples. 592 1 1 1 1 1 Sample and E FWHM Bhf A Q Assignment Subspectra mm/s mm/s mm/s T %
pH0 1.6/Cl 0.1M Doublet 3D1 0.38 0.54 0.34 55 Akaganeite Doublet 3D2 0.37 0.99 0.35 45 Akaganeite
pH0 6/Cl 0.02M Doublet 3D1 0.36 0.56 0.42 55 Ferrihydrite Doublet 3D2 0.33 0.98 0.42 23 Ferrihydrite Sextet 3S1 0.39 -0.05 50.2 7 Hematite Sextet 3S2 0.37 -0.27 37.4 15 Goethite 593 1- center shift relative to the midpoint of the spectrum of metallic Fe foil at room temperature; 594 EQ- quadrupole splitting, FWHM- full width at half maximum intensity for doublet subspectra, 595 Bhf -magnetic hyperfine field strength, and A-subspectral area. 596
27 a A A A A A A A A A A A A 20000 A pH0 1.6/Cl 0.1M H H H H H H H 10000 pH0 1.6/Cl 0.05M
G pH0 1.6/Cl 0.02M 20 40 60 80 b A A A A A A A A 6000 pH 4/Cl 0.1M G 0 G G
pH0 4/Cl 0.05M 3000 H
pH0 4/Cl 0.02M 20 40 60 80 c A A A A A A A A 6000 pH0 6/Cl 0.1M Intensity,offset for clarity G G
pH0 6/Cl 0.05M H 3000 G H H pH0 6/Cl 0.02M 20 40 60 80 d A H H A G A G A H G A H H H H H 6000 pH0 8/Cl 0.1M
pH0 8/Cl 0.05M 3000
pH0 8/Cl 0.02M
20 40 60 80 2 Figure 1. X-ray diffraction patterns of Fe(III) (hydr)oxides formed in the presence of 0.02, 0.05 and 0.1 M
dissolved chloride at (a) pH0 1.6, (b) pH0 4, (c) pH0 6 and (d) pH0 8. Corresponding sample name is shown next to each pattern. A = akaganeite, H = hematite, G = goethite. a pH0 1.6 6 pH0 4
pH0 6
pH0 8 4
, wt% ,
-
Total Cl Total 2
0 0.02 0.04 0.06 0.08 0.10 b
6
, wt% , - 4
4
pH0 1.6 pH 4 Total ClO Total 2 0 pH0 6
pH0 8
0 0.02 0.04 0.06 0.08 0.10 Initial dissolved Cl-, M
Figure 2. Total (a) chloride and (b) perchlorate contents in acid digested Fe(III) (hydr)oxides as a function of initial chloride concentration. . akaganeite 2 syntheticfor line with dashedand H akaganeite samples. removedFigure3. Continuum 2 b a O and OH combination bands are shown with dotted lines for purelines for shown dotted bands areO with OH combinationand Spectra are offsetareclarity.Spectrafor Corresponding
and/or and(Aka) total Reflectance, continuum removed and offset for clarity -0.5 0.0 0.5 1.0 0.6 0.8 1.0 ferrihydrite 14 0.5 27 1.2 18 1.6 17 0 100 6.1 28 0.05 17 1.9 Aka Aka 31 3.0 94 4.6 1.8 Cl chloride contentchloride tot . VNIR 1.92 - line reflectance spectra of (a) spectra reflectance H 1.99 ferrihydrite 2.0 Wavelength, 2 O (in wt %)are . Lines are drown through the samples that contain that drownsamples throughthe areLines . sample name is to shown next name sample shown 2.2 m akaganeite above each spectrum in (a). spectrumPositions eachofabove 2.30 2.33 akaganeite OH - 2.40 2.4 containing and (b)and containing 2.42 2.45 sample (pH sample pH pH pH pH pH pH pH pH pH pH pH ferrihydrite pH each 0 0 0 0 0 0 0 0 0 0 0 0 8/Cl 0.1M 8/Cl 4/Cl 0.05M 4/Cl 8/Cl 0.05M 8/Cl 4/Cl 0.1M 4/Cl 0.02M 1.6/Cl 0.05M 1.6/Cl 6/Cl 0.02M 6/Cl 6/Cl 0.1M 6/Cl 1.6/Cl 0.1M 8/Cl 0.02M 8/Cl 0.02M 4/Cl 6/Cl 0.05M 6/Cl spectrum and spectrum 0 akaganeite 1.6/Cl 1.6/Cl 0.1M) - free free a 0.8 OH pH0 1.6/Cl 0.1M OH+ H O 2 pH0 1.6/Cl 0.05M 0.4 H2O pH 1.6/Cl 0.02M ClO4 OH 0 0.0
-0.4 4 6 8 10 12 b pH0 4/Cl 0.1M 0.8 OH pH 4/Cl 0.05M OH+ ClO OH 0 H2O 4 pH 4/Cl 0.02M 0.4 H2O 0 OH
0.0
-0.4 c 4 6 8 10 12
0.8 pH0 6/Cl 0.1M pH 6/Cl 0.05M OH+ 0 0.4 H2O pH0 6/Cl 0.02M H2O ClO4 OH
0.0
Reflectance, continuum removed and offset for clarity for and offset continuum removed Reflectance,
-0.4 4 6 8 10 12 d pH 8/Cl 0.1M 0.8 0
pH0 8/Cl 0.05M OH+ 0.4 pH0 8/Cl 0.02M H2O OH OH H2O OH
0.0
-0.4 4 6 8 10 12 Wavelength, m Figure 4. Continuum removed reflectance spectra from 2.5 to 13.5 µm of Fe(III) (hydr)oxide formed at (a) pH0 1.6, (b) pH0 4, (c) pH0 6 and (d) pH0 8. Spectra are offset for clarity. Corresponding sample name is shown next to each spectrum. Bands are marked with the dotted lines and assignments are shown in the figure and summarized in Tables S3-S6. a Experimental Simulated Akaganeite 55% Akaganeite 45% 0.4
TC/BC-1 0.2
0.0 -10 -5 0 5 10
b Experimental Simulated Ferrihydrite 55% Ferrihydrite 23% Hematite 7% Goethite 15% 0.2
TC/BC-1
0.0 -10 -5 0 5 10 Velocity, mm/s
Figure 5. Mössbauer spectra and fit subspectra for (a) pure akaganeite sample pH0 1.6/Cl 0.1M and (b) akaganeite-free sample pH0 6/Cl 0.02M collected at RT [TC – total counts, BC – baseline counts]. 1.00 1.0
0.8 0.99
Robert Sharp crater pH0 1.6/Cl 0.1M 0.6 pH0 1.6/Cl 0.05M
Reflectance (VNIR), continuum removed continuum (VNIR), Reflectance
Reflectance (CRISM), continuum removed continuum (CRISM), Reflectance 0.98 1.8 2.0 2.2 2.4 Wavelength, m
Figure 6. Continuum removed CRISM spectrum of akaganeite at Robert Sharp crater (see Carter et al. [2015] for details) plotted together with continuum removed VNIR reflectance spectra for akaganeite-containing samples pH0 1.6/Cl 0.1M and pH0 1.6/0.05M.