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Laboratory-simulated acid-sulfate weathering of basaltic materials:

Implications for formation of sulfates at Meridiani Planum and Gusev

crater, Mars

D. C. GoldenESCG/UTC-HS, Houston, Texas, USA

Douglas W. Ming and Richard V. MorrisAstromaterials Research and Exploration Science Directorate, NASA Johnson Space Center, Houston, Texas, USA

Stanley A. MertzmanDepartment of Geosciences, Franklin and Marshall College, Lancaster, Pennsylvania, USA

Received 14 April 2005; revised 14 July 2005; accepted 9 August 2005; published 2 December 2005.

[1] Acid-sulfate weathering of basaltic materials is a candidate formation process for thesulfate-rich outcrops and rocks at the MER rover Opportunity and Spirit landing sites. Todetermine the style of acid-sulfate weathering on Mars, we weathered basaltic materials(olivine-rich glassy basaltic sand and plagioclase feldspar-rich basaltic tephra) in thelaboratory under different oxidative, acid-sulfate conditions and characterized thealteration products. We investigated alteration by (1) sulfuric-acid vapor (acid fog),(2) three-step hydrothermal leaching treatment approximating an ‘‘open system,’’ and(3) single-step hydrothermal batch treatment approximating a ‘‘closed system.’’ In acidfog experiments, Al-, Fe-, and Ca-sulfates and amorphous silica formed fromplagioclase-rich tephra, and Mg- and Ca-sulfates and amorphous silica formed from theolivine-rich sands. In three-step leaching experiments, only amorphous Si formed fromthe plagioclase-rich basaltic tephra, and jarosite, Mg-, and Ca-sulfates and amorphoussilica formed from olivine-rich basaltic sand. Amorphous silica formed under single-stepexperiments for both startingmaterials. On the basis of our experiments, jarosite formation inMeridiani outcrop is potential evidence for an open system acid-sulfate weathering regime.Waters rich in sulfuric acid percolated through basaltic sediment, dissolving basaltic phases(e.g., olivine) and forming jarosite, other sulfates, and iron oxides. Aqueous alteration ofoutcrops and rocks on theWest Spur of the Columbia Hills may have occurred when vaporsrich in SO2 from volcanic sources reacted with basaltic materials. Soluble ions from the hostrock (e.g., olivine) reacted with S to formCa-, Mg-, and other sulfates along with iron oxidesand oxyhydroxides.

Citation: Golden, D. C., D. W. Ming, R. V. Morris, and S. A. Mertzman (2005), Laboratory-simulated acid-sulfate weathering of

basaltic materials: Implications for formation of sulfates at Meridiani Planum and Gusev crater, Mars, J. Geophys. Res., 110, E12S07,

doi:10.1029/2005JE002451.

1. Introduction

[2] The Athena Science Instrument Payloads onboard theMars Exploration Rovers (MER) Spirit and Opportunityhave provided new insights on the aqueous history at theirrespective landing sites in Gusev crater and MeridianiPlanum. Measurements made with the Athena payloadindicate that a variety of aqueous processes as well asvarious degrees of alteration have occurred at the twolanding sites.[3] Aqueous alteration in Gusev crater ranges from minor

alterations on the ‘‘plains’’ to highly altered materials in the

Columbia Hills, which are located about 2.6 km to thesoutheast of the Spirit landing site. Light-toned rocks on theGusev crater plains appear to have coatings or an alterationrind that may have resulted from limited aqueous alterationon the surfaces of basaltic rocks [McSween et al., 2004;Haskin et al., 2005]. Hematite and high Fe3+/Fe(total) occurat the surfaces of some rocks [Morris et al., 2004], and highconcentrations of elements highly mobile in water (i.e., S,Cl, and Br) occur in rock veins, vugs, and coatings [Gellertet al., 2004; Haskin et al., 2005]. Soil in trenches that wereexcavated with the rover’s wheels exhibited stratigraphyhigher in Fe3+/Fe(total) ratios and mobile element concen-trations (i.e., S and Mg) than surface soils, suggestingthe translocation/transportation of these phases by water[Haskin et al., 2005]. Correlation between Mg and S suggest

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, E12S07, doi:10.1029/2005JE002451, 2005

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the formation of Mg-sulfates in these ‘‘mature’’ regoliths,possibly by the transportation of Mg and S by water and thesubsequent evaporation of water and the precipitation of Mgand other sulfates [Haskin et al., 2005]. One scenario for theformation of rock coatings or rinds and translocation ofmobile elements in Gusev crater is that thin films of watermobilized elements and hydrated rock surfaces, formingnanophase particles of iron oxides. These thin films of watermight have occurred briefly at the Martian surface duringperiods of climatic change, e.g., changes resulting fromdiurnal and seasonal changes or during times of highobliquity [Squyres et al., 2004a; Yen et al., 2005].[4] Outcrops (layered and massive) and rocks on the West

Spur of the Colombia Hills appear to be extensively alteredas suggested by their relative ‘‘softness’’ as compared tocrater floor basalts, high Fe3+/Fe(total), iron mineralogydominated by nanophase Fe3+ oxides, hematite and goe-thite, and high Br, S, and Cl concentrations beneath outcropsurfaces [Ming et al., 2005a, 2005b]. The discovery ofgoethite in West Spur outcrops is very important to under-standing the history of water in Gusev crater, because it canonly form in the presence of water in contrast to hematitethat can form by aqueous and non-aqueous processes.Positive correlations exist for Ca vs. S and for Mg vs.Cl, which suggest that the S and Cl phases in the outcropmaterials may be Ca-sulfates, and Mg-chlorides [Ming etal., 2005a, 2005b]. These outcrops and rocks undoubtedlyformed by the alteration of basaltic rocks, volcaniclasticmaterials, and/or impact ejecta by solutions that were rich involatile elements (i.e., Br, Cl, S) [Ming et al., 2005a, 2005b].However, it is not clear whether aqueous alteration occurredat depth (e.g., metasomatism), by hydrothermal solutions(e.g., associated with volcanic or impact processes), byaqueous vapors from volcanic emanations (i.e., acid fogweathering), or by low-temperature solutions movingthrough the West Spur materials. The layered outcrops onthe West Spur of the Columbia Hills appear to be sedimen-tary in nature and deposited in a fluid, e.g., air fall depositsfrom volcanic ash or impact materials. Regardless of theiremplacement process, liquid/vapor water had a major rolein the alteration of outcrops and isolated rocks on the WestSpur of the Columbia Hills.[5] The occurrence of jarosite ((K,Na,H3O)Fe3

(SO4)2(OH)6), other sulfates (e.g., Mg sulfates), and hema-tite along with siliciclastic materials in Eagle, Fram, andEndurance crater outcrops of sedimentary materials arestrong indicators of aqueous processes at Meridiani Planum[Squyres et al., 2004b; Clark et al., 2005; McLennan et al.,2005]. Jarosite can only form by aqueous processes undervery acidic conditions [van Breeman, 1980; Bigham andNordstrom, 2000]; e.g., acid-sulfate weathering conditionsresulting from the aqueous oxidation of Fe sulfides [e.g.,Fernandez-Remolar et al., 2005] or by sulfuric acid alter-ation of basalts by solutions associated with SO2-richvolcanic gases [e.g., Morris et al., 1996]. It is plausiblethat acidic solutions rich in sulfur (and Fe2+ or Fe3+) reactedwith basaltic sediments to release their structural cations.Ferrous iron underwent oxidative hydrolysis to produceinsoluble iron hydroxysulfates (e.g., jarosite) and oxides(e.g., hematite and goethite) and finally, through evapora-tion, formed siliciclastic sediments rich in other sulfatesincluding those of alkali and alkaline earths cations [e.g.,

Tosca et al., 2005]. The Meridiani outcrops appear to have acomplex diagenetic history as suggested by episodes ofcementation and recrystallization, formation of hematite-rich spherules (interpreted to be concretions), and dissolu-tion and formation of crystal mold vugs in outcrops[McLennan et al., 2005]. All of these diagenetic featuressuggest water played an important role in formation of theMeridiani outcrop subsequent to the deposition of thesulfate-rich siliciclastic sediments.[6] Much of the aqueous alteration likely occurred early

in the history of Mars. The outcrops at Meridiani representthe upper stratum of a complex sequence of layers that isdissected by Middle and Late Noachian cratered plains[Arvidson et al., 2003], suggesting that these materialsmay have formed several billion years ago. The highlyaltered Columbia Hills are older than the flood basalts thatcover the floor of Gusev crater near the landing site. Theaqueous alteration may have occurred prior to the emplace-ment of the flood basalts of likely Hesperian age [Golombeket al., 2003], suggesting that the alteration in the ColumbiaHills may have occurred over 3 billion years ago.[7] The high abundance of S is obvious evidence that

sulfate has played a major role in aqueous processes at bothrover landing sites. The sulfate-rich outcrop at MeridianiPlanum containing jarosite and Ca-Mg-sulfates has an SO3

content of up to 25 wt.% [Rieder et al., 2004; Clark et al.,2005]. The interiors of rocks and outcrops on the West Spurof the Columbia Hills have up to 8 wt.% SO3 [Ming et al.,2005b]. Soils at both sites generally have between 5 to 14wt.% SO3 [Gellert et al., 2004; Haskin et al., 2005]. Afternormaliziation of major element compositions to a SO3-freebasis, the bulk composition of these materials are basalticwith exceptions in the Columbia Hills, suggesting thesurface materials are derived from basaltic precursors byacid-sulfate alteration. Several hypotheses have been sug-gested for the aqueous formation of sulfate-bearing phaseson the surface of Mars, including (1) the oxidative weath-ering of ultramafic igneous rocks containing sulfides[Burns, 1988; Burns and Fisher, 1990]; (2) sulfuric acidweathering of basaltic materials [Morris et al., 1996, 2000];and (3) acid fog (i.e., vapors rich in H2SO4) weathering ofbasaltic or basaltic-derived materials [Clark and Baird,1979; Banin et al., 1997]. All three processes involveacid-sulfate weathering environments. Other candidate pro-cesses not involving acid-sulfate conditions might includethe dissolution and movement of soluble components bywater (e.g., groundwater, water thin films, flowing orstanding water) and the subsequent formation of evaporiticminerals depositions, e.g., Mg- and Ca-sulfates and chlor-ides [Moore et al., 1978, 1987; Clark et al., 1982].[8] Although all of these candidate processes may have

been important locally on Mars, we suggest that theprimary alteration process is acid-sulfate, oxidative weath-ering of basaltic materials. Laboratory simulation weath-ering of basaltic materials under putative Mars-likeweathering conditions offers an important method tocomplement or supplement the MER observations inunderstanding the past (and present) aqueous processeson Mars [e.g., Banin et al., 1997; Tosca et al., 2004].The objective of this study is to conduct simulated Mars-like weathering experiments in the laboratory to deter-mine the weathering products that might form during

E12S07 GOLDEN ET AL.: ACID-SULFATE WEATHERING OF BASALTIC MATERIALS

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oxidative, acidic weathering of Mars analog basalticmaterials. We have selected three possible oxidativesulfuric acid weathering environments to simulate: (1)sulfuric acid vapor (acid fog) weathering, (2) sulfuricacid weathering approximating an open hydrologic sys-tem, and (3) sulfuric acid weathering approximating aclosed hydrologic system. Acid fog simulation wasdesigned to maintain a very low water content, whereasthe weathering experiments approximating open andclosed systems were done at high water (high liquid:solidratios) and high acidity contents at elevated temperaturesto accelerate the process to achieve measurable results ina relatively short time.

2. Materials and Methods

2.1. Materials and Instrumental Methods

[9] Two unaltered basaltic samples from the Island ofHawaii were used as starting materials for the acid-sulfateweathering experiments. One sample was an olivine-richsand (HWPC100) obtained from the Puna Coast. Thesecond sample was a plagioclase-rich tephra (HWMK725)obtained from near the summit of Mauna Kea Volcano. AScintag XDS 2000 X-ray diffractometer using Cu-Karadiation operated at 45 kV and 40 mA current was usedto characterize the mineralogy of random powder mountsover the range of 2 to 70� 2q. Morphological, mineralogical,and chemical properties of the basaltic samples were char-acterized by a JEOL 5910LV scanning electron microscope(SEM) equipped with an energy dispersive spectroscopy(EDS). Quantitative chemical analyses of phases in polishedsections were conducted by EDS analysis using a 100 nmbeam, 15 kV acceleration voltage, 10 mm sample workingdistance, and detector at a 35� angle. Calibration standardsof olivine and plagioclase were used for ZAF correctionsof the respective phases. Major element chemistry ofstarting materials was determined by X-ray fluorescence(XRF) spectroscopy using a 9:1 Li2B4O7-sample flux-fusion procedure to prepare glass disks and a Philips2404 XRF spectrometer equipped with a 4 KW Rh X-raytube [see Boyd and Mertzman, 1987]. TransmissionMossbauer spectra were obtained at room temperature(�293 K) on Ranger Scientific spectrometers [Morris etal., 1985, 1989, 2000]. Absorbers for transmission experi-ments were made by dispersing powdered samples(<150 mm) in epoxy to a density of 50–100 kg/m2 of Fe.Mossbauer parameters, derived by fitting folded spectra totheoretical line shapes using an in-house computer pro-gram (JSCFIT), are the isomer shift (IS) relative to themidpoint of the spectrum of metallic iron foil at 293 K,quadrupole shift (QS), hyperfine field strength (Bhf), andline widths (W).

2.2. Acid-Sulfate Weathering Experiments

2.2.1. Sulfuric Acid Vapor (Acid Fog) Reactions[10] Samples were prepared in two ways: (1) polished

thick sections of natural particles and (2) natural (‘‘as is’’)particles (0.5–1.0 mm). For polished thick sections, ‘‘asis’’ basaltic particles were embedded in epoxy, mountedon a glass slide, and only the exposed surface waspolished. Polished sections were examined by SEMbackscatter imaging and EDS analysis prior to sulfuric

acid vapor weathering experiments. A 23-mL Teflon-linedhydrothermal vessel (Parr1 bomb) was half filled with 4-mm diameter quartz-glass beads. Two mL of concentratedH2SO4 was placed at the bottom of the reactor. Thicksections were placed with sample side up on top of thequartz-glass beads, well above the concentrated H2SO4.The Teflon-lined container was closed tight and placedinside a stainless steel casing and heated to 145�C in anincubator oven for 144 hours. At the end of the exper-iment the containers were cooled to room temperatureand opened to recover the exposed section. The boilingpoint of H2SO4 is 338�C at which the vapor pressure is101.3 kPa. The vapor pressure at 145�C is extremely low(�0.004 kPa); hence appreciable quantities of sulfuricacid vapor are not expected to condense as bombs werecooled to room temperature during the recovery ofreaction products. Sections were stored at 56% relativehumidity (RH) for 72 hours and then carbon coated priorto SEM analysis. A blank epoxy thick section (nosample) treated similar to the sample thick sectionsshowed that Si from quartz beads did not contaminatesurfaces of thick sections although epoxy became brittleand dark after the treatment.[11] In separate experiments, granular basaltic particles

were exposed to hot sulfuric acid vapors (145�C) in a closedvessel in a similar fashion to the polished thick sections asdescribed above, except that basaltic particles were placedon a small platinum dish, which was placed on the glassbeads. At the end of the experiment the particles wererecovered and immediately characterized by X-ray diffrac-tion (XRD) analysis. Particles not used for the XRDanalysis were stored at 56% relative humidity (RH) for72 hours and characterized again by XRD analysis.Samples stored at 56% RH were C-coated. The morphol-ogy and chemistry of C-coated samples were character-ized by SEM and EDS analyses.2.2.2. Oxidative Sulfuric Acid Alteration ofBasalt in a Three-Step Batch ExperimentApproximating an Open System[12] An open hydrologic system was approximated by

placing 100 mg of basaltic materials, 10 mL of 0.45 Msulfuric acid solution, and 2 mL of 31% H2O2 in aTeflon-lined vessel incased in a stainless steel bomb (�9 mgof O2 was added per mL of solution). Hydrogen peroxide waschosen as the oxidizing agent because superoxides andperoxides have been suggested as candidates for thepurported oxidant in Martian surface materials [e.g., Zentand McKay, 1994], and it will not cause appreciablechange in ionic strength or alkalinity of leaching solutions.Two particle sizes were used for individual experiments:(1) 0.5–1.0 mm and (2) <53 mm formed by completelygrinding the 0.5–1.0 mm fraction to pass through a 53-mmsieve (ground materials will have more reactive surfacearea). Reaction vessels were placed into an incubatoroven and heated to 145�C for 48 hours and then cooledto room temperature before removing the reactants fromthe Teflon containers. Solids were separated from thesolution by carefully decanting the solution. A 2-mLportion of the solution was saved for chemical analysis(leachate OS-1). Two mL of H2O2 (31%) was added tothe remaining leachate and transferred into a secondTeflon-lined reaction vessel containing 100 mg of fresh


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basaltic materials. The reaction vessel was heated to145�C for 48 hours. Samples and solutions (leachateOS-2) were separated as described above. This procedurewas repeated a third time (leachate OS-3). Solid residuesfrom each step were saved in a �20�C freezer to arrestany further reaction and freeze-dried for XRD, SEM,EDS, and Mossbauer analyses and leachate samples(leachates OS-1, OS-2, and OS-3) were centrifuged at10,000 rpm for 10 min in a IEC-B-22 Centrifuge toremove particulates and analyzed for Na, K, Ca, Mg, Si,Al and pH. A GBC 903 Atomic Absorption Unit wasused to measure solution Na, K, Ca, Mg, Si, and Al. Thesamples were again centrifuged for longer time periodsand analyzed for Na, K, Ca, Mg, Si, and Al. Si appearedto decrease suggesting that it was in a colloidal state(H2SiO4)n; however, the other elements did not changeindicating that they were in solution.[13] The procedure described above is an approximation

for an open system because the starting material is refreshedand the leachate solutions are reused at each step, increasingthe concentration of dissolved species [e.g., Barth-Wirschinget al., 1990].2.2.3. Oxidative Sulfuric Acid Alteration ofBasalt in a Single-Step Batch ExperimentApproximating a Closed System[14] A closed hydrologic system was approximated by

placing 100 mg basalt (0.5–1.0 mm), 10 mL of 0.45 Msulfuric acid solution, and 6 mL of H2O2 (31%) in a Teflon-lined stainless-steel vessel (�27 mg O2 was added per mLof solution). The reaction vessel was heated at 145�C for144 hours. Residues and the final leachate (leachate CS)were analyzed at the end of the experiment as describedabove for the three-step leaching experiments.[15] Here we define a ‘‘closed system’’ as any system

where reactants are added to the system (i.e., volatilephases such as S, Cl, and Br) and no products (exceptwater and other gaseous phases via evaporation) aretransported out of the system during or after the reaction.Perhaps there is rarely a natural system that approaches aperfect closed system (i.e., no additions and no losses),

but it is used here for comparison with the three-stepleaching experiments.

3. Results

[16] The relative abundance of phases in HWPC100 basal-tic sand is olivine > glass > plagioclase feldspar > pyroxene >titanomagnetite (Figure 1). Olivine in HWPC100 is Mg-rich(Fo80Fa20) as estimated by EDS analysis of representativeolivine grains. The relative abundance of phases inHWMK725 basaltic tephra sample is glass > plagioclasefeldspar > olivine > titanomagnetite (Figure 2). Plagioclasefeldspar in HWMK725 was Ca and Na rich (Ab50An46Or4).HWMK725 ismoreAl and Si rich than anyMartianmeteoriteand represents an extreme possible composition on Mars;whereas HWPC100 is similar in chemistry to the ‘‘Peace’’outcrop on the northwest flank of Husband Hill in theColumbia Hills of Gusev crater [Ming et al., 2005b]. Bulkcompositions (XRF analysis) of starting materials reflect thehigh Mg in olivine for HWPC100 and high Al and Na inplagioclase feldspar for HWMK725 (Table 1).[17] Secondary phases that formed after exposure to sulfu-

ric acid weathering conditions for acid fog, three-step, andsingle-step batch leaching experiments are listed in Table 2.Products identified by XRD analyses immediately afterexperiments ended and after exposure to 54% RH for72 h are listed to illustrate that crystallinity and hydrationstate for Mg, Ca, and Al sulfates depend on environmen-tal conditions. Details on the identification and character-ization of weathering phases for each starting material aredescribed next.

3.1. Olivine-Rich Basaltic Sand HWPC100

3.1.1. Sulfur Acid Vapor Reactions of Basalts:Natural Particles[18] X-ray amorphous magnesium sulfates formed as

reaction products after HWPC100 olivine-rich basalt par-ticles were exposed to the sulfuric acid vapors. X-rayamorphous magnesium sulfates were assumed to haveinitially formed because of the absence of peaks in XRD

Figure 1. Secondary electron backscatter images of epoxy imbedded sectioned HWPC100 grains:(a) general view of particles containing olivine, pyroxene (px), and plagioclase feldspar in a glass matrixand (b) plagioclase feldspar (pf) and olivine (ol) in a glass matrix.


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diffractograms for samples that were analyzed immediatelyafter the vessel was cooled as compared to samples exposedto 54% RH where hexahydrite (MgSO4.6H2O) was identi-fied as the only secondary alteration phase (Figure 3). SEMand EDS analysis indicated that HWPC100 particles werecoated mostly by Mg-sulfates and similar to phases thatformed on polished surfaces (see Auxiliary Material1

Figure 1).3.1.2. Oxidative Sulfuric Acid Vapor Reactions ofBasalts: Polished Thick Sections[19] Polished thick sections exposed to sulfuric acid

vapors permitted observation of the extent that phases inthe original basaltic materials altered and the phases thatformed on their surfaces (Figure 4). Glass appeared to be theleast reactive phase in the HWPC100 sample; plagioclasefeldspar was more reactive as suggested by etch featuresand abundant Ca-sulfate crystals associated with plagioclasefeldspar (Figure 4). The glass’s apparent resistance toalteration may be a factor of reaction kinetics due toparticle size, i.e., glass particles are larger than the plagio-clase particles. Olivine was the most reactive phase inHWPC100; in many cases the olivine was completelyreplaced by Mg sulfates (Figure 4d). Ca-sulfate crystalsreadily formed on much of the surfaces of basaltic sandgrains after exposure to sulfuric acid vapors. Most Ca-sulfate crystals were <2 mm in size (Figures 4a and 4b),while a few large gypsum crystals (25–50 mm) were alsofound near grain surfaces (Figure 4a). Olivine underwentintensive alteration and leaching leaving an amorphous silicaresidue and MgSO4.nH2O on crystal surfaces (Figure 4d).Magnesium sulfate is the most abundant salt observed inthe olivine-rich basalt; however, the highly hydrated Mg-sulfate crystals were difficult to image by SEM because

of their instability in the microscope’s electron beam andvacuum. Alunogen (Al2(SO4)3.17H2O) was a rare productafter alteration of HWPC100. Alunogen was identified byEDS analysis (Al and S) and their plate-shaped crystalmorphology (see below). In summary, phases formed byacid fog alteration of HWPC100 were MgSO4�nH2O,Ca-sulfates (gypsum and probably anhydrite), amorphoussilica, and minor alunogen.3.1.3. Oxidative Sulfuric Acid Alteration of Basaltsin Three-Step Batch Reaction ExperimentsApproximating an Open System[20] Unreacted basaltic sand HWPC100 was rich in

olivine and glass with small amounts of plagioclase feld-spar, pyroxene, and titanomagnetite. Starting materials forexperiments were the 0.5–1.0 mm and <53 mm size

Figure 2. Secondary electron backscattered images of epoxy imbedded sectioned HWMK725grains: (a) general view of particles containing plagioclase feldspars, olivine, and titanomagnetite inthe glass matrix, (b) plagioclase feldspar (pf) and olivine (ol) grains in the matrix of glass (gl), and(c) titanomagnetite (tm).

Table 1. Bulk Composition of Basaltic Starting Materials for

Sulfuric Acid Weathering Experiments

Oxide HWPC100,a wt.% HWMK725,b wt.%

SiO2 46.09 51.62TiO2 1.81 2.32Al2O3 9.29 17.86Fe2O3T

c 14.89 11.47MnO 0.20 0.23MgO 17.73 3.06CaO 7.52 6.07Na2O 1.54 4.38K2O 0.32 1.91P2O5 0.18 1.03Total 99.95 100.16% LOI 0.95 3.80FeO 11.55 5.35Fe2O3 2.05 5.53Fe3+/Fe(Total) 0.14 0.48Cl 0.132 0.00

aBasaltic sand, Kalpana, Puna Coast, Island of Hawaii.bBasaltic tephra, Mauna Kea volcano, Island of Hawaii.cTotal Fe as Fe2O3.

1Auxiliary material is available at ftp://ftp.agu.org/apend/je/2005JE002451.


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fractions of HWPC100. For both starting materials, theolivine completely reacted in the first leaching as shownby the absence of XRD peaks for olivine (Figure 5). Onlyrelatively acid resistant plagioclase feldspar and somepyroxene remain as the mineral phases in the residues afterthe first 48-hour treatment. Fe- and Ca-sulfates formed asreaction products after the second and third leachingsteps for both 0.5–1.0 mm and <53 mm starting materials(Figure 5). The XRD mineralogy of these sulfates phaseswas jarosite, gypsum, and anhydrite (note: anhydrite wasonly detected by XRD analysis in the <53 mm material; seeAuxiliary Material Figure 2). Very broad peaks at around 4 Ad-spacing (18–32 �2q) suggested the formation of amorphoussilica as an alteration product of HWPC100 (Figure 5).Jarosite, gypsum, and Mg-sulfate crystals were observed inSEM images (Figures 6c and 6d). The jarosite is a hydronium-sodium variety [(H3O)0.95Na0.05Fe3(SO4)2(OH)6)] as sug-gested by the low Na and trace K compositions as measuredby EDS.[21] The Mossbauer spectra for untreated and treated

HWPC100 samples are compared in Figure 7. The oct-Fe2+

olivine doublet is no longer present in the treated sample,in agreement with the absence of the phase in XRDdata. The oct-Fe3+ in the treated sample is assigned tojarosite and supports the XRD analysis. Although theolivine has been completely altered, the glass has not asshown by the presence of the oct-Fe2+ doublet from glass.No magnetic or nanophase oxide phases were detected. AMB spectrum of sulfatetic tephra sample HWMK515 from

near the summit of Mauna Kea Volcano in Hawaii [Morriset al., 2000] is shown for comparison in Figure 7. Thespectra for the sulfuric acid treated HWPC100 and naturalHWMK515 are remarkably similar. HWMK515 likely

Figure 3. X-ray diffraction patterns of untreated and acidfog treated HWPC100 basaltic sand grains (0.5–1.0 mm).The untreated sample consists mostly of olivine (ol) and atrace of titanomagnetite and pyroxene. No additional phaseswere identified in the acid fog treated samples at time zero.Hexahydrite (hx) was the only phase to form in treatedsamples after exposure to 54% RH conditions for 72 hours.

Table 2. Secondary Weathering Products and Remaining Residual Phases After Exposing Two Basaltic Materials to Simulated Sulfuric

Acid Weathering Conditions

Acid-Sulfate SimulatedEnvironment (Particle Size)

XRD Analysis:Immediately After Experiment

EndedXRD Analysis:

After 72 h Exposure to 54% RHSEM/EDS Analyses:a

After 72 h Exposure to 54% RH

HWPC100 HWMK725 HWPC100 HWMK725 HWPC100 HWMK725

Acid fog, natural particles olivine, plag.feldspar

AlH(SO4)2,Fe2(SO4)3,

amorph. silica,plag. feldspar

hexahydrite,olivine,

plag. feldspar

alunogen,hexahydrite,voltaite,


MgSO4.nH2O,amorph. silica

alunogen,halite,

amorph. silica

Acid fog, thick sections NAb NA NA NA MgSO4.nH2O,amorph. silica,Ca-sulfate,gypsum,

minor alunogen

alunogen,Ca-sulfate,gypsum,

amorph. silica

Three-step leachingexperiments0.5–1.0 mm jarosite,

gypsum,amorph. silica,plag. feldspar,pyroxene

plag. feldspar NA NA jarosite,gypsum,

amorph. silica,MgSO4.nH2O

amorph. silica

<53 mm jarosite,anhydrite,gypsum,

amorph. silica,plag. feldspar,pyroxene


NA NA jarosite,gypsum,

amorph. silica,MgSO4.nH2O

amorph. silica

Single-step batchexperiments0.5–1.0 mm and <53 mm amorph. silica,

plag. feldspar,pyroxene


NA NA amorph. silica amorph. silica

aPrimary phases (plag. feldspar, olivine, pyroxene) are not listed for SEM/EDS analysis.bNA, not applicable. Experiments were not conducted.


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formed when hydrothermal solutions enriched in sulfuricand hydrochloric acids (volcanic in origin) percolatedthrough tephra materials [Morris et al., 2000].3.1.4. Sulfuric Acid Alteration of Basalts in a Single-Step Batch Experiment Approximating a Closed System[22] Starting material for these experiments were the

same as for the three-step reaction experiments discussedabove. Only plagioclase feldspar and pyroxene remained asresidual mineral phases in the single-step experiment resi-due after sulfuric acid treatment of HWPC100 startingmaterials (Figure 5). A very broad peak at �4 A d-spacing(18–32 �2q) suggests the formation of amorphous silica asan alteration product.[23] Scanning electron micrographs of acid-sulfate treated

HWPC100 particles showed leached glass surfaces; how-ever, the morphology of the original glass shard wasmaintained (compare unaltered glass shard in Figure 6a toaltered glass shard in Figure 6b). The olivine has beencompletely dissolved while leaving behind a precipitate ofamorphous silica on particle surfaces (Figure 6b). Theamorphous silica phase was assumed on the basis of the

XRD analysis (see above) and the high Si content at thesurface as detected by EDS analysis.

3.2. Plagioclase Feldspar-Rich BasalticTephra HWMK725

3.2.1. Sulfuric Acid Vapor Reactions of Basalts:Natural Particles[24] Particles of plagioclase-rich basalt HWMK725 were

partially altered upon exposure to sulfuric acid vapors.Reaction products analyzed immediately after treatmentwere transient Al and Fe sulfate phases; the peaks markedT1–T4 in Figure 8 are assigned to aluminum hydrogensulfate (AlH(SO4)2) and an anhydrous iron sulfate (X-raypowder diffraction file - PDF: 21-0922). After exposure toambient conditions (RH � 56%) for 72 hr, XRD peaks werepresent that corresponded to alunogen, hexahydrite, andvoltaite (K2Fe5Fe4(SO4)12.18H2O) (Figure 8). Presumably,these crystalline salts formed by hydration and reprecipita-tion of initially X-ray amorphous and transient phases.[25] In SEM images and by EDS analysis, Al- and S-rich

platy crystals were observed adhering to individual grains of

Figure 4. SEM secondary electron images of sulfuric acid vapor treated olivine-rich HWPC100 basaltsample exposed to 56% RH for 72 hours. (a) A large gypsum (gy) crystal (approximately 20 mm across)has formed on the surface of a plagioclase feldspar crystal. (b) Small Ca-sulfate crystals (light-tonedcrystals) have formed on the surface of a plagioclase feldspar (pf) crystal. The plagioclase feldspar hasbeen etched by the sulfuric acid vapor treatment. (c) Mg-sulfate (ms) crystals have formed near theinterface of a particle and epoxy that suggests the mobility of Mg to this interface and the subsequentprecipitation of the Mg sulfate. (d) Mg-sulfate (ms) crystals and amorphous silica have completelyreplaced an olivine crystal. The surround glass (gl) appears to have experienced little alteration at thesurface as suggested by the lack of precipitates.


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HWMK725; the platy crystal morphology was similar inmorphology to platy crystals that formed on surface of thicksections after exposure to sulfuric acid vapors (see below).Presumably, these crystals correspond to the alunogendetected by XRD analysis. Halite crystals were also ob-served in SEM images of reacted HWMK725, indicatingthe presence of Cl-bearing phases that were present in theoriginal starting material (see Table 1). These samples werenot washed after residues were separated from the reactingsolutions; hence the halite formed during the evaporation ofsmall amounts of aqueous solution remaining with theresidue.3.2.2. Oxidative Sulfuric Acid Vapor Reactions ofBasalts: Polished Thick Sections[26] Etched (or leached) olivine surfaces in HWMK725

thick sections show that olivine is extensively altered afterexposure to H2SO4 vapor (Figure 9b). Plagioclase feldsparand glass appear to be etched less (Figures 9b and 9c).[27] Although not seen in the XRD analysis of the acid

vapor treated natural grain samples (section 3.2.1), nearlyall polished surfaces of HWMK725 thick sections weredecorated with Ca-sulfate crystals of 1–2 mm in size(Figure 9). The Ca sulfate appears to be crystalline, althoughit was difficult to differentiate the crystal habit (i.e., differ-entiate anhydrite and gypsum) because of their small sizes.Models of our hydrothermal system suggest that anhydritewould be the stable phase to form above 40�C, and thereforeat 145�C anhydrite would be the expected phase. In someinstances, Ca-sulfate crystals appeared to be more abundant

on the plagioclase feldspar crystals (Figure 9c). However,there were tephra particles where glass appeared to haveundergone greater leaching as suggested by the abundanceof Ca-sulfate crystals on their surfaces (Figure 9d). Thesurface precipitation of Ca sulfate appears to be a function ofthe amount of Ca in the phase being leached, i.e., more Casulfate nucleated at the surfaces of particles with high Cacontents (e.g., plagioclase feldspar, pyroxene or glass withhigh Ca contents). Surfaces of nearly all tephra particleswere covered with amorphous silica as suggested by thedetection of high Si by EDS analysis. Plate-shaped alunogenwas found in abundance near the surfaces and edges ofgrains (Figure 9a). Alunogen formation near the grain-epoxyinterface suggests migration of solvated Al3+ along grainsurfaces and subsequent nucleation at the surface. Alunogenplates were occasionally found lying flat on glass surfaces,although, most basaltic grain surfaces were not impregnatedby the Al sulfate. In summary, the primary phases thatformed on the surfaces of HWMK725 were Ca sulfate,alunogen, and amorphous silica.3.2.3. Oxidative Sulfuric Acid Alteration ofBasalts in Three-Step Batch ReactionExperiments Approximating an Open System[28] The XRD analysis (Figure 10) of solid phases from

the three-step leaching experiments for HWMK725 showsplagioclase feldspar as the predominant residual mineralafter each step for both starting materials (0.5–1.0 mm sizefraction and for <53 mm powder of bulk sample). A broad,weak intensity peak between 18–32 �2q was assigned toamorphous silica.3.2.4. Sulfuric Acid Alteration of Basalts in aSingle-Step Batch Reaction ExperimentApproximating a Closed System[29] The XRD pattern of solid phases from the single-step

batch experiment HWMK725 shows that plagioclase feld-spar is the predominant residual mineral for both particulatestarting materials (Figure 10). The broad hump near 4 Ad-spacing (18–32 �2q) is more intense than for residualphases in the three-step leaching experiments, suggestingthat a higher proportion of amorphous silica is present asan alteration phase.

4. Aqueous Chemistry of Simulated SulfuricAcid Weathering Environments

[30] The experiments with three steps of leaching allowsimulation of solutions moving through the basalt, dissolv-ing ions and transporting them to other locations. Precipi-tation of phases from solution depends on the degree ofsolution saturation with respect to the forming phase,solution pH (i.e., degree of neutralization of acidic solu-tions) and the activity of water (e.g., degree of evaporationof solutions). The single-step and the sulfuric acid fogexperiments simulate isochemical alteration (i.e., closedsystem alteration except with respect to input of the acid-sulfate alteration agent). Cations are not leached from thesystem.[31] The three sequential leaching steps that simulated the

open system allowed for oxidation and evaporation of acidicsolutions between steps. Cation concentrations in leachatesincreased as the leaching solution scavenged more cationsand the acidity was neutralized by fresh basalts in each step

Figure 5. X-ray diffraction patterns for air-dried residuesof three-step and single-step leaching experiments ofolivine-rich basalt sample HWPC100 (0.5–1.0 mm). Theuntreated pattern is for the original HWPC100 startingmaterial prior to sulfuric acid treatment, OS-1 is the residueafter the first ‘‘leaching’’ treatment in the three-step leachingexperiment, OS-2 is the residue after the second ‘‘leaching’’treatment in the three-step leaching experiment, OS-3 is theresidue after the third and final ‘‘leaching’’ treatment in thethree-step leaching experiment, and CS is the residue aftertreatment in the single-step batch experiment (ol, olivine;gy, gypsum; hx, hexahydrite; j, jarosite, pf, plagioclasefeldspar; px, pyroxene). See color version of this figure inthe HTML.


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for both HWMK725 and HWPC100 (Figures 11a and 11b).The relative concentrations of cations were different inextracts of olivine-rich basalt versus plagioclase feldspar-rich basalt. Leachate compositions were controlled by thecomposition of the most soluble components in each sam-ple. Mg and Fe concentrations were higher in HWPC100leachates where olivine (�Fo80Fa20) is a major component(Figures 11a and 11b). The complete dissolution of theolivine in 0.45 M H2SO4 in HWPC100 is consistent withthe susceptibility of olivine to acid attack [e.g., Grandstaff,1986; Wogelius and Walther, 1992] and likely accounts forthe very high Mg level observed in the HWPC100 leachate.Although the Fe and K concentrations are relatively low inthe three-step leachate for HWPC100 (Figure 11b), theyformed insoluble jarosite (very low Ksp) in the presence ofacidity and sulfate anions [e.g., Baron and Palmer, 1996]. Kincorporation into the jarosite is suggested by the decrease

in K concentration for the OS-2 and OS-3 leachates ascompared to the OS-1 leachate (Figure 11b). Al and Kconcentrations are higher in the HWMK725 because pla-gioclase feldspar (�Ab50An46Or4) is a major componentcontributing cations into solution (Figures 11a and 11b).[32] The pH of the solution increased in the three-step

experiment as the leachate passed through each batch ofbasalt (Figures 11c and 11d). The pH rise in the HWPC100leachates was larger than the pH rise in HWMK725leachates. The pH rise in the olivine-rich leachate mightsuggest that the acid neutralization by olivine was muchmore effective than that by plagioclase feldspar. One factormay be the release of Mg2+ in olivine, compared to Al3+ inplagioclase feldspar. The Al3+ cation is an acid producingcation compared to neutral Mg2+ [e.g., Banin et al., 1997].Silica (silicic acid) has a very high pK1 value (9.66 [Weast,1974]) and is able to sequester H+ ions causing the pH to

Figure 6. SEM secondary electron images of olivine-rich basaltic sand HWPC100 (<53 mm) prior toand after treatment in the single-step and three-step leaching experiments. (a) Untreated sampleillustrating the shape and texture of unaltered glass (gl) shards along with finer materials. (b) Residueafter single-step batch experiment that illustrates a leached surface (ls) of a glass shard and the formationof amorphous silica (as) on the surface of the shard. (c) A large gypsum crystal and small Mg-sulfatecrystals have formed after the third leaching step of the three-step leaching experiment. These phaseslikely formed from evaporation of solutions remaining adsorbed to particles after solutions were decantedand separated from the residue. (d) Jarosite (j) crystals that have formed after the third leaching step in thethree-step leaching experiment. These crystals are composed of Fe and S with minor amounts of Na. Thelack of K and minor Na suggests that the variety of jarosite that formed in the open system washydronium jarosite.


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rise; hence the higher Si concentration may also contributeto the pH rise in HWPC100 solutions.[33] The leaching of Ca, Mg, Na, K, Al and Fe in the

single-step batch experiment was similar to the leaching ofcations in the three-step leaching experiment, except that thetotal cation concentrations in the 3rd leachate in the three-step leaching experiment were higher than for the single-step leachate (Figures 11a and 11b). The secondary mineralprecipitation was limited to amorphous silica for bothbasaltic materials in the single-step leaching experiments.[34] It is important to note differences between the results

for the three-step and single-step leaching experiments maybe a kinetic effect and/or a consequence of different water torock ratios. Focusing on only HWPC100 experiments, theextent of dissolution may become limited by kinetics oncethe near surface regions of the particles have been leached.If so, this would limit the availability of cations, particularlyfrom the glass, and not permit the Fe3+ concentration to besufficiently high to precipitate jarosite. Jarosite may nothave formed in the single-step leaching experiment becausethe water to rock ratio in our experiments was sufficientlyhigh that the aqueous solutions were undersaturated withrespect to jarosite precipitation and the pH did not rise intothe jarosite stability field (see below). Therefore we cannotexclude jarosite formation in a closed system if environ-mental conditions permit exceeding the solubility product of

the sulfate. The water to rock ratio cannot be too low,however, or the basalt will buffer the pH a value above thatrequired for jarosite precipitation.[35] The release of ions for ground starting materials

(<53 mm powders) and the alteration products that formedwere similar to the experiments conducted with 0.5–1.0 mmmaterials. As would be expected, the increased surface areadid result in slightly higher concentrations of elements intosolution. The 0.5–1.0 mm sized material better representeolian basalt sands that may have been present and subse-quently altered on Meridiani Planum (see discussion below).Solutions were not collected for the acid fog experimentsbecause of the near absence of water in the system. Amor-phous silica, Ca sulfates, alunogen, and amorphous Mg-sulfates formed at the surfaces of polished thick sections ofthe Mars analog basaltic particles after exposure to sulfuricacid vapor. This suggests that Si, Ca, Mg, and Al werereleased into ‘‘solution’’ where they reacted with H2SO4

(aq) to form various sulfates along with amorphous silica.

5. Discussion

5.1. Acid Fog Weathering

[36] Aluminum-, Mg-, Ca-, and Fe-sulfates and amor-phous silica are the phases that formed after basaltic tephraand sand were subjected to oxidative, sulfuric acid vapors inour experiments. These results are consistent with theexperimental results of Banin et al. [1997] and Tosca etal. [2004]. Laboratory-simulated acid weathering of pala-gonitic tephra resulted in the formation of gypsum andalunogen [Banin et al., 1997]. They hypothesize that the toplayer of Mars ‘‘soil’’ may have formed by extremely slowongoing weathering interactions at the atmosphere-rock

Figure 7. Mossbauer spectra (293 K) for olivine-richbasaltic sand HWPC100 (0.5–1.0 mm) prior to and aftersulfuric acid treatment in the three-step leaching experi-ment. The spectrum of the untreated sample was dominatedby glass and olivine. Jarosite and glass were the dominantphases in the MB spectrum after sulfuric acid treatment. Anatural jarosite-rich tephra sample from near the summit ofMauna Kea Volcano in Hawaii has a very similar MBspectrum when compared to the sulfuric acid treatedHWPC100.

Figure 8. X-ray diffraction patterns of untreated and acidfog treated HWMK725 basaltic tephra grains (0.5–1.0 mm).The untreated sample is mostly plagioclase feldspar (pf).The treated sample X-rayed at time zero (i.e., immediatelyat completion of experiment) had a few additional peaksthat were probably due to transient phases (T1–T4)aluminum hydrogen sulfate and an anhydrous ferric sulfate.Alunogen (al), hexahydrite (hx), and voltaite (vo) were theprimary sulfates that formed after exposing the acid fogtreated samples to 56% RH conditions for 72 hours. Seecolor version of this figure in the HTML.


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interfaces. The reactions are driven by acidic volatiles(e.g., SO2, Cl) deposited from the atmosphere and thenreacted with the mineral surfaces in the dry Mars envi-ronment. Tosca et al. [2004] subjected synthetic basalticanalogs derived from Mars Pathfinder soil and rockcompositions to various acidic solutions and subsequentevaporation. Crystalline Mg-, Fe-, Ca-, and Al-sulfates(but not jarosite) were identified as reaction productsalong with the formation of secondary ferric oxidephases, which formed via rapid Fe oxidation underrelatively high pH levels buffered by basalt dissolution.Amorphous silica was also identified as a ubiquitousproduct on particle surfaces.[37] A major difference between the studies of Banin et

al. [1997] and Tosca et al. [2004] and our experiments isthat we did not react the basaltic materials in solution, butinstead sulfuric acid vapors interacted with the materialsthat maintained the activity of water near zero. Anotherimportant difference is that we preformed experimentsunder hydrothermal conditions (145�C) compared to exper-iment temperatures near 25�C for Banin et al. [1997] andTosca et al. [2004]. Our goal was to create environmentalconditions where sulfuric acid vapors reacted with basalticmaterials without any leaching of solutions and the higher

temperatures simulated hydrothermal conditions as well asenhance the kinetics of dissolution and neoformation ofsulfates. Small amounts of water inherent to the basalticsamples used in our experiments and in the concentratedsulfuric acid will contribute to the activity of water, but thewater either reacted with the basaltic materials to formhydrated phases or was lost by evaporation when exposedto room temperature conditions after opening hydrothermalvessels.[38] Amorphous or transient sulfates appear to form first

in our acid fog experiments with a very low water activity.Crystalline sulfates formed after exposure to ambient con-ditions (RH � 54%). No doubt, RH has a major role incontrolling the mineralogy of hydrous sulfates. This isespecially the case for the Mg sulfates where modestchanges in the RH will result in different hydration statesor mineralogy [e.g., Vaniman et al., 2005]. It is plausiblethat acid fog weathering on Mars resulted in the formationof amorphous sulfates; these sulfates may still be amor-phous, depending on the subsequent interaction of water(e.g., atmospheric) with the sulfates and the amount of waterwithin the surface materials. New data currently beingacquired by the OMEGA instrument onboard the MarsExpress orbiter may provide new insights on the sulfate

Figure 9. SEM secondary electron images of sulfuric acid vapor treated plagioclase feldspar-richHWMK725 basalt samples exposed to 56% RH for 72 hours. (a) Platy alunogen (al) crystals formed atthe interface of the basaltic tephra grain and epoxy (ep). Olivine (dark crystals) and plagioclase feldsparlaths are in a matrix of glass (gl). Ca-sulfate crystals decorate the surface of the grain. (b) Amorphoussilica (as) has completely replaced olivine crystals (dark crystal casts). Elongated rod-shaped crystals nearthe top of this image are gypsum (gy). The surfaces of the glass (gl) and replaced olivine crystals aredecorated with Ca-sulfate crystals. (c) Small white Ca-sulfate crystals decorate the surfaces of plagioclasefeldspar (pf) and glass (gl), and alunogen (al) plates have formed near the edge of the particle. (d) Ca-sulfate crystals decorate the surface of glass that was initially rich in Ca. An olivine cast (dark area notdecorated with Ca-sulfate crystals) has been replaced by amorphous silica (as).


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mineralogy on Mars [Bibring et al., 2005; Gendrin et al.,2005; Langevin et al., 2005].

5.2. Three-Step Leaching ExperimentsApproximating an Open System

[39] Jarosite, Mg-, and Ca-sulfates and amorphous silicaformed from the olivine-rich basaltic sand and amorphousSi formed from the plagioclase-rich basaltic tephra duringacid-sulfate alteration in three-step (hydrothermal) leachingexperiments. The formation of jarosite is analogous to theformation of jarosite in basaltic materials on Mauna Keavolcano in Hawaii under oxidizing, hydrothermal condi-tions [Morris et al., 1996, 2000]. The sulfuric acid solutionswere thought to be a result of interactions of SO2-richvolcanic gases. Acid-sulfate solutions percolated up throughthe basaltic tephra, dissolved Fe and other cations, andprecipitated jarosite when environmental conditions permit-ted its formation. The Fe mineralogy of some Mauna Keasulfatetic tephra is very similar to the Fe mineralogy in ourthree-step leaching experiments of the olivine-rich basalticsands used in these experiments (see Figure 7).[40] In several cases, hematite occurred with the jarosite

in Mauna Kea sulfatetic tephra, suggesting that Fe oxideeither co-precipitated with the jarosite under hydrothermalconditions or they are hydrolysis products of jarositedissolution. The pH of the system plays a major role indefining the stability field between jarosite and hematite (orgoethite). The pH values in our experiments were lowerthan 1 in most cases (Figures 11c and 11d); Fe3+ likelyremained in solution at these low values. Jarosite only

precipitated in the three-step leaching experiments (OS-3);which had the highest pH values (Figures 11c and 11d).Jarosite is stable in the pH range �0.75–3.5; below thisrange jarosite dissolves and above this range it hydrolyzesto form hematite and/or goethite (e.g., stability diagram ofBurns and Fisher [1990]). Thus, in our three-step leach-ing experiments, pH values were not high enough toinitiate jarosite dissolution and subsequent hydrolysis ofFe to form Fe oxides or oxyhydroxides (e.g., hematite,goethite). We surmise that if we repeated our three-stepleaching protocol enough times that the continued disso-lution of basaltic materials would neutralize the acidityuntil the pH of the system favored the formation of ironoxides and oxyhydroxides.

5.3. Single-Step Batch Reaction ExperimentApproximating Closed Systems

[41] Only amorphous silica formed under single stepexperiments. Apparently, the closed system solutions didnot achieve supersaturation with respect to ferric sulfates,such as jarosite. Movement of solutions through basalticmaterials (i.e., open hydrologic systems) allows for theneutralization of the acid by the dissolution of basalticcomponents. The inability of the basalt to neutralize theclosed system solutions limited the formation of phasessuch as jarosite (which is soluble at very low pH), but Sireached saturation with respect to the formation of amor-phous silica. If the solid to solution ratio was lower in ourexperiments, then it may have been possible to form jarositeand other sulfates. We did not test a range of solid:solutionratios; hence we cannot rule out the possibility of formingjarosite in a closed system.

6. Implications for Sulfate Formation onMeridiani Planum

[42] Jarosite, hematite, and other sulfates (e.g., Mg sul-fates) occur along with siliciclastic sediments in outcrops atMeridiani Planum. A leading candidate for the formation ofthese sediments is the evaporation of acid fluids that haveinteracted with and altered olivine-bearing basaltic materials[Tosca et al., 2005]. A key but reasonable assumption is thatwaters rich in sulfuric acid interacted with basaltic sedi-ments in Meridiani Planum. Sulfuric acid waters could formby the interaction of volcanic emissions (i.e., SO2, Cl, Br)with water vapor or liquid. Key questions on the formationof these sulfate-rich sediments are how much acidic waterinteracted with the basaltic sediments and whether solutionsmoved through these sediments and removed soluble con-stituents (i.e., open hydrologic system) or if these sedimentswere altered under isochemical conditions (i.e., closedhydrologic system).[43] On the basis of our experiments, the formation of

jarosite in the Meridiani outcrops may reflect only slightneutralization of acidity to move the Fe3+ ion into thejarosite stability field, similar to what we observed in ourthree-step leaching regime. Waters rich in sulfuric acid (e.g.,groundwater) may have moved through basaltic sedimentand dissolved olivine-rich basaltic phases in the sediments.Dissolved Mg, Ca, Na, K, and residual Fe3+ were trans-ported causing further neutralization of the solution toprecipitate iron oxides. Gypsum (or anhydrite) was one of

Figure 10. X-ray diffraction patterns for air-dried residuesof the three-step and single-step batch experiments for 0.5–1.0 mm plagioclase feldspar-rich basalt sample HWMK725.The untreated pattern is for the original HWMK725 startingmaterial prior to sulfuric acid treatment, OS-1 is the residueafter the first ‘‘leaching’’ treatment in the three-step leachingexperiment, OS-2 is the residue after the second ‘‘leaching’’treatment in the three-step leaching experiment, OS-3 is theresidue after the third and final ‘‘leaching’’ treatment in thethree-step leaching experiment, and CS is the residue aftertreatment in the single-step batch experiment. All peaks inthis sample are for plagioclase feldspar. A very broad peak(18–32 �2q) is assigned to amorphous silica. See colorversion of this figure in the HTML.


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the sulfates to initially precipitate because of its lowsolubility while other alkali metal and alkaline earth sulfatesprecipitated during further evaporation of solutions [e.g.,Tosca et al., 2005]. This type of weathering is supported bythe occurrence of jarosite and the dearth of olivine (oct-Fe2+

in olivine as detected by MB [Klingelhofer et al., 2004]) inthe siliciclastic sediments. There is some oct-Fe2+ in Mer-idiani outcrop that is interpreted as pyroxene [Klingelhoferet al., 2004]. If so, the pyroxene is more resistant thanolivine to attack by sulfuric acid. Although there is onlyindirect chemical evidence for feldspar in Meridiani sedi-ments [Clark et al., 2005], the phase would also be moreresistant to alteration than olivine on the basis of our three-step leaching experiments. This mineralogical description issimilar to that for our three-step leaching of HWPC100where jarosite formed at the expense of olivine dissolutionand plagioclase feldspar remained as the primary residualphase. By implication, the Meridiani outcrop could haveformed by a similar process. A possible caveat is that thebulk composition of the outcrop may not be compatiblewith significant additions of Fe, Mg, and other cationsaccompanying acid-sulfate solutions. Alternatively, our ex-perimental open-system conditions could be mimicked by a

closed system process on Mars where the composition ofthe basaltic precursor and water to rock ratio werefavorable for dissolution of silicate Fe2+ under acid-sulfate conditions to soluble Fe3+ and subsequent precip-itation of jarosite and other phases when their solubilityproducts become exceeded by, for example, partial neu-tralization of acid-sulfate solutions during dissolution ofbasic basaltic materials.[44] The Meridiani outcrop contains two populations of

hematite; fine-grained hematite dispersed within the outcropmatrix and hematite-rich spherules (1–6 mm) embedded inthe outcrop [Klingelhofer et al., 2004; Herkenhoff et al.,2004]. Possible hematite formation processes are covered indetail by Tosca et al. [2005] and McLennan et al. [2005]. Intheir view, insoluble sulfates (e.g., jarosite) and evaporatetype minerals form first in acid-sulfate weathering environ-ments. Iron oxide formation is the result of oxidativehydrolysis of Fe that occurs as the pH increases in responseto acid neutralization by the reaction with basic basalticmaterials. Although we did not form iron oxides or oxy-hydroxides in our short-term experiments, it is possible thattheir formation was precluded because our acidic solutionswere not allowed to react long enough to neutralize the

Figure 11. Solution chemistry of leachates for basaltic samples HWMK725 and HWPC100 aftersulfuric acid treatments in the three-step and single-step batch experiments (OS-1, OS-2, and OS-3 are theleachate after first, second, and third ‘‘leaching’’ steps, respectively, in the three-step leaching experiment;CS is the leachate after sulfuric acid treatment in the single-step batch experiment). (a) Elementalconcentrations for plagioclase feldspar-rich basaltic tephra HWMK725 (0.5–1.0 mm). (b) Elementalconcentrations for olivine-rich basaltic sand HWPC100 (0.5–1.0 mm). (c) Solution pH for plagioclasefeldspar-rich basaltic tephra HWMK725 (0.5–1.0 mm). (d) Solution pH for olivine-rich basaltic sandHWPC100 (0.5–1.0 mm).


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acidity and raise the pH into the stability field for hematiteor goethite.

7. Implications for Sulfate Formation in GusevCrater

[45] Evidence for aqueous alteration processes involvingS, Cl, and Br range from minimal for rocks on the Gusevplains to extensively altered outcrops and rocks in theColumbia Hills. Specific S, Cl, or Br minerals have notbeen identified by the mineralogical instruments onboardthe Spirit rover, unlike the identification of jarosite inMeridiani outcrops. Chemical compositions (APXS data)and Fe mineralogy and oxidation state (Mossbauer spec-trometer data), however, are being used to constrain thepossible mineralogy of these volatile-bearing phases. Forexample, the positive correlation of Mg and S in someGusev soil trenches suggests the mobilization and precipi-tation of Mg-sulfate phases [Haskin et al., 2005]. Moreextensive alteration appears to have occurred in the Colum-bia Hills where some outcrops have high S, Cl, and Br insubsurface rock that was exposed by the Rock AbrasionTool (RAT). Positive correlations between Ca and S in someoutcrops (e.g., ‘‘Clovis’’) suggest the occurrence of Ca-sulfates [Ming et al., 2005a, 2005b]. Recently, the SpiritAPXS has detected high Mg and S concentrations in severaloutcrops (e.g., ‘‘Peace’’) that suggest the possible occur-rence of Mg-sulfates [Ming et al., 2005b].[46] Acid-sulfate weathering is a leading candidate pro-

cess for the formation of the purported occurrence of Mg-and Ca-sulfates in the Columbia Hills. Most of the outcropsexamined by APXS on the West Spur of the Columbia Hillsexhibit small differences in the primary rock formingelements Al, Si, and Fe. The major differences are theoxidation state of Fe, Fe-bearing phases, and especially theamounts of S, Cl, and Br when compared to the olivine-richbasalts on the ‘‘plains’’ of Gusev crater near the landing site.The aqueous alteration in these materials may be isochem-ical weathering under closed hydrologic systems or acid fogconditions. The major products in our acid fog weatheringexperiments were alunogen, Mg-sulfates, Ca-sulfates, andamorphous Si. There have been no suggestions (e.g., APXScorrelations of Al and S) for alunogen or Al-sulfate forma-tion in the West Spur rocks or outcrops. Amorphous silica isa common product of the sulfuric acid weathering ofbasaltic materials; however, secondary silica has yet to beidentified in the Columbia Hills. The possible occurrence ofCa- and Mg-sulfates suggests sulfuric acid weathering ofbasaltic materials that may have been rich in olivine,pyroxene and plagioclase feldspars. This hypothesis issupported by the low abundance of olivine in the WestSpur outcrops and rocks as determined by MB [Morris etal., 2005]. Oct-Fe2+ interpreted as pyroxene is detected inoutcrop rocks at levels comparable to rocks in the Gusevplains. Plagioclase feldspars are likely phases in some of theWest Spur outcrops, but their occurrence has not yet beenmade by direct mineralogical measurements by MER instru-ments; only assumed by APXS normative chemical calcu-lations [Ming et al., 2005b].[47] One scenario for aqueous alteration on the West Spur

is that vapors rich in SO2, HCl, HBr, and water interact atsurfaces of rocks and outcrops, which are basaltic in bulk

composition and are presumably volcaniclastic or impactdeposits [Ming et al., 2005a, 2005b]. Acidic vapors diffusedinto the rocks and outcrops and react with the primaryphases (mainly olivine). Soluble ions from the host rockreacted with S (and likely Cl and Br) to form Ca-, Mg- andother sulfates along with iron oxides and oxyhydroxides.[48] Although the experimental results presented here are

used to support or strengthen the current knowledge of theMartian surface processes, a caveat is that the knowledgebank is still evolving and any conclusions drawn here canonly be firmly validated by more in-depth mineralogical andgeochemical characterization of Martian surface materials(e.g., 2009 Mars Science Laboratory Mission, Mars samplereturn missions, etc.).

[49] Acknowledgments. This work was funded by the NASA MarsFundamental Research Program through a grant to D.W.M. We thank BenClark and an anonymous reviewer for their very helpful reviews.

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��D. C. Golden, ESCG/UTC-HS, Mail Code JE 23, 2224 Bay Area

Boulevard, Box 7, Houston, TX 77058, USA.S. A. Mertzman, Department of Geosciences, Franklin and Marshall

College, Lancaster, PA 17604, USA.D. W. Ming and R. V. Morris, Astromaterials Research and Exploration

Science Directorate, NASA Johnson Space Center, Mail Code KX, 2101NASA Road 1, Houston, TX 77058, USA. ([email protected])


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