Marine Chemistry 177 (2015) 687–696

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Marine Chemistry

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Amorphous As-sulfide precipitates from the shallow-waterhydrothermal vents off Milos Island (Greece)

A. Godelitsas a,⁎, R.E. Price b, T. Pichler c, J. Amend d,e, P. Gamaletsos f,g, J. Göttlicher g

a School of Science, University of Athens, Panepistimioupoli Zographou, 15784 Athens, Greeceb School of Marine and Atmospheric Sciences, SUNY Stony Brook, Stony Brook, NY, USAc AG Geochemie & Hydrogeologie, Fachbereich Geowissenschaften, Universität Bremen, D-28334 Bremen, Germanyd Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089, USAe Department of Biological Sciences, University of Southern California, Los Angeles, CA 90089, USAf School of Mining and Metallurgical Engineering, NTUA, Zographou Campus, 15780 Athens, Greeceg Karlsruhe Institute of Technology, ANKA Synchrotron Radiation Facility, 76344 Eggenstein-Leopoldshafen, Germany

⁎ Corresponding author.E-mail address: agodel@geol.uoa.gr (A. Godelitsas).

http://dx.doi.org/10.1016/j.marchem.2015.09.0040304-4203/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 14 May 2015Received in revised form 9 September 2015Accepted 14 September 2015Available online 19 October 2015

Keywords:Arsenic sulfideOrpimentAmorphousHydrothermalMarineAegean SeaMilos IslandCycladesHellenic Volcanic Arc

Amorphous orpiment-like As-sulfides (As2S3) are the most common As phases precipitating in hydrothermalsystems, yet there is a lack of information regarding their solid-state characterization. Using a combination ofoptical, SEM–EDS, micro-Raman and XANES/EXAFS applications, we investigated yellow-orange As- and S-richsediments occurring in the shallow-water hydrothermal system off the coast of Milos Island, Greece. The precip-itates have several morphologies, but are dominantly colloidal. Intriguing “biological” morphologies also exist(e.g., cell-like (~10 μm), spirals (~20 μm), and rounded “cinnamon bun” shapes (~20 μm)). SEM–EDS data indi-cated that the precipitates have an As:S ratio similar to orpiment (average = 0.58, range 0.51–0.63; n = 8).Micro-Raman spectra indicated that orange colored precipitates appear to be dominated by poorly crystallineand/or amorphous arsenic sulfideswithmicro-amounts ofmore crystalline orpiment and impure sulfur. The yel-low sediments also contained crystalline elemental sulfur in the form S8. Bulk As K-edge XANES spectra of the As-sulfide precipitates proved a valence of As corresponding to orpiment-type (As2S3) compounds (−1 to +3).EXAFS fitting results indicated that the studied material exhibits an amorphous orpiment-like structure withAs ions coordinated by 3 sulfur atoms (CN = 3.0). The As–S interatomic distance of the first shell is calculatedat 2.279 Å and the Debye–Waller factor (σ2) is 0.00427. These data suggest that the modeled structure ofthe studied precipitates is slightly S-deficient and ordered only in the first shell around As, resembling anorpiment-type structure, whereas higher shells are not present andmust be disordered. The disorder phenome-non may be strictly produced either by the existence of occasional As–S–As bridges with As–As bonds or by theoccurrence of As–O–As bridges, causing twisting of the AsS3 pyramids in the initial orpiment structure. Thisdistortion in the higher coordination shells of the structural sheets creates the amorphous orpiment.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

One of the primary sources of arsenic (As) to the oceans is from hy-drothermal venting (e.g. Breuer and Pichler, 2013). However, removalof arsenic, either as a discrete arsenic mineral or adsorption ontoe.g., iron oxyhydroxides at submarine hydrothermal vents is essentialfor reducing the highhydrothermalfluxof this toxic element to seawaterand is an important part of the geochemical As cycle (e.g. Pichler et al.,1999a; Pichler et al., 1999b; Price and Pichler, 2005; Dekov et al.,2013). Orpiment (As2S3) and realgar (As2S2) are the primary crystallinearsenic minerals precipitating in hydrothermal environments (Dekov

et al., 2013). Orpiment is commonly deep orange to yellow colored andoccurs mainly in low-temperature hydrothermal veins, hot springs andfumaroles. Thermodynamically, it is stable across a range of Eh and pHconditions (Pokrovski et al., 1996), but is more stable under reducing,acidic conditions. Precipitation typically takes place through the reactionof aqueous As(III) (H3AsO3) and sulfide (H2S or HS−). In hydrother-mal systems precipitation of amorphous “orpiment” (Eary, 1992),i.e., amorphous As-sulfide phases with a similar stoichiometry to orpi-ment, may be more common than precipitation of crystalline orpiment(Kilias et al., 2013; Dekov et al., 2013).

Amorphous As-sulfides have been reported for several active terres-trial geothermal systems, including Kamchatka (Migdisov and Bychkov,1998) andNewZealand (e.g.,Weissberg, 1969; Jones et al., 2001;Ullrichet al., 2013). Amorphous As-sulfide solid phases from the Uzon caldera,Kamchatka Peninsula Russia were investigated, using conventionallab X-ray techniques to identify relevant crystalline phases (realgar,

Fig. 1. (a) Location of Milos Island and other calc-alkaline volcanoes (shaded) along theAegean Island Arc (dotted line). (b) Milos Island and the location of Palaeochori Bay. Stip-pled offshore areas around the island are mapped gas emissions by echo sounding(Dando et al., 1995).

688 A. Godelitsas et al. / Marine Chemistry 177 (2015) 687–696

orpiment, alacranite, uzonite; Migdisov and Bychkov, 1998). Earlyinvestigations of precipitates from Champagne Pool in New Zealand,mentioned the existence of an orange to red-orange solid phase highlyenriched in As (Weissberg, 1969). Jones et al. (2001) and Ullrich et al.(2013) performed conventional XRD and SEM–EDS, but did not providea definite mineralogical characterization. There are reports of amor-phous As-sulfides co-occurring with crystalline As-sulfides (orpimentand realgar) at deep-sea hydrothermal fields (Dekov et al., 2013 andreferences therein), but the precise solid-state characterization of theAs solid phases, including the determination of the As oxidation state,was not demonstrated. In a recent study (Kilias et al., 2013), PXRD-amorphous orpiment-type As sulfides were reported for sulfide/sulfatechimneys located at approximately 550 m depth in the submarine cal-dera of the Kolumbo volcano (north of Santorini Island). Dekov et al.(2013) showed that orpiment in hydrothermal systemsmay precipitateboth inorganically or biogenically. Furthermore, amorphous to poorlycrystallineAs(III) sulfides, similar to orpimentmayprecipitate in aqueousmedia due to microbial reduction of arsenates (AsVO4

3−) (e.g., Newmanet al., 1997; Newman et al., 1998), and amorphous As2S3 filamentousnanotubes can be formed due to the presence of As(V) and S2O3

2−

under anaerobic conditions (Lee et al., 2007).Those investigations illustrate 1) the importance of amorphous

orpiment in hydrothermal systems, and for removing arsenic from sea-water, and 2) a lack of information about the precipitates themselves,particularly with respect to mineralogy, valence state, and structure.More research is needed to fully characterize these naturally occurringcrystalline and/or amorphous orpiment-type solid phases in hydrother-mal environments, especially if this characterization can help elucidatethe redox changes necessary for their precipitation. Furthermore,in-depth understanding of the nature of these precipitates will help im-prove thermodynamic modeling and prediction of their occurrence.

Off the coast of Milos Island, in the Hellenic Volcanic Arc (HVA, seee.g. Papanikolaou, 1993; Royden and Papanikolaou, 2011), dischargeof acidic (pH ≈ 5), hot (up to 115 °C), highly sulfidic (up to 3 mMH2S) hydrothermal fluids takes place, with the strongest dischargesoccurring in Paleochori Bay (e.g. Valsami-Jones et al., 2005; Bayraktarovet al., 2013) offshore in the southeast of the island (Fig. 1). Recently,elevated As were reported for the fluids, with concentrations reachingup to 78 μM (Price et al., 2013). This is approximately 3000-times thatof the local seawater and moreover the highest value reported for anymarine hydrothermal system at present. Generally, the hydrothermallyinfluenced marine sediments in Palaeochori Bay are characterized bythe occurrence of yellow-orange, white, or brown microbial mats andhydrothermal precipitates on the seafloor sediments (Fig. 2a). Thesecolored sediments are significant for the hydrothermal system, andthe different colors may be related to different sediment temperatures.Whitematsmake up the largest hydrothermal areas throughout the bayand display mid-range temperatures (e.g. ~45 to 85 °C). In the lowertemperature areas (~30 to 35 °C), brown colored manganese and/oriron oxide deposits are present, typically surrounding the whitepatches. The hottest areas (N85 °C), typically found in the middle ofthe white sediments, are often characterized by a bright yellowish toyellow-orange precipitate (Fig. 2). Field observations indicated thatthe unusual precipitates are probably formed bymixing of the overlyingseawater with hydrothermal fluids in the upper few centimeters of thesediment (Fig. 2). Previous research suggested these precipitates to beorpiment-like arsenic sulfide deposits (Price et al., 2013), but no fullcharacterization was carried out. Very recent works have also indicatedorganic Cu-complexation in the Milos marine sediments, as well as thepresence of various sulfides and sulfates (such as cinnabar, pyrite,As-pyrite, alunite, barite) without mentioning the occurrence of amor-phous As-sulfides (Kleint et al., 2015; Kati et al., 2015; Callac et al., inpress; Kotopoulou et al., 2015).

The purpose of the present work is to provide insight into the min-eralogy and geochemistry of the yellow and orange As-bearing marinesediments on Milos Island, by making use of new synchrotron-based

and micro-Raman measurements. A better understanding of their As-sulfide solid phases will aid to our understanding of their precipitationand ultimate fate. Furthermore, the entire HVA is characterized by sig-nificant As discharges primarily associated with hydrothermal ventingrelated to volcanic activity (e.g., Pichler et al., 1999b; Varnavas andCronan, 2005; McCarthy et al., 2005), and the precipitates off Milosseem to exhibit unique biogeochemical characteristics among theother marine hydrothermal sediments of the HVA. Thus, investigatingthe Milos precipitates may help understanding the anomalous As-richcharacter of the HVA.

2. Methods

Arsenic-bearing marine sediments of yellow to yellow-orange colorwere sampled at 4.5 m water depth, at the site corresponding to the“Rocky Point (RP)” site designated in Price et al., 2013 (Figs. 1 and 2).This site is located approximately 100 m offshore, nearby a large, sub-aerially exposed rock. Samples were collected into 50 ml falcon tubesby SCUBA divers and kept refrigerated until analysis. The sedimentswere initially examined using a Zeiss Axio Imager.Z1mmotorized opti-cal microscope in transmitted and reflected light, and a Zeiss stereomi-croscope connected to a PC.

X-ray diffraction patterns in powdered samples (PXRD) wereobtained using a Siemens D5005 (currently Bruker AXS) diffractometerwith CuKα radiation at an accelerating voltage of 40 kV. The identifica-tion of crystalline phases was obtained with data from ICDD and the

Fig. 2. (a) Photograph of the marine sediments investigated in this study. P-O (Palaeochori-orange), P-W (Palaeochori-white), and P-B (Palaeochori-background). (b) Close-up of P-O,indicating the areawhere the yellow-orange As-sulfide precipitateswere sampled. (c) Optical image of the yellow-orange As-sulfide compound covering quartz grains and phyllosilicates.(d) Core of the As-bearing sediment showing a typical precipitation in the upper few centimeters of sediments with advection of hydrothermal fluids.

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evaluation was performed with EVA software from Siemens (currentlyBruker AXS) for semi-quantitative analysis aswell as using theMATCH!version 1.9a software.

Bulk geochemical analyses for major and trace elements of theAs-bearing sediment were performed using a Perkin Elmer ICP-OESand a Perkin Elmer Sciex Elan 9000 ICP-MS following a LiBO2/LiB4O7 fu-sion and HNO3 digestion. In addition, a separate 0.5 g split was digestedin HNO3:HCl mixture (1:3) and analyzed by ICP-MS for precious andbase metals.

Scanning Electron Microscopy–Energy Dispersive Spectrometry(SEM–EDS) investigation of carbon-coated free surfaces of the yellow-orange precipitateswas performed using a Jeol JSM-5600 SEMequippedwith an Oxford EDS and a Camscan CS 44 SEM equipped with a Phillipsenergy dispersive system (EDS-PV 98) to obtain semi quantitative ele-mental analyses. Laser micro-Raman spectroscopy (Kaiser HoloLab5000)was performedusing 532-nmexcitation and 10mWpower at the samplesurface.

X-ray Absorption Fine Structure (XAFS) spectra were obtained fromthe X-ray beam line of the Laboratory for Environmental Studies(SUL-X) of ANKA synchrotron Radiation Facility (KIT, Germany) usingpowder of As-bearing sediment pressed to pellets with cellulose

(e.g., ANKA Instrumentation Book, 2012). Natural minerals of knownAs oxidation state (e.g., Helz et al., 1995; Foster et al., 1998; Savageet al., 2000; Takahashi et al., 2003; O'Day et al., 2004; Hattori et al.,2005; Lee et al., 2007; Burton et al., 2013; Handley et al., 2013),such as arsenopyrite (FeAsS), orpiment (As2S3), and arsenates(i.e., annabergite: Ni3(AsO4)2·8H2O and scorodite: FeAsO4·2H2O), aswell as synthetic As3+ compounds such as As2O3 (arsenolite) andNaAsO2 (sodium meta-arsenite), were used as reference materials.Spectra were measured at the As K-edge (11,867 eV). Energy wascalibrated for the As K-edge XAFS measurements to 11,919 eV (1stderivative of the Au L3-edge, Aumetal foil). The spectra were processedusing the ATHENA and the ARTEMIS software packages (Ravel andNewville, 2005). The EXAFS equation parameters (such as amplitudefactor) have been defined according to the theory (e.g., Rehr andAlbers, 2000 and references therein). For the overall EXAFS fitting pro-cedure crystal cell data from an orpiment reference from the literature(Mullen and Nowacki, 1972) were imported from its atoms.inp filefor the FEFF calculation. First shell parameters were used as crystallineanalogs for the EXAFS fitting of the studied sample. In particular, selec-tive single scattering (SS) paths of a known orpiment reference (Mullenand Nowacki, 1972) were used for the EXAFS fitting.

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3. Results and discussion

3.1. Optical and PXRD characterization

According to optical images taken, by using a stereomicroscope, theyellow-orange sediments found in Palaeochori Bay mainly consist ofsub-rounded quartz grains along with lesser amounts of phyllosilicateminerals, partially to completely covered by thin, yellow to orangeprecipitate (Fig. 2c). The bulk of the sediment therefore consists mostlyof silica/silica minerals, with lesser amounts of the orange precipitate.This observation is supported by the PXRD patterns, which indicatethe presence of abundant quartz together with phyllosilicates, namelychlorite- and illite-group minerals (see Supplementary Data; Fig. S1).No detectable PXRD-crystalline As sulfide phases, such as orpiment(As2S3), crystalline As4S4 (neither realgar/α-As4S4 nor alacranite andpararealgar), duranusite (As4S), uzonite (As4S5) and dimorphite(As4S3), were observed.

3.2. Bulk geochemistry

The chemical analyses of the studied As-bearing sediment showed, asexpected, abundant SiO2, due to quartz (84.64 wt.%) and phyllosilicates,as well as Al2O3, Fe2O3 and K2O (6.72, 1.77 and 1.36 wt.% respectively),due to phyllosilicates and Fe-sulfides/-oxides. Concerning trace ele-ments, As and Ba were the most abundant (1403 μg g−1 and981 μg g−1 respectively), followed by Cr (89 μg g−1), Zr (54 μg g−1), Sr(48 μg g−1), Rb (48 μg g−1), Ni (46 μg g−1), Tl (42 μg g−1), V(41 μg g−1) and Zn (27 μg g−1). The studiedmarine sediment containedthe highest concentrations of As among relevant sediments sampled inthe entire HVA (Fig. 3). For example, As concentrations in the westernpart of the HVA, and particularly in surface siliceous and carbonatesediments obtained from Methana Island, exhibit an average of20.45 μg g−1 and 6.67 μg g−1, respectively (Huebner et al., 2004). In

Fig. 3.Upper Continental Crust (UCC)-normalized (according to Rudnick and Gao, 2003) spiderical positive anomaly of the Milos Island (Paleochori Bay) As-bearing sediments relative to otMethana siliceous sediments (Huebner et al., 2004): red thick line & solid square; Average val& solid lozenges; Milos Island sediments (Cronan and Varnavas, 2001; Varnavas and Cronan, 2blue background (Palaeochori Bay), dark blue thick line & cross (Rivari); Yali Island sediments (Island sediments (Varnavas and Cronan, 1991; Varnavas and Cronan, 2005): gray thick line & cgreen thick line & open triangle (Nea Kameni islet), light green thick line & closed triangle (Pa“channel” located between Palaea and Nea Kameni islets).

the eastern part of the HVA significant As enrichment was found incoastal sediments from Yali Island, with an average of 356 μg g−1, andKephalos Bay of Kos Island, with an average of 48 μg g−1(Varnavas andCronan, 1991, 2005). In the central part of the HVA, previous studiesfor the submarine hydrothermalfields ofMilos Island showed an averageof 9 μg g−1 at Voudia Bay, 57 μg g−1 at Palaeochori and 108 μg g−1 atRivari (Cronan and Varnavas, 2001; Varnavas and Cronan, 2005). Mea-surements of the As concentration in metalliferous sediments fromSantorini caldera showed an average of 493 μg g−1 at Nea Kameni and425 μg g−1 at Palaea Kameni as well as 145 μg g−1 at the area of thechannel located between the two small islets (Varnavas and Cronan,1988). In the above case, freshly precipitated Fe-oxides have been con-sidered as scavengers of hydrothermal As into the sediments; therewas no reference for specific As sulfide compounds (Handley et al.,2009; Handley et al., 2013). Thus, the Milos sediment up to nowcontained the highest reported values for marine hydrothermal sedi-ments from the HVA (Fig. 3). It is also significant to emphasize the Wconcentration in the studied As-bearing sediment (5.8 μg g−1) and therelevant positive geochemical anomaly (see Supplementary Data;Fig. S2). The W/Th ratio of 1.65 is much higher than the value reportedfor the volcanic rocks in the middle of the HVA (e.g., 0.07 for SantoriniIsland, Kirchenbaur and Münker, 2011; Kirchenbaur et al., 2012). Thus,further research is needed to clarify the W supply in the eastern Milossediments.

3.3. SEM–EDS characterization

SEM images demonstrate that the yellow-orange precipitates fromMilos are primarily coating quartz sediment grains (Fig. 4 upper).While several types of precipitates were observed, the most frequentlydistinguishedmorphologywas colloidal. Moreover, As sulfides togetherwith framboidal Fe sulfides were observed, indicating a possible biolog-ical influence. For example, long, cell-like morphologies were found

diagram of trace elements for hydrothermal sediments showing an extreme As geochem-her sediments from the HVA (modified after Gamaletsos et al., 2013). Average values ofues of Methana carbonate sediments (Huebner et al., 2004): darker red-brown thick line005): blue thick line & plus symbol (Voudia Bay), light blue thick line & asterisk in lighterVarnavas and Cronan, 1991; Varnavas and Cronan, 2005): gray thick line & open circle; Koslosed circle; Average values of Santorini caldera sediments (Varnavas and Cronan, 1988):laea Kameni islet), yellow thick line & green triangles with yellow filling color (Santorini

Fig. 4. Representative SEM images of the As-bearing shallow-water marine sediment showing the As- and framboidal Fe-sulfides (middle) precipitated onto quartz grains (upper). Dem-onstration of potentially biogenic As-sulfide filaments and spirals with characteristic shape (lower).

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(Fig. 4middle). SEM imaging of sediments that were made up of partic-ularly hard crusts in the field reveal spiraling and ‘cinnamon bun’ typemorphology (Fig. 4 lower), also indicate a potential biological influence.However, it appears that the precipitates have no clear crystal habits,i.e., are amorphous, as also indicated by PXRD. In a very recent study(Kilias et al., 2013), PXRD-amorphous orpiment-type As sulfides werereported for sulfide/sulfate chimneys located at approximately 550 mdepth in the submarine caldera of the Kolumbo volcano (north of Santo-rini Island). However, in the above case the physicochemical conditionswere significantly different (Carey et al., 2013; Kilias et al., 2013), com-pared to the shallowmarine hydrothermal sediments of Milos Island. Incase of the Kolumbo's chimneys the As2S3 compounds formed straight,curved and branching filaments with ringed grooves in microscale(SEM–EDS), implying similar biogenic origin. SEM–EDS measurementsindicated that the precipitates were at least similar to orpiment inrespect of their As:S ratio. For example, orange hydrothermal precipi-tates revealed an average As/S ratio of 0.58 (range 0.51–0.63; n = 8).However, these measurements do not give a full characterization of

the amorphous As-sulfide precipitates. Therefore, thorough methodsbased on synchrotron X-rays are necessary.

3.4. Micro-Raman characterization and solid-state sulfur (S) speciation

To further determine the nature of these yellow-orange As-sulfideprecipitates, and give insight into S solid-state speciation, laser micro-Raman analyses were performed. Results concerning the orange partof the material indicate the presence of multiple phases; with the dom-inant phase represented by poorly crystalline and/or amorphous formof orpiment, accompanied by micro-amounts of more crystalline orpi-ment (Fig. 5). This interpretation is based on the observations that1) the narrowest, highest-intensity peak in the Raman spectrum is at351wave numbers, which is just slightly downshifted from the positionof the most intense peak at 355 wave numbers of a well crystallinesample of orpiment from Hunan, China, 2) the very broad peak thatdominates the spectrumof the orange precipitate appears to encompassthe 4main peaks of crystalline orpiment at 291, 309, 355, and 382wave

Fig. 5. Laser micro-Raman spectra of orange (upper image) and yellow (lower image)hydrothermal precipitates from Milos island, together with natural reference materials(crystalline orpiment from Hunan (China) and crystalline sulfur from Italy).

Fig. 6.Upper image: Normalized As K-edge XAFS spectra of the studied sample fromMilosIsland (As-bearing marine sediment: red-colored line) and the natural crystallineorpiment reference (orange-colored line); Lower image: Normalized As K-edge XANESspectrum of the studied sample from Milos Island in comparison with spectra of naturalminerals (i.e., As2S3 (orpiment); Ni3(AsO4)2·2H2O (annabergite): green-colored line;FeAsO4·2H2O (scorodite): olive-colored line; FeAsS (arsenopyrite): magenta-coloredline), as well as of selected synthetic As(III) compounds (i.e., As2O3 (arsenolite): blue-colored line; NaAsO2 (sodiummeta-arsenite): cyan-colored line) used as referencemate-rials. Vertical gray-colored lines indicate the energy positions of thewhite linemaxima forsample and references with As of different valences: sample and orpiment reference at11,869.4 eV (dotted line), As−1 at 11,868.4 eV from arsenopyrite, As(III) at 11,871.1 eVfrom As2O3 and NaAsO2, As(V) at 11,874.4 eV and 11. 874.6 eV from annabergite andscorodite spectra, respectively (all solid lines).

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numbers, and 3) the weak peak in the orange precipitate at 193 wavenumbers could be downshifted analog to the peak at 201wave numbersin orpiment. Peak broadening of two or more peaks in a spectrum ofpoorly crystalline material can produce the kind of broad spectral enve-lope shown in the spectrum of the orange precipitates (Pasteris, J.,unpublished data).

In contrast, yellow-colored sediment appears to be a mixture of thepoorly crystalline and/or amorphous orpiment and well crystalline ele-mental sulfur in the form of S8 (i.e., sulfur atoms in 8-fold rings), whichis themost common form of elemental sulfur (Pasteris et al., 2001). Theposition of peaks characteristic of this S8 solid aremarkedwith an “S” onthe spectrum of the orange precipitate in Fig. 5. The peaks labeled 232and 491 wave numbers in the orange precipitate could be slightlyupshifted analogs of the two strong S8 peaks seen in the Milos yellowprecipitates, perhaps due to some solid solution effects.

3.5. XANES/EXAFS characterization and solid-state arsenic (As) speciationand structure

A more complete characterization of the poorly crystalline and/oramorphous orpiment (detected in microscale by Raman spectroscopyin both yellow and orange parts of the As-bearing marine sediments)was attempted bymeans of bulk synchrotron-based spectroscopic tech-niques and particularly by As K-edge XANES/EXAFS measurements(Figs. 6 to 8).

With increasing As valence, the absorption edges of the referencematerials were relatively shifted towards higher energies (Foster et al.,1998; Savage et al., 2000; Takahashi et al., 2003; O'Day et al., 2004;Lee et al., 2007; Langner et al., 2011; Burton et al., 2013; Handleyet al., 2013; Kilias et al., 2013). The peak maximum of the white lines

for As−1 (i.e., arsenopyrite), and As(III) species (i.e., As2O3, NaAsO2),as well as of thewhite lines of As(V) species (i.e., annabergite, and scor-odite) appeared at 11,868.4, 11,871.1, 11,874.4 and 11,874.6 eV, respec-tively. In agreement to previous As K-edge XAFS investigations onvarious geological samples (Savage et al., 2000; Takahashi et al., 2003;Hattori et al., 2005) thewhite lines of all the sulfide referencematerials,and the As-bearing marine sediment, clearly appeared at lower energyvalues, than those of the non-sulfide references (arsenolite, sodiummeta-arsenite, annabergite, and scorodite). XANES spectral results forthe Milos yellow-orange sediments indicate that the As valence of thestudied material corresponds to the As valence of orpiment-like(As2S3) compounds (Fig. 6—lower image). The white line maximumof the As K-edge XANES spectra of the As-bearing marine sedimentand the natural orpiment (As2S3) have the same energy value of11,869.4 eV (Fig. 6). In either case, the comparison between the normal-izedAsK-edgeXANES spectrumof theAs-bearingmarine sediment, andthe spectra of the reference materials, revealed that the oxidation stateof As in the studied material ranged from −1 to +3, and that the Asoxidation state of the precipitateswas lower than that of abioticmineralarsenates. These are consistent with XANES results from a recent study

Fig. 7. The Fourier transform (FT) of χ(k) of the As K-edge EXAFS spectrum of the As-bearingmarine sediment (red-colored line) of Milos Island together with the FT of the orpiment ref-erence (orange-colored line), using the ATHENA software (a); the experimental FT of χ(R) of the As K-edge EXAFS signal of the studied As-bearingmarine sediment comparedwith the FTEXAFS signal of the As2S3 (orpiment) reference, using the ATHENA software package (b); the experimental FT plot in themagnitude part of χ(R) of the As K-edge EXAFS signal of the stud-ied As-bearing marine sediment (red-colored line) together with the literature (Mullen and Nowacki, 1972) orpiment's single scattering (SS) paths (i.e., SS paths of S atom at 2.2839 Å(orange-colored solid line), at 1.8488 Å (gray-colored dotted line), at 1.9779 Å (light gray-colored dotted line), and at 2.4561 Å (dark gray-colored dotted line) interatomic As–S distances,respectively), using the ARTEMIS software (c); the experimental FT plot in the real part of χ(R) of the As K-edge EXAFS signal of the studied As-bearingmarine sediment (red-colored line)together with the literature orpiment's SS paths (Mullen and Nowacki, 1972), using the ARTEMIS software package (d).

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on PXRD-amorphous orpiment-type As sulfides of Kolumbo's chimneys(Kilias et al., 2013).

The EXAFS results, revealing details on the nature of the studied As-sulfide precipitates, are presented in Figs. 7 and 8. The Fourier transform(FT) of χ(k) of the As K-edge EXAFS spectra of the As-bearing marinesediment and the natural orpiment (As2S3) are analogous (Fig. 7a), indi-cating significant similarities in the local structure of As-atoms in bothmaterials (see also Lee et al., 2007; Kilias et al., 2013). The experimentalFT of χ(R) of the As K-edge EXAFS signal of the studied As-sulfide, com-pared to the homologs signal of orpiment, indicates that the first shell(first neighbor) of the As central atom is sulfur (Fig. 7b). However, it isnotable that the FT of χ(R) of natural — crystalline— orpiment showeda second shell peak, reasonably corresponding to As, which was notclearly established in the studied Milos material due to its disordered/amorphous nature. Prior to EXAFS fitting, the experimental FT plottedin the magnitude part of χ(R) of the As K-edge EXAFS signal of thestudied sediment, compared to single scattering (SS) paths of a knownorpiment reference from the literature (Mullen and Nowacki, 1972),suggests that the scattering from the nearest neighbor S atom substan-tially contributed to the EXAFS in the FT in this region (Fig. 7c). Inparticular, the scattering from the nearest neighbor S atom at the dis-tance of 2.2839 Å (calculated from EXAFS spectra processing on thebasis of Mullen and Nowacki (1972)) seems to contribute to a greaterextent to the EXAFS in the FT in this region than to that of an S atomat 2.4561 Å, and much more than other S atoms at comparable neigh-boring distances (i.e., 1.8488 Å, 1.9779 Å). The interatomic distance tothe shell corresponding to SS paths is related to the position of thefirst peak of the As K-edge EXAFS signal of the studied material. This issupported by the experimental FT plot in the real part of χ(R) of the

As K-edge EXAFS signals (Fig. 7d), shows the complicated phase rela-tionship between the SS paths themselves and the first coordinationshell of the studied material. Moreover, as the first coordination shellis approximately the same between the studied material, the measurednatural orpiment (Fig. 7b), and the relevant crystalline phase as report-ed (Mullen and Nowacki, 1972; see Fig. 7c), and no other coordinationshell is present at higher distance (Fig. 7b and c) in the EXAFS signalof studied sample, it is evident that the As-bearing marine sedimentmost likely contains an amorphous-like component. All above men-tioned evidence support the findings on amorphous As2S3 solid, asHelz et al. (1995) have concluded from their EXAFS data, too. CrystallineAs2S3 has a layered sheet-like structure composed of two dimensionallypolymerized As2S3 pyramids, while realgar consists of large, ordered,oligomeric As4S4 species (Mullen and Nowacki, 1972). Therefore, withFT window set of [2:13.5] and the fitting range set of [1.3:2.3], theEXAFS fitting results additionally indicate that the studied As-bearingsediments contain an amorphous (As2S3) orpiment-like structure withAs ions coordinated by 3 sulfur atoms. Thus, the coordination numberis three (CN=3.0)whereas the further calculations yield anAs–S inter-atomic distance of the first shell at 2.279 Å (Table 1). The best-fit of thefirst peak contribution, reflecting the As–S covalent bond (e.g., Langneret al., 2011), is represented by the radial distribution function (RDF) ofthe first shell of the As-bearing marine sediment plotted in χ(k) space(Fig. 8a), and also in the magnitude (Fig. 8b), real (Fig. 8c), as well asin the imaginary part (Fig. 8d) of χ(R) space. The best-fit results areshown in Table 1 giving the coordination number (CN), interatomic dis-tance (R), and the Debye–Waller factor (σ2) of the obtained As-bearingmarine sediment. Currentfitting results are in excellent agreementwiththat of Helz et al. (1995), who investigated a structurally analogous

694 A. Godelitsas et al. / Marine Chemistry 177 (2015) 687–696

amorphous orpiment. Furthermore this is contrary to the highly crystal-line orpiment EXAFS results of Foster et al. (1998) study,which reportedall the coordination shells of As–S (2.28 Å), As–As (3.19 Å), As–S(3.45 Å), and As–As (3.57 Å) bonds, as well as of O'Day et al. (2004),who verified weak and visible scattering beyond the first shell of the Satom, respectively. It can be suggested that the calculated structure ofthe studied material is slightly S-deficient and is ordered only in the ap-peared first shell around As (Figs. 7b and 8), resembling orpiment-typestructure, whereas higher shells are not present as they will be disor-dered similar to previous work (Helz et al., 1995). In accordance to theconclusions of Helz et al. (1995), the disorder phenomenon in thestudied samplemay be strictly produced either by the existence of occa-sional As–S–As bridgeswith As–As bonds or by the occurrence of As–O–As bridges. This causes twisting of the AsS3 pyramids in the initial orpi-ment structure (Mullen and Nowacki, 1972) and subsequently the finaldistortion in higher coordination shells in the structural sheets of anamorphous orpiment.

4. Conclusions

Arsenic is ubiquitously enriched in hydrothermal fluids, creating anet source of arsenic to the oceans. However, arsenic concentrationsare buffered by adsorption onto hydrothermally precipitated ironoxyhydroxides and As-bearing minerals such as orpiment, amorphousorpiment, or realgar. While many investigations at present describethe occurrence of such minerals, no rigorous evaluation of the solid-state character of these precipitates has been attempted. Yellow toyellow-orange As-bearing hydrothermal sediments off the coast ofMilos Island located in the Hellenic Volcanic Arc (HVA), Greece, containamorphous orpiment-like As-sulfide (As2S3) precipitates. Arsenic en-richment in these sediments more or less reflects the arsenic enrich-ment in the entire HVA. Previous investigations indicated that thelikely source of As was through leaching of the greenschist facies meta-morphic basement rocks transpierced by an abundance of secondaryquartz and calcite veins rich in pyrite (Price et al., 2013). It is possiblethat these veins were deposited from As-rich magmatic fluids, similarto some epithermal Au deposits. However, anomalous W enrichmentas well as the W/Th ratios in the Milos sediments, indicate the possibil-ity of a magmatic source, which could also be contributing As to thehydrothermal fluids. The vast As enrichment in the hydrothermal sys-tems of the entire HVA, as compared to remarkable geogenic As releasein other areas of Greece, has been reviewed recently by Gamaletsos et al.(2013).

According to SEM–EDS the precipitates exhibit characteristic “bio-logical” morphologies including cell-like (~10 μm), spirals (~20 μm),and rounded “cinnamon bun” shapes (~20 μm). These morphologiesare similar to those found in the shallow-submarine arc-volcanoKolumbo (Kilias et al., 2013), and deep-sea precipitates (Dekov et al.,2013), although the role of biological activity for precipitation remainsuncertain.

Micro-Raman spectroscopy indicated, in addition to poorly crystal-line and amorphous orpiment, the presence of crystalline elemental sul-fur S8. Most yellow-colored sediments were dominated by crystallinesulfur, but also had abundant poorly crystalline orpiment, which wasthe dominating hydrothermal precipitate in the orange-colored sedi-ments. Downshifted and broadened peaks in the micro-Raman spectracompared to those in crystalline orpiment suggest that the granulesmay consist of a not only poorly crystalline, but perhaps also of a chem-ically impure form of orpiment. These impurities may contribute to theamorphous nature of the precipitates. Bulk As K-edge XANES/EXAFSspectra proved that the valence of As corresponds to orpiment-type

Fig. 8. The first shell, reflecting the As–S covalent bond, EXAFS fit (Radial DistributionFunction — RDF) of the As-bearing marine sediment plotted in χ(k) space (a), and alsoin the magnitude (b), real (c), as well as in the imaginary part (d) of χ(R) all together,using the ARTEMIS software package.

Table 1Structural parameters from EXAFS fit of the amorphous As sulfide occurring in theAs-bearing marine sediment of Palaeochori bay (Milos Island, Greece).

As-bearing marine sediment

CN 3.0R (Å) ±0.001 2.279σ2 (Å2) ±0.0012 0.00427

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(As2S3) compounds and that the material posses an amorphousorpiment-like structure with As ions coordinated by 3 sulfur atoms(CN= 3.0). The As–S interatomic distance of the first shell is calculatedat 2.279 Å and the data suggest that the structure is slightly S-deficientand is ordered only in the first shell while higher shells are not evidentandmust be disordered. This distortion in the higher coordination shellsof the structural sheets contributes to the formation of the poorly crys-talline and/or amorphous As-sulfide. Thus, by combining micro-Ramanwith XANES/EXAFS measurements, we are able to understand that theamorphous nature of the orpiment-like precipitates offMilosmay resultfrom not only chemical impurities, but also disorder in higher shellstructures. This conclusion is suggested to apply tomany of the existinghydrothermal arsenic sulfide precipitates, still to be characterized.

Acknowledgments

We gratefully acknowledge Prof. J. Pasteris (Washington University,St. Louis) for collaboration in the laser micro-Raman spectra and thesynchrotron Light Source ANKA (KIT, Karlsruhe) for provision of beamtime at the SUL-X beam line. Partial funding of this projectwas providedby the University of BremenMARUM Postdoc fellowship (to R.E. Price),MARUM Unforeseen Incentive Funding (to R.E. Price and S. Bühring),and National Science Foundation/USA (to J. Amend). Special thanks toA. Vichos and the Artemis Bungalows, Diving Center Milos, and theSirocco Restaurant for logistical support in Milos.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.marchem.2015.09.004.

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