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949 DUCANADA BUIIETIN DEL'ASSOCIATION MINERATOGIOUE rHE CANADIAN InINERALOOTST OFCANADA OFTHEMINERALOGICAT ASSOGIATION JOURNAL Volume33 October1995 Pafi 5 Thz Catndinn M incralagist Vol. 33, pp. 949-960(1995) SPECTROSCOPY S K. AND L.EDGEX.RAYABSORPTION OF METALSULFIDES AND SULFATES: APPLICATIONS IN MINERALOGY AND GEOCHEMISTRV DIEN LI, G. MCHAEL BANCROFI ANDMASOUD KASRAI Departmentof Chcmistry,The Universityof WestemOnnrio, Inndon, Ontario N6A 587 MICHAELE. FLEET* DepartuEntof Earth Sciences,The Universityof WesternOntario, Lonton, Ontario N6A 587 XNGHONG FENG am KIM TAN CanndianStn/:hroton RadiationFacility, SynchrotronRadi.ationCenter,Universityof Wisconsin, Stoughton,Wisconsin53589,U.SA. AssrnAcr S K- and Ledge XANES spectraof pyrrhotite Ger-S), pynte (FeSt, the tliospinels ca:rollite (CuCo2Sf and linnaeite (Co3S/, andMg, C4 Sr andBa sulfateshavebeenobtainedusing synchrotronradiation.The spectraofthese metal sulfidesare interpretedbasedon qualitativemolecularorbital (MO) - energy-bandmodels,andindicatethat the metal 3d crystal-fieldband below the conductionbandminimum has significant S 3s- and 3p-like densityof states@OS), becauseof strongparticipation ofthe metal 3d electronsin the bondingofthe metalswith sulfur. The S K- andl-edge spectraof sulfatesareinterpretedusing MO theory. The S K- and Z-edgesshi'ft to higher energyby about 10 eV from sulfides (S2) to sulfur (S9, sulfite (Sa) and sulfates (Stu), providing a powerful and independenttechniquefor the determinationof oxidation state of sulfur. For semiconductingmetal sulfides,the S K- andi-edges shift linearly to higher energyby 2-3 eY with increasein the energyband gap (Es), andcurvilinearly to lower energywith increasein reflectivity. s, oxidationstateof sulfrr, energyband-gap. Keyvords: X-ray absorptionspectroscopy,sulfides,sulfate. SovrMerns Nous avonsobtenu des spectresd'absorptionX (XANES) aux seuilsK et Z du soufrepour la pyrrhotite (Fer-S)" pynte @eS), les thiospinellescarmllite (CuCo2S/ et linilaelte (Co3S), ainsi que pour les sulfatesde Mg, Ca Sr et Ba, en utilisant une sourcede rayonnementsyncbrotron.Nous interpr6tonsle spectrede ces sulfures de m6taux i la lumidre de-modbles qualitatifsdel'dnergiedesorbitesmoldculaires.D'aprbscesspectres,la bandede lorbite 3d desatomesm6talliques,d'un niveau * To whom correspondence shouldbe addressed. 950 Tr{E cANADIANMrNERALocrsr d'6nergieinfdrieur d celui de la bandede conduction,contientune contributionimFortantedesdensit6sd'6tatssemblablesaux orbites3s et 3p du soufre,i caused'uneparticipationimportantedes6lectrons3d desmdtauxdansles liaisons de cesm6taux avecle soufre.Nous interpr6tonsaussiles specresd'absorptionaux seuilsK et L dessulfatesen utilisant le moddledesorbites mol6culaires.Irs absorptions^aux seuilsK etl, du soufresontd6cal6esversune6nergieplus 6lev6epar environ 10eV enpassant de sulfures(S2J au soufre(Sf, sulfite (Sa) et sulfate(Se). Cettetecbniqueconstitueraitdoncun outil puissanter ind6pendant dansla d6terminationde 1'6tatd'oxydationdu 56ufr9.fans ls casdessulfuresmdtalliquessemiconducteurs, 1esabsorptionsaux seuilsK et L du soufresontd6ca16s de fagonlin6airevers une dnergieplus 61ev6epar 2-3 eV avecune augmentationen 6nergie dans la s6parationdes bandes(Eu), et de fagon "curvilin6aire" vers une 6nergieplus faible avec une augmentationd4nsla 16flectivit6. (Iraduit par la R6daction) Mots'cl6s: spectroscopied'absorptiondesrayonsX, sulfures,sulfates,oxydationdu soufre,s6paration6nerg6tiquedesbandes. Ixrnooucnotrl Sarpms ANDE)pERnm.lrALME"rHoD The fust X-ray absorption specfium (XAS) was reportedby Kossel (1920).However,XAS was not as widely usedasX-ray diffraction QRD) in physicsand chemistry because of ditficulties in interpretation, limitations of exciting light sources,and the indirect 416 implicit nature of the structural information obtainable.In the qarly 1970s,bright and polarized synchrotronradiation, tunable over a wide range in photon energy, becameavailable for XAS measurements, and a short-range single-electron scattering theory for EXAFS was developed(Sayerset al. 7970, Stern 1974,Sayers1975).Sincethen,XAS has been applied in mineralogy and geochemistry,biology and material sciences,as well as physics and chemisny. XAS is element-specificand applicable to a wide selection of elements and density of states @OS) components.It canbe usedto studygases,liquids, and crysrallineand amorphoussolids; it providesi::formation on short-rangestructureand electronic structure, and it allows rapid acquisition of high-resolution spectra. XAS has been applied in mineralogy and geochemistryto determine the local geometrical arrangementsof transition elementsin silicates and oxides,electronicstructureof metal sulfrdes,structures of amorphousmaterials,nucleationand crystall2ation of mineralsin aqueoussolutions,and mechanismsof chemisorptionreactions at mineral-liquid interfaces. These and other applicationsof XAS in mineralogy and geochemisty have beenreviewed tn Calaset al. (1984, 1987),Brown er aI. (1988, 1989),Brown & Parks(1989)andBrown (1990). We have reportedhigh-resolutionS K- and L-edge X-ray absorption near-edge structure (XANES) of ZnS, CuFeS,andCu2FeSnSoQ,i et al. 1994c),Zn,Cd and Hg monosulfidesQ) et al. 1994a) and coppercontainingsulfides (Li et al. L994b),and investigated the unoccupiedS 3s-, 3p- and 3d-like statesin the conductionband of these sulfides. In this paper, we presentand interpret S K- andLedge XANES specfia of pyrrhotite and pyrite, carrollite and linnaeite, and Mg, Ca, Sr and Ba sulfates, and discuss various applicationsof thesespectra. A1l mineral samples were obtained from the Departmentof Earth Sciences,Univenity of Westem Ontario, and the Departmentof Mineralogy, Royal Ontario Museum. Some sulfates investigated were analytical-grade reagents. All mineral samples were shown by powder X-ray diffraction (PXRD) and electron-microprobeanalysis (EMPA) to be single-phase.The samples were finely ground in air to approximately 10 pm in size and spread uniformly on conductingcarbon tape supportedon a stainless-steelsample holder of about 15 mm in diameter for S K- and l-edge measurements.The S K-edge XANES spectra were collected with a Double-CrystalMonochromator@CM) at a chamber pressnreof about 10{ t-orrand at room temperature, with 0.2 eV intervalsand2 secondsfor eachdatapoint. The K-edgeXANES spectrawere calibratedusing the K-edgeof native sulfur at 2472.0 eY. The DCM used InSb (11l) monochromatorcrystalsand has an energy resolution of about 0.8 eV at 2460 eV; its detailed designandperformancearedescribedelsewhere(Yang et al. 1992). The S l-edge XANES spectrawere taken on the Grasshopper beamline (Bancroft 1992) at room temperatureand chamberpressureof about l0{ torr. The Grasshopper beamlineusesgrazingincidencewith a grating of 1800 grooves/mm and has an energy resolutionof about0.2 eY at 160 eV. The spectrawere calibratedby the first sharp peak at 162.7 eV in the Ledge specfrumof native sulfur. All S K- and Ledge spectra \ilere recorded by Total Electron Yield (TEY) and fluorescence modes as a function of photon energy using synchrotronradiation. The spectrarecordedin these two modes were found to be very similar', and only the TEY spectra are presentedhere. The DCM and Grasshopperbeamlines are affiliated with the CanadianSynchrofton Radiation Facility (CSRD at ttreAladdin storagering, University of Wisconsin.The storage ring operates at either 800 MeV with a currentof about 60-180 mA. or 1 GeV with a culrenr of 40-80 mA. S XANES SPESTRAOF SIJLFIDESAND SULFATES Rrsulrs Pynhotite Figure 1 showsthe S K- andl-edge XANES spectra of pyrrhotite, togetherwith the Fe K-edgespectrumof syntheticFe1_*S. The S l-edge spectrum\ras alignedto zero by subtractingthe S 2gp binding energy @E) of FeS at 161.2 eY, and the S K- and Z-edgespecfa are correlatedusing the S Ka, X-ray-emissionenergy at 2307.8 eV. The Fe K-edgespectrumof Fe1-rSwas digitized from Sugiura(1984), and alignedto zero by subtractingthe Fe ls BE calculatedas the sum of the Fe 2912 BE at 710.1 eV and Kcq X-ray emission energy at 6403.8 eV. The S K-edge spectrum of pyrrhotite is also similar to previous results (Sugiura et al. L974,Sugrura1981,Sugiura& Muramatsu1985, Kitamuraet al. 1988). Pyrrhotitehas a NiAs-type sffucture,in which each Fe atom is octahedrallycoordinatedby S, and eachS d1 t I v) .H p >'r li K-edge 951 atom is triangular-prismaticallycoordinatedby six Fe atoms. The six Fe2+3d electronsin pyrrhotite have high+pin {"--el configuration,sothat the majority spin tq" and e5 bands are filled, whereas15s minorit! s!"ln t!" Uaia is partly filled, and the minority spin eP UanOffcompteteiy enpty (Iossell 1977,Saklopouloi et al. I984).In the Fe K-edgespectrum,a weakpeaka, usually called a pre-edgefeature, is assignedto the transition of Fe ls elecfions to the unoccupied 3d orbitals (t$" and e!1. rnis transition is formally forbidden b} the qulntum selection rules, but local distortionofthe coordinationoctahedronaroundthe Fe atom mixes the Fe 3p and 3d orbitals, and makesthis Fe ls -+ 3d fransitionweakly allowed. Peaka in both S K- andZ-edgespectracolrespondsto peaka in the Fe K-edge spectrumwirhin 10.5 eV, and is assiguedto transitionsof S ls and2p electons to S 3p- and3s-like states,respectively,mixed into fhe unoccupiedFe 3d crystal-field bands in the band gap. Peak c in the S K-edge spectrumis assignedto transition of S ls electronsto S 3p-like states.In the Z-edge spectrum, peak b is due to the fransition of S 2p electronsto S 3s-like states,and peakss and dr are attributedto transitionsof S 2p electronsto t2sand e" bandssplit from empty S 3d statesQ'i et al. 1994c).Theseresults indicate that the Fe 3d crystal-field band in the band gap is hybridized with S 3p and 3r states,and the conduction-bandminimum is characterizedby Fe sandp-like stateswhoseDOS are also hybridizedwith S 3s-, 3p- and even 3d-1ke states.This is contrary to the recentcalculationsof electronicbandsof cubic and tetragonalFeS, which indicate that the Fe-S bond is mainly ionic with little covalent mixing (Welz & Rosenberg1987). l{ +t -o Pyrite anl marcasite H Figure 2 comparesthe S K- and l-edge XANES ofpyrite, and the Fe K-edgespectrumofFeS, spectra o i-edge digitized from Drlger et al. (L988). The S Z-edge +) spectrumis alignedta zeroby subtractingthe S 24p tr o BE at L62.4eV (Hyland& Bancroft1989),andthe S a K-edge spectrum is correlated with the L-edge spectrum by the S Kcrl X-ray emission energy at 2307.8 eY. The Fe K-edgespectrumis alignedto zero using the Fe ls BE calculatedby addingFe 24pBE at ff-edge 706.5eV andKcq X-ray emissionenergyat 6304.8eV. The partial DOS of S 3p and Fe 3d statesin the fint -10 0 10 20 emptybandof pyrite, digitized from Bullett (1982)and aligned by calibrating the Fermi level (Ep) to zero (eV) Energy energy, are included for comparison.The S K- and Ec. 1. S K- and Ledge XANES spectra of pyrrhotite, t-edge specta of native sulfirr are also shown as togetherwith Fe K-edgespectrum.The S K- and Ledge dashedcurvesfor comparison.Our resultsarein good BE spectraarealignedto zeroenergyby the S 1.rand2p37,2 agreementwith previouswork on pyrite (Sugiura1981, (161.2e9, respectively,wherethe S ls BE is calculated Sugiura& Muramatsu1985). from the sum of S 24pBE and S Kcrl X-ray emission In the pyrite structure,eachFe atom is octahedrally energyat 23M.8 eY. The Fe K-edge spectrumis aligned BE coordinated by S atoms with an Fe-S distance of to zero by Fe ls BE calculatedby addingthe Fe 2p371 of 710.1eV andKat X-ray emissionenergyof 6403.8eV. 2.26 L" and each S atom bonds to another S atom H 952 TIIE CANADIAN MINERALOGIST t, rn +) .H p >t f{ (g h +) K-edge S 3p DOS I I I I fr a' H o 'al Z-edge \a 1", dl so that peak a in the S Z-edgespectrumalso seemsbe attributableto the transition of S 2p electronsto S 3slike statesin this empty band.Peakb in the S l-edge spectrumis assignedto the transitionof S 2p electrons to the antibondingS 3s-like statesat the conductionbandminimum, andpeaksc and dt are assignedto the empty S 3d-lke e and t2 states,respectively(Li et al. L994c).Peaksa and b in the S l-edge spectrumare eachsplit by 1.2eV, owing to the spin-orbitinteraction of S 2p orbitals. These results show that the first unoccupiedstateof pyrite is the Fe2+crystal-fieldband stronglymixed witl S 3p- and 3s-like states,and there is a strong overlap betweenthe Fe p-like statesand S 3s- and 3p-like states'at the conduction-band minimum. The S K- andZ-edgespectraof marcasite,the other import4ntpolymorphof FeS2,arevery similar to those of pyrite, except for a sffi of about 0.2 eV to lower v) tt energy,which is alsoin agreementwith the elecftoniclf-edge structure calculation of pyrite- and marcasite-type sulfides@ullett 1982).The S K- andl-edges of pyrite andpyrrhotite:re very similarbecausetheybothhave the FeS$ clusterasthe basicstructuralunit. However, there are fwo significant differences.First, peak a in 0 10 20 the S K- and Z-edgespectraof pyrite shifls to higher Energy (eV) energy by about 1.5 eV. This mainly reflects the difference in Fe-S bond distances in pyrite and Flc. 2. S K- andZ-edgeXANESspectraof pyrite,together pyrrhotite. The MO-energy band calculation @ullett with Fe K-edgespectrum. The S K- andl-edge spectra 1982)indicatesthat the unoccupiedFe en sub-bandis are alignedto zero energyby the S ls and 2p32BE to of (162.4eY),wherethe S ls BE is calculated by the sum displaced higher energyby about 1.2 eV^because of the S 241288 andS Ko, X-ray emissionenergyat the differencein Fe-S bond distance:2.38 A in mono2307.8eV. The Fe K-edgespectrumis alignedto zero clinic pyrrhotite versus 2.26 A in pyrite, in good by Fe ls BE calculated fromthesumof Fe 2proBBat agreementwith our S K- and Ledge spectra.Second 706.5eVandFeKcr,X-rayemission energyat6403.8eV. peak a is significantly more intensein the S K-edge Thecalculated (Bullett1982)arealso spectrumof pyrite than in the spectrumof pyrrhotite; S3p andFe3dDOS includedfor comparison, by placingtheFennilevel(Ep) thuspeaka is much moreintensethanpeakc in pyrite, at zeroenergy.The S K- andl-edge spectraof native whereaspeak a is somewhatweaker than peak c in sulfur(dashcurves)arealsocompared to thecorrespond- pyrrhotite; also, peak a in the S ,L-edgespectum of ing spectra of pyrite. pyrite is moreintensethanin pyrrhotite. This indicates strongermixing of S 3s and 3p statesinto the Fe 3d band in pyrite, which is apparently related to the at 2.08A andthreeFe atoms.Pyrite is a semiconductor presenceof the Sl- ion, giving overlappingof the antibecausethe 3d crystal-field band lies in the band gap. bonding S po" band with the empty Fe e" (3d) band. The Fe2+3d electronshave low spin {n--e!configura- The similarity betweenthe S K- and Z-edfe spectraof rion, andthe filled fn andempfye! Uandsaie separated pyrite and native sulfur (Fig. 2) also supports tlis by about 0.7 eY, regardedas ttreindirect En of pyrite interpretationof the spectal featuresand emphasizes (Bullett 1982).On the otherhand,thereis no complete that the molecularS]- ion is a dominantfeaturein the distinctionbetweenthe antibondingSpo* bandandthe elecftonicstructureof pyrite. emptyFe eg(3d) band,eventhoughthe former orbitals conribute principally at the top of the first unoccupied Carrollite and linnaeite band (GoodenoWh1972,Tossellet al. L98L,Bullen 1982).By comparisonwith the calculaledDOS, peaka The S K- and Z-edgeXANES spectraof carrollite in the S K-edgespectrumis assignedto tle ftansitionof andlinnnsile axesimilar, andonly the carrollite spectra S ls electrons to the S 3p-like states in the first are shown in Figure 3, togetherwilh the Co and Cu unoccupiedFe enandS po* band,in agreementwith Fe K-eAgespectrafrom the literature.The S l- andK-edge K-edgespectrum.Although the calculatedDOS of the spectraarealignedto zeroby the2472BE at 162.3eY S .r- andd-like statesaxenot available,peaka in both S (Nakai er c/. 1978)and S ls BE calculatedby the suK- and l-edge specta are alignedwell within 0.4 eV, of. S 24p BE and S Kar X-ray emissionenergy at F H 953 S XANES SPECTRAOF STJLFIDESAND SI]LFATES the pre-edgeregion in the Cu and Co K-edgespecfra. Henceopeak a in the S K- and Z-edge spectraare assignedto fansitions of S Ls and2p elecftonsto the S 3p- and 3s-like states,respectively,mixed into the ou' or cA" bands(or both). The interpretationsofpeaks b, co d1 and d2 in the S K- and L-edge spectraare also similar to those for pyrite, pyrrhotite and the other metal sulfides (Li et al. L994a,b, c). The S K- and L-edgespecfraof carrollite arein good agreementwith the model of the qualitative bonding (Goodenough 1968, Vaughan et al. l97L) and MO calculations (Vaughan& Tossell1981)forthiospinels,butprovide new experimental information on the bonding and conduction-band structure of carrollite. First, the unoccupiedCo and Cu 3d crystal-field bandshave a large contribution of S 3p statesand somefeaturesof S 3s states,and the metal3d elecfronsare involved in the bonding of the metals with S atoms. Second,the conduction-bandminimum, characterizedby metal sp states(Goodenough1968, Vaughanet al. l97l), has high DOS of S 3s- and 3p-like states,also indicating strong mixing of the metal sp statesand S 3s- and 3pJike states. u +) H h h d h {J p h s o +) A |r o vt -lo Energy (eV) FIG.3. S K- and Ledge XANES spectraof carrollite, togetlerwitr Cu andCo K-edgespecm.TheS K- and l-edge spectraarc alignedto zero energyusing S 1,r andZhn BE (162.3eV: Nakaiet al. 1978),wherethe u) S ls BE is calculated fromthesumof theS 24pBE ar.d +r S Kcq X-ray emissionenergyat 2307,8eY. The Cu andCoK-edgespectra aredigitizedfrom CJtwnock et aL (1990),andre-aligned to ourspecha by about5 eV shifts >t mainedgesweretakenas F{ to higherenergy,because 1foe d F{ zeroenergyin &unack et al. (1990). +, H p F{ S ff-edge S .[-edgo AC E Al'' ii,i\i A'i Li'i!r. I I I I I I / | t\r I 14.2IIgO --1-' I Gypsum -f' /ivi', I I ---.4------ lr4tr I 2307.8eV, respectively.The Co andCUK-edgespectra , rllr r I o Anhydrite (1990) aredigitizedfrom Chamocket al. andrealigned +) L.l'i\^t _ J - with the S K- and l-edge specfiaby an approximate p4 5 eV shift towardhigherenergy,becausethe main edge t{o a / iI i\ was takenas zero energyin Chamocket al. (L99A). p Celestite Carrollite hasthe normal spinel structue, with lowspin Co3+occupyinghalf of the octahedralsites, and Cu} occupyingone-eighthofthe tetrahedralsites.The , l\r rr Cof at the octahedralsiteshasbonding (os) and antibonding(os') molecularorbitals forrnedby overlapof ',t70 Cor+ en, 4s and 4p orbitals wilh S 3s and 3p orbitals. 160 190 -12" "-)li'N:rz 1 8 0 probably Filled orbitals remain esssafially nonEnergy (eV) bonding.Tor Cu2*at the tetrahedralsites,the e orbitals are non-bonding,whereast2,4s and 4p orbitals form (dashcurves) and S l-edge (solid curves) o4 and 04* bands(Iossell & Vaughan1992).The pre- Ftc. 4. S K-edge XANES spectra of synthetic MgSOa'2H2O, gypsum, peak edge in the Cu andCo K-edgespectraof carrollite anhydrite, celestite and barite. The S K- and Ledge is assignedto Cu ls + 3d and Co ls -->3d transitions, spectraarecorrelatedby the S Kcr X-ray emissionenergy respectively.Peak a in both S K- and Z-edgespecta at ?307.8eV and the S ls BE for K-edgespectr4 and the arc well aligned,and also approximatelycorrespondto 2g12BE for Le.dgespechaH Ajil' l-i,'i\i\ 954 TIIE CANADIAN MINERALOGIST TABLE 1. Sample MgSO4 .2H2O Label S K- AND'-EDGEXANES Assigments SPECTRA OF SOMESI'LFATES K-edget (eD A ^ S ls - rz (3plike) 2481,5 E Gypsum A c S ls - rz (3p-like) 248t,5 E Anhydrite A c S lr - ,, (3p-liks) Z-edgs8 (eD Assignments 170.5 17t.6 t73.7 181.0 S !p- + a, (lx-like) S ?ra t ar (3r-like) Sb*'tz 0PAsifte) S2Pyz-tz 6P6s-like) S2p-s (36-19.1 170.3 171.3 172.9 180.2 S 27* + S ?ra S2P-t2 S2P-e t7t.2 t72.2 r79.2 180.2 S 2p - ar 13t1i1"; S2pa-6 0p8dike) Sbw-6 @p&like) Sbtn- e Qd'like) S2pto- s 66-1il..1 t71.1 172,0 r73.2 179.5 180.4 S 2p - ar 13t1i1.; S?e!2+6 (3p8r-fiks) S2pn-h (3pBslike) S 2p*- s Qd-1iY.1 S 2p'o+ 2 66-1iYu1 170.9 173.9 180.5 181.6 S 2P * a' 13t111", S 2p+4QpBs-\ke) S 2Pn- e (M-like) S 2p,"- s (37-1Y.1 LIZ.J Ltt-+ Celestite Barite A c S 1r - rz (3plike) 2481.3 A c S h- t, (3plike) E eThe enor in fte fugs ar (13-llke) ar (3slike) S,Bs'ltke) 3d-like) is estimatedar i0.1 eV. andtr-edgephotonenergymeasurements Mg, Ca, Sr and Ba sulfates Figure 4 comparesthe S K- (dash curves) and l-edge (solid curves) XANES spectra of MgSOa.2H2O,gypsum (CaSO,.2H2O), anhydrite (CaSO), celesrite(STSOJandbarite (BaSO), atigned by the S Kcrt X-ray emissionenergyat 2307.8eV and the S ls BE for K-edgespectraand2py2BE for l-edge spectra. The peak positions and assignmentsare srrmmalizedin Table 1. The assignmentsarebasedon comparison with the S K- and Z-edge spectra of Na2SOa(Dehmer 1972, Sutherland et al. 1,993), gaseousSFa@odeur& Hirchcock1987)and SO2CI2 (Hirchcocket al. L987),and Si K- and L-edgespectra of SiF4@riedrichet al. 1980)and cr-quartzQ-i et al. 1993).PeakA in the Ledge spectrais assignedto tle transitionof S 2p electronsto S 3s-like al states,andit is split by about 1.2 eV, owing to the spin-orbit interaction of S 2p orbitals.PeakA is absentin the K-edge spectra,becausethe S ls + 3s transitionis forbidden by the selectionrules. PeakC in the K-edgespecfa is attributed to the fransition of S ls electronsto the S 3pl3s-like t2 states.PeakE is assignedto the S 3dlike e and t2 state$,and is weak in the K-edgespecta becauseofthe forbiddetrS ls + 3dfansitions. We also observesome systernaticchangesin the S K- and Ledge spectra from MgSOa.2H2Oand gypsum to anhydrite, celestite and barite. For MgSOa.2HrOand g)4)sum,which contain structurally bound H2O, peak A in the Ledge spectrashifts to lower energy,the first peak of this doubletis weaker, and there is no spin-orbit splitting of peak E. For anhydriteand celestite,the spin-orbit splitting of peak E is resolved.For barite, peak A also shifts to lower energy, as for gypsum, but is not discemibly split, whereas peak E moves to higher energy and is apparentlysplit by the spin-orbit interaction of S 2p orbitals.PeakC in the K-edgespectratendsto shift to lower energyoand the post-edgefeaturesbecomemore complicated from MgSOa.2H2Oto barite, probably relatedto the greaterback-scatteringeffrciency of the heaviercationsbeyondthe fust shell. A second-order Ca L-edgedoublet is discernibleat about 174.6 and 175.4 eY in the Z-edge spectra of gypsum and anhydrite,andthe correspondingBa doubletappearsas the low-energy shoulder to peak E in the L-edge spectrumof barite. DlscussIoN Chemicalshifts of S K- and L-edgesand oxidation stateof suwr We havereporledS K- andt-edge XANES spectra of severalmetal sulf,idesand sulfate minerals in this study and previouspapers(Li et al. 1994u b, c). The fust peak in the S K- and Ledge XANES spectra correspondsto the first unoccupiedstate.The positions 955 S XANES SPFTTRAOF SIJLFIDESAND STJLFATES TABLE2. S K- AND Z-EDGES, ENERGY BAND GAP (EE) AND REFLECTMTY R %) OF SIJLFTDES, SULFOSALTSAND SULFATES Mineral Formula .K-edgex i-edge* (eV) (ev) Sphalerite WurEite Greenockite Metacinnabar Orpiment Realgar Cinnabar Proustite Stibnite Molybdenite Pyrrhotite Covellite Tetrahedrite Stannite Chalcocite Enugite Bornite Cubanite Chalcopyrite Linnaeite Carrollite ZnS ZnS 2473.4 v+73.2 cds 2472.3 Hgs 247t.1 Asrs, 2470.8 AsnSn 247t.1 HgS 247t.1 AgrAsS, 2471.1 sb2s3 2470.7 MoS, 2469.4 Fq-,s 2469.8 CUS 2471..1 Cu,rSboS,, 2470.9 CurFeSnSo 2470.8 curS 2470.1 CqAsSn 2n0.1 Cu5FeSo 2470.0 CuFerS, 2469.5 2469.6 CuFeS, Co.Sn 2469.8 CuCorSo 2469.7 163.6 r63.5 t62.2 162.0 161.6 161.8 161.8 162.0 t61.7 161.9 t6r.7 162.5 t62.1 162.3 162.2 TO.O Pyrite Marcasite FeS, FeS, 2471.3 2471.2 163.0 163.0 Sulfiu sE 2472.0 163.3 u77.4 168.6 NarSO, Badte Celstite Anhydrite Gypsum BaSO, 2481.1 SrSOo 2481.2 CaSO4 2481.3 CaSO4.2HrO2481.5 MgSOo.2HrO 2481.6 MnSO4 2481.2 Fdo" 2481.t CoSq 2481.5 Niso" 2481.5 NqSrO, 2479.3 2470.8 161.6 161.7 161.7 161.6 E, (eD 3.8 3.9 2.5 2.3 2.4 2.2 2.Q 2.0 r .t 1.0 1.0 2.0 l.E 1.5 1.1 1.0 1.0 0.7 0.7 0.9 0.9 R (7a) Symbols 16.4 16.3 18.7 25.2 23.r 19.4 28.1 29.3 38.3 M.0 38.9 19.3 33.0 29.0 30.2 26.0 24.4 40.2 38.8 44.1 41.9 Sp Wz Grn Mcin Op Rg Cin Pro Stb Mo Po Cv Tet Sta Cc Eng Bn Cub Ccp Lin Car 55.0 51.6 Py Mar 1,70.9 171.1 171.2 171.3 171.6 171.8 t7t.5 171.4 171.5 169.5 162.9 aTheerror in the lGedgeandl-edge photonenergymeasurements is estimatedat +0.1 eV, of the first peakof all thesematerials,togetherwith the band gap, En, and reflectivity of metal sulfides, are summarizedin Table 2. Figure 5 shows that the S K- and Ledges shift toward higher energy in the order from sulfides to sulfite and sulfates,and unambiguouslyquantitatively distinguishbetweentheseoxidation states.The energy sffi from 52- to SG speciesis about 10 eV for both S K- andt-edges (Fig. 6). The chemical shift in the X-ray absorptionedge dependson the initial and final statesinvolved in the transition. If a valence electron is removed from an atom, the screening of core electons by valence electrons is reduced, and the core energy-levels become more tightly bound. Therefore, the binding energyofthe inner-shellS 2p increaseswith increase in its oxidation state,asis evidentin the XPS chemical shift. On the other hand, comparing Sn+compounds and clusters,the more oxidized speciesof sulfur form a stonger bond with the same ligand, promoting greateroverlapof sulfur andligand orbitals.This gives more stable bonding orbitals, and less stable antibonding orbials. Consequently,the S K- and tr-edges are expected to shift toward higher energy with increasein the oxidation state of sulfur. The correlations in Figure 6 unambiguously determine the 956 TTIE CANADIAN MINERATOGIST Sulfates 172 q) a0 .I AR C) ts{ 164 0 160 2468 2472 2476 24AO S K-edge ("V) Flc. 5. Conelation of S K- and Ledges in sulfides (open circles and solid triangles), sulfur (solid square),sulfite (inverted triangle) and sulfates(solid circles). The data usedin this figure arelisted in Table 2. oxidation state and form of sulfir in any kind of substanceand any state of matler. For example, in sodiumthiosulfate(NazSzO:),it is commonlyaccepted that therearetwo different sulfur species,52- and Sft; the S K- andZ-edgeXANES spectraof this compound are plotted in Figure 6 as an empty and solid square, respectively. Cleady, one of the sulfur species in NqS2Q is betterdescribedas S5+,andthe otheroneis S-, similar to sulfur in pynte (Huffman et al. 199'1, Vairavamurthyet al. L993, 1994). The shifts of S K- and Ledges with oxidation state of sulfur arc very similar to those of the S 2p BE as measuredby )(PS. Figure 7 shows the correlations (solid circles)of S K- and.L-edgeswith the S ls and2p BE, respectively.The experimentalS K- and Z-edge dataare shownas solid circles,and are comparedwith the ideal correlationsfor S K- and Z-edge values set equal to the respectiveS 1s and 2p BE (open circles and straight lines). The S 2p BE were measuredby )(PS and cited from Hyland & Bancroft (1989) for 2444 o o) u0 H S t-edge H S K-edge 2480 So+ . 1 . 2476 2472 o) 3il',", 1 6 8 E q) v) 1 7 2- go- {7"u* 0) a0 rd g) I' . 164 . {5yr cn 160 -3-2-101234567 Oxidation state of sulfur Flc. 6. Variation ofS K-edge(solid dots,left-handscale)andS l-edge (opencircles,righthand scale)with oxidation stateof sulfur. The S K-edge(solid squares)and S Ledge (opensquares)of the two sulfr speciesin Na2S2O3 are alsoplotbd on this diagram, identifying oneof the speciesasS5+,andttre otheras S-. The S K- andLedges for SL are averagesof 22 metal sulfides, and the data for Sto are averagesof 10 sulfates (Iable l). 957 S XANES SPECTRAOF ST]LFIDESAND SULFATES 24AO o e 2476 o a0 o d z+zz a 2468 tz24BA 2470 2472 2474 2+76 247A 24AO 24€2 S 1s Bindrng Energy (eV) BE, shift toward higher energywith oxidation stateof sulfur. However, the sffis of S K- and l-edges are larger, becausethey dependon fhe sffis of the unoccupiedfinal states,as well as the BE of the inner shells.It is very difficult to quantitativelyevaluatethe contributionsofthe initial and final statesto the shifts of S K- and Z-edges.However,as shownin Figure 7, the BE shifts of S ls and 2p munly contributeto the S K- and l-edge shifts, but the shifls of the final states also make someconftibution.As an example,the S 2p BE of ZnS and CuFeS2,both of which containSF, are very similar, but the S K- and tr-edgesof ZnS shift to much higher energythan thoseof CuFeS2.Therefore, the shifts of the final unoccupiedstires mwt make a significant contributionto the edgepositions. Chcmicalshirtsof S K- anl L-edgesversus the Eu and reflectivity of metal suffides -9,16E 0) $c) roo Figure 8 showsa positive linear correlationbetween the S K- and.L-edgesof metal sulfides;evenfor metal v, sulfides containing only SL, the S K- and ,L-edges also shift significantly. Although small differences probably exist owing to different geometical and 't I 00 168 70 electronicsffuctures,the inner Is and2p shellsmustbe XPSS Zprr, lfudlng Energy (eV) very similar for different sulfides. Clearly, the Ftc. 7. Conelations between S K-edgeandS ls BE (a) and chemicalshifls in the S K- andtr-edgesdependmainly S l-edgeandXPSS 2g72BE@)of CuFeS2, ZnS,FeS2, on the final states,that is, the first unoccupiedstates. The first unoccupied states arc the metal 3d band nativesulfur,FeSO3 andFeSOo. mixed with S 3s- and 3p-like statesin the bandgapfor fransition metal (Fe, Co, Ni, Cu and Mo) sulfides, ZnS, FeS2 and native sulfur, and from Flggl et aI. and are antibonding S 3s- and 3p-like statesat.the (1993)and Richardson& Vaughan(1989)for FeSO, conduction-bandminimum for Zn, Cd, Hg, As and Sb and FeSOa.The S K- andLedge valuesof FeSO, are sulfideswith firlly occupiedd orbitals.Therefore,both assumedto be equal to thoseof NaSO3.The S ls BE S K- and Ledges are at a lower energyfor the forrner was calculatedby addingthe S 2p BE and the S Kcrl (empty circles), and move to higher energy for the X-ray emissionenergyat 2307.8 eY.In general,the S latter (filled circles). The chemical shifts in the K-edgeand S 1.rBE, as well as the S L-edgeandS 2p S K- and Ledges are relatedto the crystal chemisty, ..o 164 164 163 a0 d I 162 *.1 (n tol 2469 2471 S K-edge 2472 (eV) 2473 2474 Frc. 8. Correlation of the S K- and.L-edgepositions of metal sulfides. Abbreviations of mineral names arekevedto Table 2. 958 TIIE CANADIAN MINERALOGIST OJ bt 50 Rg (o u0^ Op a a C" >. u0 x40 Pro stb ( () Ir Xb BaCo/.4 tr1 h8 0! 2469 2470 Oin uarrt py 2471 S K-edge 2472 (eV) 2473 2474 2469 2470 2471 2472 S l(-edge (eV) 2+73 2474 FIc. 10. Correlation of reflectivity with S K-edge for metal sulfides.The reflectivity values axefrom Anthony er al, (1990), with abbreviationsof mineral names keyed to Table2. (g a0^ Rg >' q) Q1 ilg c" c6p q n tol 162 163 164 S Z-edge (eV) Ftc. 9. Correlationof S K-edge(a) andS Ledge (b) witfr energybandgap(En)ofmetalsulfides.TheE* valuesare from Shuey(1975),andabbreviations of miri'eralnames arekeyedto Table2, semiconducting metal sulfides. Figure 10 shows the correlation of the S K-edge position with the reflectivify R (7o) of these metal sulndes, where the reflectivity values are cited from Anthony et al. (1990). Pyrite and marcasite depart from the correlations of Figures 9 and 10, presumably because they contain Slspecies, rather than simple 52- species. ACKNOWLEDGEMM.{TS We thank F.C. Hawthorne and an unnamed referee for helpful comments, D. Dillon, F.J. Wicks, T. Ottoway and R. Ramik for provision of mineral samples. This study was supported by NSERC. We 61s6thank the staff at the Synchrotron Radiation Center (SRC), the University of Wisconsin for their technical assistance, and the National Science Foundation (NSF) for the support of the SRC. electronicstucture, En - and reflectivity of thesesemiconductingmaterials. Figure 9 shows the correlation of the S K- and L-edgeswith the E* of metalsulfides,wherethe Endata (1975)andwereestimated-from aremainlyfrom Sh-uey optical spectra.Fifft, it is apparentthat for transitionRm'nnwcps metal sulfides that have small electrical resistance (Vaughan& Craig 1978),both S K- andl-edges lie at ANTHoNY,J.W., BrDEArrx,R.A., Buon, K.W. & Nrcuor,s, M.C. (1990): Handbook of Mineralogl. I. Elements, lower energy,and the bandgap is smaller.For Zn, Cd, Suffidesand Sulfosalts.Mineral Data Publishing,Tucson, Hg, As and Sb sulfides, which usually have higher Arizona. electricalresistrnce(Vaughan& Craig 1978),the band gap is larger, and both S K- and Z-edgesare also at a higher energy.Second,for all sulfidesstudied,both S BeNcnor"r,G.M. (1992): New developmentsin far UV, soft X-ray research at the Canadian syncbrotron radiation K- and l-edges are linearly correlated with En. In facility. Can.Chem.News44(6),1,5-22. general,the correlationsof S K- and Z-edgeswith the band gap of metal sulfides seem quite acceptable becausesomeof the small deviationsfrom linearity are BoDEuR,S. & [Irrcscoc& A.P. (1987): Inner- and valenceshell excitation of SFo studied by photoabsorptionand probably due to the present use of approximateEn electron energy loss spectroscopy.Chem. 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