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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.
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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
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band gap of metal sulfides seem quite acceptable
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probably due to the present use of approximateEn
electron energy loss spectroscopy.Chem. Phys. lll,
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467479.
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