,1.157
BUILETII'IDEL'ASSOGIATION
DUCANADA
MINERATOGIOUE
rHE GANADIAN
TWNETIATOGlsil
JOURNAT
OFTHEMINERATOGIGAT
ASSOCIATION
OFCANADA
Volume33
December1995
P^rt6
The Carndian Minerab gist
Vol. 33,pp. I 157-1166(l 995)
ELECTRON.LOSS
NEAR.EDGE
STRUCTURE
GLNES)AS A PROBEOFVALENCE
AND COORDINATION
NUMBER
LATIRENCEA.J.
GARVIE
Depar*rcnt of Geology,Arizorn State University,Tempe,Artzorn 85287-14M, U.SA..
PETERR. BUSECK
DepartuEntsof Geolagyand Chzmistry,Arizona Snte University,Tempe,Arizona 85287-14M, U.SA.
AI-ANJ. CRAVEN
Deparxmentof PltysicsandAstronomy,Universityof Glnsgow,GlasgowG12 8QO U,K.
ABSTRACT
We used parallel electron energyJossspectroscopy(PEELS) in a scanningtransmissionelectron microscope(STEM)
equippedwith a cold field-emissiongun to acquirehigh-resolutionspectra-Detailedanalysisofelectron-lossnear-edgestructure
(EINF.S) of core-lossedgesprovide.schemicalinformation aboutthe oxidation stateand site symmetryof the excited aroms.
In many cases,the ELNBS exhibits a shapethat is dependentupon the nearest-neighborcoordination and representsa
'frngerprint" of this coordination.
Gaudefroyite,Ca4I\,tn3@OtlCO3XO,OfD3,containsseverallight elementsan4 as such,is
ideal for a demonstrationof the fingerprint technique.We studiedthe core-lossedgesfrom gaudefroyitein the 0 to 1000eV
energy-lossrange,and illustate the uniquenessof the specfiaby comparisonwith thosefrom materialspossessinga rangeof
structuralandelectronicproperties.The B K-edgefrom gaudefroyiteis consistentwith 3-fold-coordinatedB and distinct from
the edgefor zt-fold-coordinated
B. The C K-edgeis distinct from that in elementalC, but consistentwith that atnibutedto the
carbonateanion. The Mn lrr-edge exhibits ELNES indicative of octahedratlycoordinatedMfi; it is distinct from the edge
shapesfor the other Mn oxitlation stafes:The O K-edgeis rich in structurearising from O in the BOl, CO3-,and Mn}Ofigroups,with little contributionfrom the predominantlyionic Ca-O bonds.
Keyword*: parallel elecfronenergy-loqsspechoscopy,PEELS,electron-lossnear-edgestructure,ELNES, coordinationfingerprinq valencefingerprint, gaudefroyite,,
Sovna.mr
Nous noussommesservisde la spectroscopiede la perte d'6nergiedes6lecuons(PEEI,S)en paralldleavecun microscope
6lectroniquei ftansmissionavec balayage,muni d'un canond'6missionI froid pour obtenir des spectresde hauterdsolution.
Une analysedetatll6e,de la structue du spectrede perte d'6lecfons prOsdu seuil d'absorption(ELNES) du noyau donnede
I'information i propos de l'6tar d'oxydationet la sym6triedes sitesdes atomesexcit6s.Dans plusieurscas,le spectreELNES
I 158
TIIE CANADIAN
MINERALOGIST
possddeune fotme qui d6pendde la coordinencedes atomesvoisins et qui fournit donc une "empreintedigitale" de cese
coordinence.La gaudefroyite, CaaMn3@O)3(CO3)(O,OIT)3,
contient plusieurs6l6ments16gers,et sert d'exemple6loquent
de la puissancede la technique.Nous avons6tudi6les seuilsde perte d'6nergiedu noyaude la gaudefroyrteda"s I'intervalle de
0 n 1000 eV, et nous illustrons la sp6cificit6 des spectresen les comparanti ceux de mat6riauxposs6dantune gammede
propri6t6sstructuraleset 6lectroniques.l,e seuil K de I'atomeB dansla gaudefroyrteconcordeavec le cas du bore i coordinence3, et diffdre de celui du bore Acoordinence4. l,e seuil K de l'atomeC sedistinguede celui du carbone6l6mentaire,mais
il concordeaveccelui qui caract6riseles groupesanioniquescarbonat6s.
possbdeun specueELNES
Le seuil!3 du manganbse
typique du Mn3t en coordinenceocta6drique.Ce spectredifldre de ceux des autrei valencesde Mn. k seuil K de I'oxygbne
possddeune structureriche en detailsi causedescontributionsde I'oxygbnedesgroupesBOh CO3-,et Mn3+O3-,mais sans
contributionappr6ciabledesliaisonsCa-O, b caractdreplut6t ionique.
Mots-cl6s:spectroscopieen parallblede la perted'dnergiedesdlectrons,PEEIS, structurede la perted'6lectronsprbsd'un seuil,
ELNES, "empreintedigitale" de la coordinence,"empreintedigitale" de la valence,gaudefroyite.
(EXELFS). Energy-lossfeatures at higher energies,
such as the K-edges of Ca (-4000 eV) and Mn
Elecron energy-loss spectroscopy (EELS) (-6535 eV), me not amenableto EELS study in a
conductedwith a hansmission electron microscooe TEM, but can be recorded by X-ray absorption
(TEM) can be usedfor quantitativechemicalanaly;is spectroscopy(XAS) (e.g.,Manceauet al. 1992).Aose
at the nanometerscale;it hasthe addedadvanrrgeover to the edgethreshold,the ELNESrepresentstransitions
many cornmonanalytical techniquesof being able to to unoccupied states modified by the immediate
record infomation about the light elements.Analysis environmentof the atom. In many cases,the ELNES
of the fine structure of EELS spectra offers the exhibits a shape reflecting the nearest-neighbor
possibility of determiningimportant crystal-chemical coordination.Sucha shapeis referredto asa coordinapropertiessuchasthe oxidationstate,coordination,and tion "fingerprint"; similarly, for cerlain elements,a
site symmetry.As an exampleof tle power of EELS in ' fingerprint of the oxidation stateis observed.At highmineralogy, we present specfa for gaudefroyite, a energyresolution,certain elementssuch as Fe and Ti
complex boro-carbonate,and comparethem to those exhibit edge featuresindicative of both coordination
for a range of minerals and synthetic materials of and oxidation state.
known structure.
The recent commercial availability of parallel
Gaudefroyitecontains3-fold coordinatedB and C, recording systernsfor EELS (PEELS) conductedin
7- and 9-fo1dcoordinatedCa, and 6-fold coordinated conjunction with a TEM has resulted in an increase
Mn3+.Its idealizedformulais CaoMnr@O:)t(CO31Or,in EElS-related studies. The use of PEELS, as
althoughthe Mn sitesarenot firlly occupied,but range opposedto serial EELS, results in a large gain in
from Mn .urto Mnr.3,.Substitutionof OH- for O com- collection efficiency. ELNES data can now be
pensatesfor the Mn deficiency @eukeset al. 1993). recorded rapidly and from materials that were too
Gaudefroyitehas been the subject of several studies beam-sensitiveto be collected with serial EELS at
(Jouravsky & Perrningeat 1964, Yakuboich et al. comparablespatialresolutions.
1975,Beukeset al. 1993,Garvie,Groy andBuseck,in
In this study,we demonstratehow interpretationof
prep.).It occursin the hydrothermalMn depositsnear the ELNES associatedwith a core-lossedge is used
Tachgagalt in Morocco (Jouravsky & Permingeat to obtain chemical and structural properties ftom
1964) and in the Kalahari mangane$efield in South minerals. We use the ELNES to infer the nearestAfrica. The Kalahari gaudefroyiteis a late secondary neighbor coordination of the atoms undergoing
phaseformed at high temperaturesand low pressures excitationand,whereappropriate,their oxidationstate.
during B metasomatism(Beukeset al. 1993).
Intensity maxima called core-lossedgesare caused
or Elsc"rnorlr-Loss
INrnnpnsrATroN
by transitionsof core electronsto unoccupiedstatesin
Nnen-Ercs Srnucnnrs
the conduction band. These edges sit on a monotonically decreasingbackground; the initial rise in
Core-lossedgesrecordedat low scatteringangles
intensity is called the edge tbreshold.The edgescan obeythe dipole selectionrules, which statethat during
take a variety of shapes(e.9., Colliex et al. 1985, electronexcitations,the changein angularmomentum,
Egerton1986),but within the first -30 ro 50 eV of the /, obeysA/ = tl, whereasthe spin remainsunchanged,
edge threshold, there commonly are a number of As = 0. At high scatteringangles (large momenfrrm
pronounced
peaks.This near-edge
regionof a core-loss transfers),nondipoletransitionscan contributesignifiedgeis defined as tle electron energy-lossnear-edge cantly to an EELS spectrum (Auerhammer & Rez
structure(ELNES). Extending to severalhundredeV 1989).Core-lossK-edgesarecausedby the excitation
above the ELNES is a region dominated by the of core ls electrons(l = 0) to unoccupiedstatesof
extended electron energy-loss fine structure p-like symmetry(/ = 1). The corewavefunctionsof an
hrm.ooucloN
EI.NES AS A PROBEOF VALENCE AND COORDINATIONNUMBM
atomin a-atom.
solid showlittle differencein energyfrom an
isolated
On the other hand, the energiesand
character of the outer bonding and antibonding
orbitals,which forrn bandsof orbitals,aregovernedby
the bonding and oxidation state of the atoms. Thus,
sincethe core-lossedgesrepresentfransitionsto these
unoccupiedstates,we expect to see modifications to
the ELNES that reflect the bonding.Before excitation,
the initial stateis well defined,but in the final state,a
core hole is present. The core hole is expectedto
modi$ the final stale,but the extrentto which this is
done dependson, among other factors, the atomic
number and character of the atom @rydson 1991,
Tamuraet al. 1995).
The r"j-edges result from the excitationof2p core
sta0es(l = 1) to unoccupieds (/ = 0) andd (l = 2) states.
T\e 4s ELNES from the 3d nansition metals (TM),
including K and Ca consist of two intense peals
arising from dipole-allowedtansitions from an initial
2p63dostateto a final 2ps34n+tshte. K+ and Ca2+have
empty d-orbitalsin their initial states,so ttre excitation
is of the form 2p63d0-+ 2p53d1.The 4- and,Ir-edges
are causedby the two ways in which the spinor, can
couple with the orbital angularmomentum,l, to give
j. The ,Q-peakis caused
the total angularmomentumo
by transitions from the 2p3p level, aud the -1r.-dge,
from the 2pyplevel. Transitionsto the dipole-allowed
s-like statesonly have a small effect on the !.yedges
and are manifested as weak structures at higher
energiesabovethe /-4 peals (Abbateet al. 1992).In
the presenceof sunounding ligands, the 3d TM /r.,
ELNES will be modified by the crystal field from the
ligands.
Mermrus ANDMgrrroDs
We studieda 5-mm crystalof gaudefroyitefrom the
Kalahaxilocality. It is dark brown with a stubbyhabit
and hexagonalshape;its identity was confirmed by
powder X-ray diffraction. Mcroprobe and secondaryion massspectroscopyanalysisfailed to show signsof
2ening or inclusions. The sample was preparedfor
PEELS analysis by crushing a mm-sized piece in
acetoneand drying a drop of the finely groundmineral
in suspensionon a lacy C film supportedon a coppsr
TEM edd.
The remainingsamplesClable 1) usedin this study
come from a variety of sources as either wellcharacterizedsingle crystals or pure inorganic solids.
The metallic Mn, Ca, andCaOwerepreparedin situtn
the elecfronmicroscopeby elecfron-beamreductionof
calcite or metallic Mn containing a few wtTo of O.
After electron-beamirradiation of a -3-[m piece of
Mn grew
Mn, fingersof thin electron-beam-transparent
from the sides of the larger particle, free of the O
presentin the parentmaterial.The rest of the samples
studied, except for TiC, p-rhombohedral boron
G-Bros),and glassy8201 oreminerals.
Specfia were acquired with a Gatan 666 PEELS
specfrometerattachedto the top of a VG IIB5 scanning
fransmissionelecfron microscope(STEM). The tIB5
was equippedwitl a cold field-emission gun (FEG)
and operatedat 100 kV with an emissioncurrent of
5 pA, a probesemiangleof -11 mra{ anda collection
angleof 12.5mrad.Undertheseconditions,the resolution of the STEM PEELS systemwas -0.4 eV. The
spectrometerwas calibrated against the Ni Q-edge
from NiO, which was determinedto be at 853.2eV by
XAS (van derLaan et ql, 1986),
To reduceC contaminationin the STEM, the TEM
grid supportingthe finely crushedmaterialwas placed
on a 100Watt Ughtbulb for -10 minutesjust prior to
insertion in the STEM, A thermocoupleplacedon the
bulb gave a reading of -170'C. Failure to place
the sample on the bulb invariably resulted in the
materialduring the recording
buildup of cmbonaceous
of PEELS data which was a problem since the C Kedge from the contamination interfered witl the
TABI,E 1.MINERAIS AND SYNTIIETIC COMPOTJNDSSTUDIED BY PEEI.S
Ideal fomula
asbolane
elcile
delits
dimd
flufiiE
grudeftoyib
graphite
iudwigile
nmganiE
piemllilc
rlodcbrGite
rbodizite
rebllic@lcim
retalic lmgenese
qlcim oxide
litgnim6bide
$rhombohedml borcn
glasy boric oxid,c
(coNi) r-v(Mnozh-x(OII)z-2"+a.r,Hzo
CaCq
CaBSiOaOH
c
ca&
caatvtnl,@o3)f oj)(o,oII)3
c
Mg2FC+B05
tlvln@H
CazlvlnAluO(siOqXsizQXoID)
MnC03
(Cs,K)BeaAla(BBe)12qs
Ca
Iv1o
CaO
Ti(b.gs
FBros
8203
8 s tdt for prepardti@d€tails,
1159
Ielitislsorre
GebeIslad lndonci,a
I lrnelweddQuarry,Wal6
iel4 Sor.trhAftiQ
Kalahd tmgm*
SoulmA&ic!
uDknom
feld, SouthAfiie
Kslahd nmgrc
southw6tem GraphiE Miqe, Teru
Skye,Scotlud
Blafrktek Mr€, fuiam
Sr Ma@!, Pimonl Italy
mknom
Anninbe,Madagm
geged il.rlrr*
preged !r siae
preparcdiz nird
m
coml]mial
@l]]miel
I 160
TIIE CANADIAN MINERALOGIST
C K-edgefrom the materialunderinvestigation.
Core-loss edges were obtained from thin areas,
typically -50 nm thick, overhnngingholesin the lacy
C film. The B and C K-edges were recorded with
in0egrationtimes of -1 ,s.The ls6aining edgeswere
recordedwith integration times of 4 to 8 s. Spectra
were acquiredby scanninga focusedprobe, diameter
I nm, in a TV-rate raster over regions measuring
15 x l0 nm2.In principle, the spatialresolutionin the
lIB5 STEM is governedby the sizeof the probe,which
is typically 1 nm in diameter; probe sizes down to
2.2 A have been used to probe individual atomic
columns@rowning et al. 1993).In practice,the spatial
resolutionis dictatedby the resistanceof the materials
being studied to electron-beamdamage.An energy
dispersion of 0.1 eV was used, allowing the fine
sfucture of the edgesto be recorded.The dark current
and a backgroundof the form AR{ @gerton 1986)
were subtactedfrom beneatheachcore-lossedge,and
the effect of the asymmetryof the zero-losspeak was
deconvolutedfollowing the proceduresof Egerton.
EmcrnoN Loss Nnan-Eocs Srnucnrnss
The core-lossedgesfrom gaudefroyiteare shownin
Figure 1, and the principal energieslisted in Table 2.
Subsequentfigures comparethe individual core-loss
edgesfrom gaudefroyitewith edgesfrom compounds
0204060
that are usedto explain the gaudefroyiteELNES. The
RelativeEnergyLoss (eV)
individual edgesare describedin order of increasing
FIc.
1.
Core-loss
edgesfromgaudefroyite.
Thelettersreferto
energy losses, B and C K-edges, Ca Lr.r-edge,
prominent
features
described
in thetextandTable2. The
O K-edge,andthe Mn!.r-edge.
spectra
havebeenalignedrelativeto thefirst peaka.
Boron
In minerals,B, which is bondedalmostexclusivelv
to O or OH-, occurs in the tenahedratBOf, and the
planar riangular BO]- groups.The O atoms may be
partialy or totally replacedby OH-. The spectrafrom
gaudefroyite, ludwigite, and glassy BrO, exhibit
similar ELNES, dominatedby a prominent peak at
-193.6 eV and a broaderasymmetricalpeak centered
around 204 eV (Fig. 2). Boron in rhodizite is tetrahedrally coordinated,andits B K-edgediffers from the
t31BK ELNES. The differencesbetweenthe t3lB and
t4lB K ELNES can be usedto determinethe t31B/t4lB
ratio in minerals containing B, in both coordinations
(Saueret al. L993, Garvie 1995, Garvie et al. 1995).
Both ludwigite and glassyBrO, contain t3lB,and their
B K-edgesaresimilar to tlat of gaudefroyile.The good
match betweenthesematerialsallows us to conclude
that the B in gaudefroyiteis 3-fold-coordinated.
An understandingof core-lossedgefeaturescan be
undertakenusing a number of theoreticaltechniques
@rydson1991,de Groot 1993,R:ezet al. 1995).Of
thesen the molecular orbital (MO) methods are
intuitivd the easiestto understandbecauseparticular
featuresof the ELNES are assignedto transitionsto
specificMOs. In tum, theseMOs are govemedby the
bonding betweenthe central atom in the polyatomic
clusterandthe surroundingligands.Sincethe B K-edge
reflects transitions to unoccupiedp-like states,these
edgespresumablyreflect the p-like unoccupieddensity
of states.A MO model for BOl- indicates that the
TABLE 2. ABSOLUIE ENERGIES(eV) OFTIIE PRINCIPAL FEATURBSOF
TIIE CORE.I,oSSEDGESFROM GAUDEFROYIIE
a
b'
b
c
d
tst.e
rgo,s
iso.t
Trt
300.7
295.r
f
Note:emr in EEIS valus is f).2 ev
347,0
347.8
u9.l
351.0
:-
530.3
s31.4
534
535.1
538.9
541.6
5M.O
639,9
641.4
&2,4
651.7
653.1
ELNES AS A PROBEOF VALENCE AND COORDINATIONNT]MBER
1161
shows the first unoccupied MO to be the triply
degeneratet2 (o*) followed by the a1MO (Vaugban&
Tossell1973).T\e strongpeakat 198.0eV is assigned
to transitions to the t2 (o*) MO. The origin of the
features immedialely following the main peak are
uncertain, but they are possibly caused by higher
unoccupiedMOs from the BOf- anion andinteractions
with the sunesndingatorrs.
Finally, the spectrumfor p-B1e5(the subscriptrefers
to the number of B atoms in the unit cell) differs
significantly from the B K-edgesof t3lB or t4lB. The
lower energy of the edge onset relative to the B
K-edges from t3lB and t4lB is causedby the lower
effectivepositivechargeon the B in p-8105.In general,
edge onsets shift to higher energies as the forrnal
oxidationstateofthe excitedatomsincreases,an effect
known as the chemicalshift.
Carbon
The C K-edge from gaudefroyiteexhibits similar
ELNES to tle t3lB K-edge(Fig. 3), which is a consequenceof the isoelectronicand isostructuralnatureof
gaudefroyite
glassy-QO3
180
190
200
2t0
EnergyLoss (eV)
220
p-rhombohedral
Ftc. 2. B K-edges
fiom gaudefroyite,
boron
(P-Bros),rhodizite tt41B;(K,CS)Be4A14@,Be)pO2xl,
ludwigite(t3lB;MgzFeBOs)
andglassyB2O3C3lB).The
coordination
of theB in themineralsis shownto theright
of the spectra.
Thespectraillustratedhereandin subsequentfiguresareshownonanmbitraryscaleofintensity.
lowest unoccupiedMOs are the d'z (n*), a'r (o*),
and e' (o*) orbitals derived from B2p, B2s, and B2p
electrons, respectively (Vaughan & Tossell 1973,
Tossell 1986).On the basisof the MO scheme,peaksa
and c for the t3lB K-edge from gaudefroyite are
assignedto statesd d'z @\ and e' (o*) character,
respectively.The origin ofpeak b is likely causedby
the interaction of non-nearestneighbon, i.e., interactionsbetv/eenB and atomsbeyondttre BOI- anion
(Garvieet al.1994).
The B K-edges of t3lB and t4lB exhibit distinct
ELNES arising from the different MO structuresof
290
300
310
280
BOI- and BOf-. The rhodizite B K-edgeconsistsof an
(eV)
Energy
l,oss
initial sharp rise in intensity, with a maximum at
198.0 eV, followed by weaker fluctuations at 200.2, Frc. 3. C K-edge from gaudefroyite,cubic TiC, graphite,
diamond calcite (CaCO3),andrhodochrosite(MnCq).
203.L,and2I3.3 eV. The MO modelfor the BOagroup
LL62
TIIE CANADIAN MINERALOGIST
fte higonal CO]- and BO!- anions. A similar edge
shapeis also presentfor the N K-edgefrom the NQ
anion(Nekipelovet al. 1988).By analogywith the B K
ELNES, the principal featuresof the C K-edge from
gaudefroyitearisefrom hansitionsto unoccupiedstates
of n* (peaka) and ox (peakc) characler,respectively.
The C K-edges from rhodochrositeand calcite are
illusftated in Figure 3. The C K-edgesfrom the three
minerals are similar, thus providing the basis for a
fingerprint of the coordination.
Spectra of a selection of other carbon species
(Fig. 3) are markedly different, illustrating the
sensitivity of the ELNES to the nearest-neighbor
environment.Such differenceshave beenusedto identify
the nature of the C in interplanetary dust particles
(Keller et al. 1994), to determine the ratio of
amorphousand graphitic C in carbonaceousaerosols
(Kantnaket al. 1992),andto understandthe bondingin
transitionmetal carbides(Craven& Garvie 1995).
The increasingenergy-lossesof the C K-edge on
tslcu
) l\\,r-lnl\
going from TiC (280.4eV) ro graphite(284.2 eY) anld nuorite
the carbonateanion (288.5 eV) can be understoodin
relationto the changingchargeson the C atom.In TiC,
graphite,and CO!-, the C aons are essentiallynegative, neutral, and positive, respectively.The chemical
shift cannotbe understoodsolely from the valenceof
tIIq{
the excitedatom,ascanbe demonstrated
by comparing piemonttte.rJ\ ,/the C K-edges from graphite, which is an electrical
conductor, and diamon4 which is an insulator with
346 348 350 352 354 356 358
a band gap of 5.5 eV. In comparisonto graphite,the
EnergyLoss(eV)
C K-edge onset from diamond has shifted to higher
energylosses,reflectingthis bandgap,andthus in this Ftc. 4. The Ca l2j-edge from gaudefroyite(mca and telQa),
elementalCa (lr2]Ca),CaO (o6u1, calcite (ogu' CaCO),
case ttre chemical shift is related to differences in
datotite1tt16u'CaBiSiO4(OfDl,fluorite (t81Ca;
CaF), aad
bonding and not to changesin the valence of the
piemontite [te]Ca and tlolca; CarMnAlrO(SiO)
exciled atom.
(SirO?)(OH)1.The coordinationof the Ca is shownto the
Calciurn
SinceCa in mineralsis found exclusivelyin the 2+
oxidation state,diflerencesamongthe Ca 123ELNES
from different minerals should reflect the local
enyironment$urroundingthe Ca atoms.$y'ewantedto
confirm the coordinationnumberand site symmetryof
the Ca polyhedra in gaudefroyiteand determinethe
ratios of the coordinationsfrom the Ca 123 ELNES.
The Ca Z6-edges from severalCa-bearingmaterials
consistoftwo intensepeaks,a andb (Frg.4), with only
small variationsin the energyof the.Q-peakmaxima.
Since the Ca environnent in gaudefroyite is
complex, with distorted 7- and 9-fold coordinated
polyhedra we first discussthe spectrumobtainedfrom
CaO,in which the Ca atomssit in a regularoctahedral
environmenl with six identical Ca- O bondlenEthsof
2.405 A. The Ca 3d orbitals are split into a iowerenerry hz level and a higher-energyenlevel. The Ca
l* spectrumconsistsof two principat*peaks,a and b,
sepmatedby 3.4 eV, and two weakerpeaksoa' and b',
separatedby 1.61 eV from the following peak. In
right of the spectra.
calgite,Ca is octahedrallycoordinatedand its Ca /r,redgeis similar to that from CaO.Thus,for octahedrally
coordinatedCa with Oo site symmetry,as in CaO and
calcite,the Ca fu-edges seemto exhibit coordinationspecificELNES.
Interpretationof the gaudefroyrtoCa In3 ELNES as
weU as that from datolite and piemontite is complex
becauseof the two types of polyhedrain gaudefroyite
andpiemontiteandthe 8-fold coordinatedpolyhedrain
datoliteandfluorite. Ilimpsel et al. (L99I) haveshown
that the Ca l..t-edge from fluorile is not easyto interpret with the simplecrystal-fieldmodel.By comparing
the gaudefroyiteCa 123-edgervith that from CaO and
calcite, we can deduce that Ca in gaudefroyite is
unlikely to be locatedin a regular octahedralenvironment becauseof the differencesamongthe spectra-A
thorouehunderstandingof the ELNES, in relation to
the Ca environment requires
of the edges,
l163
ELNESAS A PROBEOF VALENCE AND COORDINATIONNTJMBER
such as has been done by Himpsel et al. (199L) for
fluorite.
TABLB 3. AISOLUTB ENERGIES (eD oF TIIE PRINCIPAL FEATURTS OF TIIE
SPECTRAIN FIGIJRE 5
Orygen
eudetuyib
Whereaswith B and C, we can explain the corresponding K-edges as causedby single coordinated
sites.O occursin severalcoordinationsand is bonded
to a variety of different atoms. In general, if a
compound containing a 'omolecularunit", such as
COa-,in which the bondingis predominantlycovalent,
is ionically bondedto a cationsuchasCq thenthe edge
shapesfrom the two atomswithin the "molecularunit"
will exhibit basically the sameELNES; there will be
little conribution to the O K-edge from the ionically
bondedcation. Thus we expectthe principal features
on the gaudefroyiteO K-edgeto arise from O in the
BO3-,CO3-,andMn3+Ol-groups,wherethe cation- O
bonding is predominantlycovalent,with little contribution from the mainly ionic Ca - O bonds.
530.3 53t.4
fiodcbcit€
glassyB2o3
mqrgrnite
5v
535.1
538.9 541.6
5X4.0
-
539.9 -
544,7
-
544.0
535.0
530.0 53t.4
544
541.5
NG: emr i! EEIJ valu6 is iO2 eV
ln order to assignthe featureson the O K-edge as
arising from particular cation - O bonds,we compare
the absoluteenergiesof the principal featuresfor the
O K-edges from rhodochrosite,glassy BrOr, and
manganite with the o K-edge from gaudefroyite
(Fig. 5, Table 3). All the main featuresin the oxide
spectraalso occurin the gaudefroyiteO K-edge.
Manganese
In nature. Mn occurs as Mn2+, Mn3*, and Mne.
Figure 6 illustrates the Mn \3-edges from asbolane
(Mn4*), gaudefroyite (Mn3*), manganite (Mn'*),
rhodochrosite(Mn2+),and Mn metal.With increasein
oxidation state, the Mn !3-edges for the non-metal
show t}ree apparentchanges:a valence-specificedge
shapeo
a decreasein the Ia 12arcaratro,and a move to
higher energy-lossesof the 4- and !-edges (Rask
et al. 1987, Garvie & Craven 1994a, b). Table 4
summarizesthe relevantdatafor the Mn !3-edges.
Manganite and gaudefroyitehave almost identical
Mn le.z ELNES, a result of the similar coordination
andoxidationstateof Mn in both minerals.The L3peak
maximum for gaudefroytiteaI 642.4 eV, peak a, lies
betweenthe values for the 4 p"ak maxima for the
Mn2+and Mn4 compounds.The four 3d electronsin
the Mn3+ compoundscausethe Mn06 octahedronto
distort,with elongationof the nno transbotds but little
differencein the four equatorialMn-O distances.This
Jahn-Teller distortion reduces the symmetry of the
octahedronfrom 06 to D46.For weak-field tetragonal
distortions, as found for the Mn06 octahedra in
manganiteand gaudefroyite,the t2Eand en MOs are
further split into four orbitals (lever 1984). The
L3-edge from gaudefroyite exhibits two partially
resolved peaks,a' and d', that are 1 eV and 2.4 eY
below the peak maximum,respectively.The partially
resolved structures on the Mn 4 ELNES from
manganite and gaudefroyite representtransitions to
525 530 s35 540 545 550
unoccupiedstatesbasedon a crystal-field splitting of
ElectronLoss(eV)
the 3d statesin weak-field Dnosite symmetry(Garvie
Ftc. 5. O K-edges from rhodochrosite(O associatedwith
& Craven1993,1994b).
CO!-; IUnCO), glassy B2O3 (O bonded to fte BO3-)
Rhodochrositehasan Mn,!3-edge characterizedby
and manganite(O associatedwith the MnO6 octahedron;
y-MnOOlD. The absoluteenergiesof the labeledfeatures a strong4 peak and a weaker& peak. The li-edge
exhibits several resolvable peaks, with a maximum
arecomparedin Table 3.
lL64
THE CANADIAN MINERALOGIST
unoccupiedstateswith tz" MO characier,andthe more
intensepeak, from tansitions to unoccupiedstalesof
es MO characler. The dominantly covalent Mn-O
bbndingin Mn4+mineralsexplains1foelyfQJike nature
of this edge(Garvie & Craven I994b), as opposedto
the quasiatomicmodel that adequatelydescribesmany
3d tramition metal (1M) /23-edges (de Groot et al.
1989, I990u b, van der Laan & Kirkman 1992).
The Mnlr.r-edge from Mn metal exhibitstwobroad
peaks centeredat @13 arrd 652,OeV. The lack of
ELNES is a result of the screeningof the core hole by
the valenceelectons.
DrscussroN
We have shown the easewith which the coordination of light elementscan be determinedfrom crystals
having nanometerdimensions.The B K-edge from
gaudefroyiteis clearly attributed to I3lB rather than
t4lB. As expected, the gaudefroyite C K-edge is
consistentwith that from the C K-edgeof the carbonate
anion. The C and B K ELNES are indicative of the
3-fold environmentand can be interpretedin the light
of a MO model for the C$ and BO!- anions.
Few investigationshave concentratedon the nearedge sfructure of the Ca 123-eAges,and these have
largely been confined to XAS studies(Himpsel et al.
1991, Borg et al. L992). The lack of prior Ca I.u
ELNES studiesresultsfrom the high-energyresolution
requiredto resolvethe ELNES. The Ca.Q3-edgefrom
635
&0
&5
650
655
ffio
gaudefroyite does not show any edge features that
EnergyLoss(eV)
could be attributedto the two coordinationsof the Ca,
Frc. 6. The Mn ^Q3-edgesfrom gaudefroyite,metallic Mn, but we were ableto showthat tle Ca in gaudefroyiteis
asbolane(Mn#), manganite(Mn3), and rhodocbrosite not dominantlyoctahedrallycoordinated.
Octahedrally
(Mn). The oxidationstateof the Mn is shownto the right
coordinated
Ca
exhibits
clearly
resolved
ELNES that
of the specfta.
may reflect the sfrengthof the crystal field, although
the overall ^Q3 stucture is similar for all the Ca
compounds.
The gaudefroyiteO K-edgeexhibits ELNES rich in
at &0.3 eV and a small pre-peak L.L2 eY lower in sfucture becauseof the many differenl O environenergy.Their energyseparationhasbeeninterpretedas ments. We assign these sfructures to the covalent
a measureof the crystal-fieldsfiengthactingon the Mn cation- O bondingwithin the structure.The O K-edge
(Garvie& Craven 1994b).
peaksat ca.53OeV areindicative ofcovalent bonding
The asbolane(nMn4+) ldn Q-edge exhibits two betweenat leastsomeof the O atomsand 3d TM.
pe4ks,a maximum at 643.8eV and a weakerpeak at
For the Mn /2,:-edge,the shapeand energyposition
64L.4 eY. For the Q-edges from Mne materials, ofthe,Q peakindicatethe oxidationstate.The valencethe lower-energy peak arises from transitions to speciflc edge shapescan be used as the basis for
determining the ratio of oxidation states in mixed
valence 3d TM compounds.Such a determination
TABLE4. ABSOLUTE
ENERGIES
OFTI{E LI-EDGE
ANDTHELdLz AREARATIOSFORSnt.rc:IEn Mnhas been demonstratedby Cresseyet al. (L993) for
BEARING COMPOI,JNDS
Fe-bearingmineralshaving mixed valencyand coordination, and Pattrick et al. (1993) for Cu-bearing
minerals. Q6mparisonof the Mn Zs-edge from
L3+dge
L3/L2area
ralio
gaudefroyitewith the sp€cm in Figure 6 demonstrates
that tle Mn is in the 3+ oxidation state.Interpretation
rhodocbrosile
640.4
4.67
manganite
642.5
2.55
of the first series ^Q3-edgescan be achievedvia a
gaudefroyite
642.4
2.46
quasiatomic
model with the inclusion of solid-state
asbolane
643.8
2.t7
effects,most notably the crystal field.
ELNES AS A PROBEOF VALENCE AND COOR.DINATIONNT]MBER
CoNIcLusroxs
PEELS conductedin a TEM is a powerful specfroscopic techniqueproviding important information on
chemicalcompositionand bonding. The possibility of
using the ELNES as a probe of the local elechonic
structurewas demonstratedby studying gaudefroyite.
Analysis of the core-lossedgesillustratesthe easewith
which elementidentification can be undertaken,even
for elementssuch as Bo C, and O, which are ftaditionally diffrcult to deal with by energy-dispersion
specfroscopy,although recent advancesin electronprobe micro-analysis hold promise for materials
containing the light elements of Z from 4 to 9
@audsepp1995,Hawthomeet al. 1995).Analysis of
the core-lossfine sfucture, andcomparisonofthe edge
shapeswith materials in which the bonding of the
elementsis known, allowedthe coordinationof B and
C and the oxidation stateof Mn in gaudefroyiteto be
determined.
Acrnqowr-mcmrmms
We tha::k Dr. J. Faitbftll of the HunterianMuseum"
Glasgow, for providing samples of rhodocbrosite
@otley 1425)andpiemontite(M8006).In addition,we
tlank Drs. A. Manceauand R.F. Martin for comments
and suggestions.PEELS data were recorded in the
Deparment of Physics and Astronomy at the University of Glasgow. LAJG and AJC thank the
Engineeringand Physical SciencesResearchCouncil
for providing funds to purchasethe Gatan666 parallel
electron loss spectrometer. Partial funding was
provided by National ScienceFoundationgrant EAR
9219376(to PRB).
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ReceivedFebrunry 15, 1995, revised manuscript accepted
August6, 1995.