THE AMERICAN
MINERALOGIST,
VOL. 43, JULY-AUGUST,
1963
INFRARED SPECTRA, SYMMETRY AND STRUCTURE
RELATIONS OF SOME CARBONATE ]\{INERALS
Hens H. Alr.on AND PAUL F. Konn, Ll. S. Atomic Energy
Commission, Wash,ington,D. C. and.Colwmbia Llnil;ersity,
I{ew York, N. Y.
Assrnecr
variations in infrared spectra of carbonate minerars are indicative of changes in symmetry of the ionic carbonate molecule. The point-group symmetry of the ion determined
from spectral considerations is generally compatible with its site symmetry. Molecular symmetry does not, however, vary systematically with crystal class. The existence of more
than one non-equivalent molecule per unit cell is reflected in certain carbonates by spectral
enrichment in the wavelength region of one or more of the fundamental cegenerate or nondegenerate vibrations. Alternative causes of spectral multiplicity are also considered; however, the enrichment has been observed only for minerals which are known or suspected from
r-ray data to contain non-equivalent carbonate ions.
INrnonucrroN
rnfrared spectroscopyhas been used successfullyby chemists and
physiciststo resolveproblems of structural arrangementof organic and
inorganicsubstances.This approachis possiblebecausemechanicalvibrations of moleculesfrom which infrared spectraoriginate are governedby
molecularsymmetry in such a way that specificsetsof vibrations can be
correlatedwith specificsymmetry operations.rt is possibleto treat ions
as discrete functional groups becausebinding forces within ionic molecules in minerals are significantly stronger than forces binding the moleculesto the rest of the structure, bearingin mind that the vibrations are
dependent on crystal structure. This provides a versatile method for
analyzing mineral structures at the molecular level.
In spite of this potential, spectral studies of minerals appearing in
journals have for the most part emphasizeddescriptivefeaearth-science
tures, and relatively little attention has been accorded effects of molecular environment on frequenciesand complexitiesof molecularvibrations.
Somemanifestationsof this dependencyhave been indicated by Halford
(1946)for calciteand aragonite;by Nakamoto et al. (1952)for carbonate
coordinationcompounds;and by Adler and Kerr (1963)for calcite-group
and aragonite-groupminerals. For the most part, however, difierences
among molecular spectra of various carbonate minerals as well as other
mineral groups, such as sulfates,phosphates,and silicates,remain unexplained.The lack of adequateunderstandingof these variations continues to be a deterrent to application of infrared spectroscopyto mineralogicalproblems.
The purposeof this and other recent papersby the writers is to provide
839
H H. ADLER AND P. F. KERR
an analysis of infrared spectral data for several carbonate minerals, in
terms of factors that have a known or suspectedinfluenceon vibrations of
the ionic carbonate moleculeCOrz-. The resuits have led us to an intriguing and rapid meansof recognizing,or at leastsuspecting,non-identical
carbonate ions in the unit celi, a task otherwise accomplishedonly by
rigorous r-ray structure analysis.
The point-group notations used here to identify molecular and site
symmetries are the usual sets of symmetry operations prescribed by
Although the HerSchoenfliesand customarily used by spectroscopists.
mann-N{auguinnotations are equally informative and are in more common usagein mineralogicalIiterature, the Schoenfliesnotations are preferred for easeof comparisonwith the character tables identifying symmetry speciesfor various molecularpoint groups.
The spectral region which includes fundamental vibrations of the
carbonatemoleculeapproximately coversthe range 6 to 16 microns.The
carbonate stretching modes,v3 and V1,ar€ found near 7 and 9 microns,
and the bending modes,v2 and V4,occur at about 11 and 14 microns,respectively. The v3 and va modes are doubly degenerate,a.e.each under
certain symmetry conditionsmay split into two distinct vibrations of different frequency.In the presentstudy only featuresof absorptionbands
correspondingto fundamental vibrations are of primary concern.
The internal symmetry of the moleculeis estabiishedby the confi.guration of nuclei comprisingthe molecule.In the carbonateion, positionsof
the singlecarbon and three oxygen atoms determine this symmetry. The
site symmetry of the moleculeis definedbythe arrangementof any molecular ion and extramolecularnuclei to which it is bonded; it is, therefore,
a function both of the symmetry of the moleculeand its externalenvironment. A simple examplemay help to clarify this point. The ideal carbonate ion has the internal symmetry Dar,.However, if neighboringCa atoms
are also considered,as in calcite,the symmetry elementswhich definethe
configurationof one carbon, three oxygen and six caicium atoms are no
IongerDrr,but D3. For dolomite and aragonite,the divalent metallic ions
provide still different site symmetries for the carbonate ion (or carbon
atom).In this study, variations in site symmetry have been observedto
alter the carbonateabsorptionspectra,leadingus to certain inferencesregarding the ionic symmetry.
\,Iuch more remains to be learnedabout the dependenceof vibrations
of functional moleculargroupsin crystalson extramolecularenvironment'
Until this knowledgeis gained,it is doubtful whether much significance
may be attached to interpretations of absorption spectra ascribed to
interactions required for crystal cohesion,such as hydrogen bonding,
which seemsto be the topic of much predictive discoursein recent mineralogicalIiterature.
INFRARED SPECTRA OF CARBONATES
84r
ExpBnrlrBwrer- Tncuurgun
Infrared spectrathat furnish the data for this paper wererecordedon a
Perkin-Elmer Model 21 double-beam infrared spectrometer equipped
with sodium chloride optics. All sampleswere preparedusing the potassium bromide pellet technique.Ordinarily, 0.85 mg of finely ground mineral was mixed and reground with 300 mg of KBr in a dental amalgamator and the mixture usedto form a pellet under vacuum at approximately
19,000psi.
l\{ineral specimenswere obtained primarilyfrom the mineral collections
of the U. S. National Museum through the courtesy of Dr. George S.
Switzer,and supplementalmaterial was furnished by Dr. JosephFahey,
U. S. GeologicalSurvey. Mineral identity was confirmed by c-ray diffraction analysis.
The cooperationof Dr. Irving A. Breger and the U. S. GeologicalSurvey in making available the spectrometerfor this investigation is gratefully acknowledged.The authors are alsoindebted to Dr. Howard Evans,
U. S. GeologicalSurvey, who kindly provided many helpful suggestions
in regard to the crystal-structure data and to Dr. Ralph S. Halford,
Columbia University, for suggestionsconcerningalternative interpretations of the spectraldata.
RELATToNS
MorBcur,an Svlruprnv AND STRUcTURAL
The structure of the carbonatemoleculewas establishedfirst by Bragg
(1914) and more recently has been confirmed by other crystal structure
studies.In the model ion, three oxygenatoms are coplanar with a central
carbon atom and are situated at the cornersof an equilateraltriangle at a
distance of approximately 1.31 A fro- the carbon atom. This form is
highly symmetricaland is consideredrepresentativeof the moleculein its
free state undistorted by asymmetrical external forces.In crystal structures, however,one might expectthe symmetry of the ion to be modified,
becauseof variable or unequal externalforces,and thesecan be expected
to produce differencesin infrared spectra.
Previogs systematic studies of relationshipsbetween molecular symmetry and vibrations of the carbonateion were accomplishedby Halford
(1946)and Nakamoto et al (1957).From theoreticalconsiderations,HaIford (1946) related spectral differencesbetween calcite and aragonite to
the site symmetry of the carbonate ion, the structural transition being
accompanied by a lowering of the site symmetry of the carbonate ion
from D3 in calcite to Cuin aragonite;with concomitantremoval of the degeneracy of the vg and v+ modes and the appearanceof vr in aragonite.
The spectraldifferencesobservedfor theseminerals at9 p. and 14 p, are,
therefore, related to differencesin molecular symmetry brought about by
a changein the environment of the carbonateion.
U2
H. H. ADLER AND P. F. KERR
Subsequently,Nakamoto et al. (1957),investigatingcarbonatecoordination compounds,determinedthat a split in the degeneratev3 carbonate
mode could result from formation of unidentate (in which one C-O bond
changescharacter)or bidentate (in which two C-O bonds changecharacter) bonds between the carbonate and metallic ions. Although effects
were found to be qualitatively similar for both bond types, the bidentate
structure producesa greater separationof the split bands. fn both cases
v1is activated, but absorptionis more intensewhen a bidentatelinkage is
formed. The vr mode apparently is unaffected,sincethe degeneracyis nol
visually resolved. For both unidentate and bidentate complexesthe effective symmetry of the ion is loweredto C2,,becausethe coordinatedand
uncoordinated oxygens are no longer symmetrically equivalent.
These early works demonstrate the influence of the symmetry of the
carbonateion on spectral complexity and indicate the possibility of applying variations in absorption patterns to elucidating molecular symmetry in carbonate minerals. We have found, however, that fortuitous
overtrapor coincidenceof bands placessomelimitation on the usefulnessof
the spectrafor this purpose.For example,the absenceof a vr doublet in
spectraof witherite and cerussite(Adler and Kerr, 1963),isotypesof aragonite and strontianite, is most likely caused by accidental degeneracy
and restrictsthe usefulnessof thesespectrain moiecularanalysis.Spectra
obtained with CsBr optics by Miller et al. (1960) show no additional
bands assignableto the vn vibration beyond the 1S-micronregion. Furthermore,in no casedoesthe v3 vibration appear to be resolvedinto two
componentbandsalthough, as reported by Halford (1946),degeneracyof
this band is reinoved in the aragonite group. Consequently,it may be
inferred that the individual v3 and v+ vibrations fortuitously coincide or
overlapfor theseminerals.It shouldbe emphasized,therefore,that molecular symmetry is not always clearly indicated by the absorptionpattern,
inasmuch as degeneracies
may remain unresolvedbecauseof overlap or
near coincidenceof band maxima or for unknown reasons.The absenceof
spectral bands is not always an indication that a particular molecular
configuration may not exist. Nevertheless,when a complex carbonate
spectrum contains split degeneratebands with v1, the symmetry of the
moleculeis Iow.
We have observedthat hydrous carbonate minerals present another
problem.Someof theseminerais,including gaylussiteand pirssonite(Fig.
1) yield absorption bands in excessof the prescribednumber of fundamentals in the 11-, 12- and 14-prregions.Becausesimilar bands are observedgenerallyin hydrous minerals,thesebands may be caused,we believe, by OH torsionalvibrations. Sincethe bands may, in somecases,be
mistaken for the vs and v4 carbonatefundamentals,their existenceis of
INFRARED SPECT:RA OF CARBONATES
N U M B E R SI N G M - I
56zI'grolt2t3t4t5
W A V E L E N G T HI N M I C R O N S
Frc. 1. Infrared spectra of carbonate minerals showing absorptions corresponding
to carbonate vibration modes,
843
H. H. ADLER AND P. F. KERR
some concernin utilizing spectra of hydrous carbonatesas an index of
molecularsymmetry. Although dehydration has been considered,this is
also likely to alter the coordination about the carbonateion and, thus,
cause further spectral changes involving carbonate bands. Therefore,
deuteratedanalogsof the hydrous mineralsmust be synthesizedbeforea
more reliable interpretation appears possible.Despite these diffi.culties
tentative interpretations of the spectra of gaylussiteand pirssonite are
given and tentative molecularsymmetriesare assignedon the basisof the
v3 and vr modes.
Tleln
1. AssonprroNs ol rrIE coa2 Ion es .l FuNcrroN ol MOr.tcur-ln
,"@
Cz"
-Number of Bands Observed
Molecular
Symmetry
Dsn
Da (site)
Syuurtnv
Vg
1
1
1
2
(vs)
(vs)
(vs)t
(s)
1(*)
1 (vw-m)
1 (s)
1 (s)
1 (s)
1 (s)
1 (s)
I (s)
2 (m-s)'z
1 (w-m)t
vs, very strong; s, strongl m, medium; w, weakl vw, very weak.
1 Two bands are theoretically possible.
2 A single band is observed for some minerals.
With the aid of tables (Herzberg,1945)denoting the number of vibrations of each speciesfor point groups with degeneratevibrations and the
symmetry types and charactersfor these point groups, it is possibleto
establishthe nature of the vibrations which may occur for various posof the carbonateion. If the symmetry of the carbonsibleconfi.gurations
ate ion (Table 1) in a crystal is identical or similar to that of the free ion,
Dsr",it is apparent that only singlebands shouldappearfor the degenerate
v3 and v4 modes,and v1 should remain inactive. Aithough the site symmetry of the carbonateion in calcite is D3, the effectivemolecular symmetry is apparently Den.Similarly, the site symmetry for dolomite is C3,
but the apparent molecular symmetry is alsoDzn(Table 3). The difference
in molecularand site symmetry for calciteresultsfrom the orientation of
the calcium atoms which are doubly coordinatedabout the oxygenatoms
and lie in a plane oblique to the molecular plane, as depicted by Bragg
(1937,p. 119).The differencein site symmetriesfor dolomite and calcite
resultsfrom substitution of a Mg atom for one of the two Ca atoms about
each molecularoxygenatom. This site symmetry changehas no effecton
the molecularsymmetry as determinedfrom the absorptionspectra.
The site symmetry for aragonite is C", but the vibrations are no longer
INFNARED SPECTRAOF CARBONATES
845
identical to those of the free molecule;hence, the molecular symmetry
appearsalso to have been modified. Since the appearanceof vr in the
aragonite spectrum indicates a molecular dipole moment, which cannot
exist if more than one axis of symmetry is present, the moleculecannot
have internal D31 s/mmetry. It is possible, however, that coordination
contributing to site symmetry Cu may produce molecular symmetry Cs'
as the latter is compatible with a dipole moment. Moreover, from sitegroup data (Table 2) it follows that site symmetry C" is also a sub-group
of molecular point groups Drn, Cro and Czo.Therefore, crystals in which
PorNrGnoupsor rnr CansoNernIoN
T rrr:rn2. PossmrcSrrusron Mor-ncur-an
Molecular
Site Group
Molecular
Point Group
D*
Caot
Czol
Cy
Cb
Ct,
Ct,
C", Cz, Cu Czo,Cao,C31,,D3, Ds1,
Cu
C",
C",
Cz"
C",
C"
1 Point groups to which a molecule with a dipole moment might belong.
carbonate ions occupy a C" site may conceivably have molecular symmetry Ds;, C", Cr, ot Cso.Other site groups compatible with molecular
point groups of the carbonate ion are given in Table 2.
As previously indicated, with C2, symmetry the carbonate molecule
may assumeunidentate or bidentate configuration in which one C-O
bond is non-equivalent o the other two. This may involve a metaloxygenbond of different characteras in Werner coordinationcompounds'
a hydrogen bond, or a different cation coordination about the nonequivalent oxygen.
The distinction between unidentate and bidentate bonding is important although both configurations produce C2osymmetry in the carbonate
molecule.Using H bond formation as an example,only the single nonequivalent oxygen is hydrogen-bonded in the unidentate structure,
whereasin the bidentate structure the two equivalent oxygen atoms participate in H bonding. According to Nakamoto et o'1.(1957)' the distinction may be possibleon the basisof the intensity of the vr vibration and
the degreeof splitting of the two vr bands. IIowever, no limiting quantitative criteria have as yet been established for distinguishing between
the two structures by infrared spectra. Recognizingone or the other of
the two bond types in minerals may be difficult.
Considerationalso might be given to the existencein mineralsof a nonplanar carbonate ion having C3aslmmetry in which the carbon atom is
846
H. H. ADLER AND P. F. KERR
displaced slightly from the molecular plane along the three-fold symmetry axis. In this case there are two symmetric vibrationS,v1 &rd V2,
and two degeneratevibrations, v3 and v+. One absorption band should
appear, therefore, for each of the four active fundamentals. The degeneracy of v+ is evidently not always resolved in minerals having C"
molecular symmetry (for example witherite and cerussite).Hence, the
existenceof a C3,-typeion cannot be confirmed,i.e., distinguishedfrom
Cu,on the basis of the fundamental absorption spectrum. However, its
existenceshould be suspectin any carbonate mineral producing a spectrum in which all four fundamentals are present, when v3 and va are
doubly degenerate.From this aspect, the possibility of a molecular
symmetry change from C", for aragonite and strontianite, to C3,, for
witherite and cerussite,should not be overlooked.Such a changemight
conceivablyresult from the crowding of electropositivechargesabout the
carbonate molecule as larger cations are introduced in the structure.
Molecular symmetry Cr must also be accorded similar consideration.
However, it should be kept in mind that both Ce,and Cl require a nonplanar carbonateion for which there is, as yet, no concreteevidence.
After considerationof the possiblesymmetry elementsinvolved, a decision about the molecularsymmetry may be basedfirst on whether the
moleculepossesses
a dipole moment.In the absenceof v1 the moleculemay
be assumedto have zero dipole moment. Therefore, carbonate spectra
lacking absorption correspondingto v1 are indicative of molecularpoint
group D31which has symmetry elementsprecluding a dipole moment.
When v1 appears in the spectrum the molecule may belong to point
group Cu or C2, (and C3oot C1, if we consider non-planar CO32- ions)
which have symmetry elementsprescribinga dipole moment. The number of bandsobservedfor the normally degeneratevibrations also may be
used as an indication of the molecular point group. However, as previously mentioned,the absorptionsresulting from removal of degeneracies
may overlap in some cases,and the resulting lack of resolution can be
misleading.Therefore,,thevalue of v3 and v4 alone is limited in determining molecular symmetry. Theoretically, the distinction between C"
and Cz, symmetry cannot be made on the basisof the observedpresence
or absenceof the split in the vr and va bands,although when the va band is
not visibly split C" symmetry is tacitly assumed. The transition to
either symmetry type removesall degeneracies
presentin the Daastate,
since C" and C2, have only non-degeneratevibrations. Furthermore, all
six vibration modesare infrared active for both point groups.
In classifyingthe carbonatemineralsaccording to indicated molecular
symmetry (Table 3), primary considerationis given to the presenceor
absenceof absorption corresponding to the vr vibration mode; likewise, a
INFRARED SPECTRAOF CARBONATES
847
secondarydistinction is made on the basisof the degeneratemodes.Pointgroup crystal symmetry data and space-groupnotations in Table 3 are
based upon Palache et al. (1951). Infrared mineral spectra yielding the
absorptiondata in this table are shown in Fig. 1, and the absorptiondata
are given in Table 4.
Comparison of molecular with crystal point-group data (Table 3)
showsno consistentvalid correlation between molecular symmetry and
crystal class. For example, Der molecular symmetry is represented by
four minerals crystallizing in the isometric, hexagonal and monoclinic
systems and C" molecular symmetry by two minerals which are respectively hexagonal(trigonal) and orthorhombic. From another standpoint,
the hexagonalmineralshave both DznandC" molecularsymmetries,and
the orthorhombic minerals have C" and Cz, molecular symmetries. Unfortunately, problems associatedwith the interpretation of spectra of
many hydrous carbonates,which are more numerous than anhydrous
species,iimit the number of examples.
Although lattice symmetry is not a satisfactory index to molecular
syrr-metry, the molecular site-group determination using space-group
data and a knowledgeof the population of the unit cell (Halford,1946),
is of some value in this respect.In many cases,however,the atomic arrangement within the unit cell can be determinedfrom crystal structure
data, and it is, therefore,not necessaryto determine site symmetry by
this method. Such information was for the most part obtained from abstracts of structural data in Strukturbericht and Structure Reports.
Referencesto original data may be found in these.reportsas well as in
Palache, et al. (1951). Site data for bastnaesite were obtained from
Donnay (1953).
The atomic arrangementin northupite is compatiblewith two possible
spacegroups,namely Tnaand On1.In eachstructure the COa2-ions are in
closelyrelated positions.For spacegroup Or,7(seeStruhturbericht,Il) the
carbonateion lies on site C3oand lor Tnaon site Cs,both of which, according to Table 2, are sub-groupsof molecularpoint groups Daaand Cao.The
spectrum however, is conformable with the vibrational requirements of
Dan,butnotC3o,symmetry inasmuchas no dipole moment is evident.
The site symmetry for pirssonite, Cr (Evans, t962), is compatible with
all possiblemolecularpoint groups listed in Table 2.
NoN-Equtver,oNr Momcur,an SrrBs IN CARBoNATES
In the foregoing,the complexity of the carbonateabsorptionspectrum
has been interpreted in terms of molecular symmetry, assumingthat all
carbonate moleculesin the unit cell are identical and that none of the
bands resultsfrom a coupling or combination effect.The number of per-
848
H. H. ADLER AND P, F. KERR
-?)-
9.;96669
HHi'
qlv
il_il
=
9Po tr
,i> h
(2, a2U7 . a ' A ' ; ' d
A
A
A
.q
vv
.?
vvv
I?
I?
I.:
N
fr
P.o
eO
o'n
!o
q,!
a
Fl
E<
v
ts
z
E
O
O
5
o
a
gsEseggE
z
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O
1l
O
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E
6
O
a
il
O
F
sRa
o c - : 1. {i !R! cE- ' ' E
ha;
O(,o
eSElp;Y
@ . Y N =
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-F:';AF-$.i
SHV\!!U
cd!
O
ho
E
o
Fr
a
sS€€€
hbN\SSr;F
YTcoJYYX-:
d*e*ddau
INFRARED SPECTRA OF CARBONATES
T.q.rr,t 4. AssonprroN BlNls o.r'ruE CansoxA,rnsLrsreo rN T.qnlr 3
Wavelength (microns)
Mineral
Locality
t,s
14.06
14.02
13.72
1 3. 8 9
or 14.44
14.o2,
1 4. 3 1
11.63 1+.41
13.74
tI.52
1 1 . 5 0 14.0514.30
t 1. 3 7
1 1. 4 0
1135
lt 43
Northupite
Calcite
Dolomite
Gaylussite
Green River, Wyo.
New Mexico (106158)1
Guanajuato, Mex. (R2408)
SearlesLake, Calif. (102876)
6.83
7. 0 2
6.95
7.07
Aragonite
Horschenz, Bohemia (R12050)
6.80
9.22
Witherite
Bastnaesite
Pirssonite
Cumberland, England (R2560)
Belgian Congo (104097)
SearlesLake, Calif. (107403)
6.97
6.93
6.72,
7. 0 7
942
9.21
9.36
1 U. S. National Museum Identilication Number.
missiblefundamental bands is prescribedfor non-linear moleculesby the
reiationship
3N-6 :
number of normal vibration modes,
where N is the number of atoms in the molecule.Hence, there can be
only six normal modes for a given carbonate molecule.These are the v3
(doubly degenerate),v1 (latent, non-degenerate),v2 (non-degenerate)
and va (doubly degenerate)vibrations. The carbonate minerals already
consideredgive rise to no more than six fundamental carbonateabsorption bands, and the appearanceof fewer than this number has been reIated to symmetry restrictions. It can be assumed,therefore, that for
these minerals all carbonate ions are identical or occupy more or less
identical sitesin the crystal lattice.
The featuresthat distinguishone mineral spectrumfrom another arise
from differing influences exerted by the crystai field on the molecule
(Adler and Kerr, 1963).If in a singlecrystal two moleculesexist with respect to their cation environmentsor their symmetriesat non-equivalent
sites, the observedspectrum may yield evidenceof this in the ideal case
through the appearanceof the simple spectrum of each of the nonidentical molecules.Consequently,the mineral spectrum may contain
more than the maximum number of bands permissiblefor an individual
mode, and in some casesdoubling of the spectral bands may result.
Spectra from which two non-equivalent carbonate ions may be postuiated on this basisare shown in Fig. 2, and the absorptiondata are given
in Table 5.
Sincethe basic r-ray data indicate that the unit cellsof huntite, parisite and shortite contain two non-equivalentcarbonatemolecules,there is
850
H. fl, ADLER AND P. F. RERR
considerablesupport for our interpretation. The structuresof barytocalcite and alstonite are not sufficiently well known to identify such dissimilar moleculargroups.Therefore,we point to the data presentedhere
as evidencefor the existenceof non-equivalentcarbonate ions in these
two mineral structures.
Unfortunately, alternative mechanismsmay account for spectral enrichment observedin the region of the carbonate fundamentars.Nakamoto et al. (1957) and Gatehouseet al. (1958)have attributed strong abT,rer,n5. AesonprroN
Dlre or CannorerBsComrnrNrNc
Monr Trrat ONe
NoN-Equw4r,nxr Morrcur,en Srrn Prn UNrr Crr,r,
Wavelength (microns)
Mineral
Composition
Barytocalcite
CaBa(COs)r
6 62,6.8+,
7.11,7.32
9 20.9 25
Huntite
MgaCa(COs)r
6 62,6.92
8.99
Parisite
CerCa(COa)rFr
690
9.19,9 27
Shortite
NarCar(COr)s
Alstonite
CaBa(COa)r
6 57,6.75,
6.88,7.09
685
83, 14.29,
37,14.74
13 46
9 . 1 8 .9 3 5
9 20,9 39
Barytocalcite-Cumberland, England (78839)r
-Cresrmore,Calil.(R11367)
Huntite
-Colombia, S. A. (R2612)
Parisite
-uleen Klver, wyo.
Jnortrle
-Northumberland,England(78840)
Alstonite
I U. S. National Museum Identification
Nunber.
sorption bands appearingat about 7 micronsin spectraof unidentate and
bidentate carbonato-complexes
to splitting of the va vibration, which is in
accord with removal of its normal degeneracy.Similar splitting has also
been observedfor basic and organic carbonatesby Miller and wilkins
(1952), Hunt, Wisherd and Bonham (1950)and Gatehouseet al. (1958).
However, further splitting of v3 has been observedonly for the hydrogen
carbonatesand has been attributed by Gatehouseet at. (t958) to interaction between neighboring CO3 groups which are hydrogen bonded.
They alsohave observeda splitting of vr in the hydrogen carbonatesand
carbonato-complexes
that, they suggest,may arise from interaction of
neighboringmolecular groups or from Fermi resonanceof v1 with lower
frequencyfundamentals.However,no splitting of v2 was observedfor the
carbonatecompoundsthey examined,and the va mode did not give rise
INFRARED SPECTRA OF CARBONATES
W A V E N U M B E R SI N C M
-I
56789tOilt2t3t4
W A V E L E N G T HI N M I G R O N S
Frc. 2. Infrared spectra of carbonate minerals indicating non-equivalent
molecules in the unit cell.
851
852
H. H. ADLER AND P. F, KERR
within the Nacl region to more than the number of bands prescribedby
removal of the degeneracy.
Consideration must also be given to additive and subtractive combinations with lattice modes to produce satellite bands which are displaced to either side of the molecular frequency. Hexter (1958) has
suggestedthat the 5.7 p band (1755 cm-l) in the calcite spectrum reported by Schaefer,Bormuth and Matossi (1926) is in accord with the
additive combination of the v3 fundamental (1432 cm-r) and the 323
cm-l lattice mode. Similarly, the band at 6.3 p (1587cm-l) may be a combination of the 155 cm-l lattice mode with the vr fundamental, and
the 8.5 p (1I75 cm-l) band may be a subtractive combination of the
257 cm-r lattice mode with v3.rt should be noted that among thesethree
combination modesonly the bands at 5.7 and 6.3 p, are strong enough to
be observedin the spectraof powderedsamplesof calcite-groupminerals
and none can be readily mistaken for fundamentals.unassignedsatellite
bands of similar intensity have been observedalso adjacent to v3 and v2
(Fig. 1) for various carbonateminerals,but they are certainly not to be
confusedwith the intense bands observedin the spectraof barytocalcite
and huntite, which are taken as evidenceof their dimolecular character.
Although none of the combination bands observedby Hexter (195S)
for calcite is of sufficient intensity to be mistaken for degeneraciesof
fundamentals,it is noteworthy that Brecher et al. (1961) have observed
in their study of a singlecrystal of cyclopropanethat combinationsof lattice modeswith molecularfundamentals can give rise to prominent absorption bands adjacent to the involved fundamental. Therefore, it is
necessaryto considerthis possibility, as well as molecularinteraction, to
account for the multiplicities of bands which are not attributable to removal of degeneracies.
It is interesting,however,that thesemultiplicities
are observed (Fig. 2) for three minerals known from r-ray evidenceto
contain non-equivalentcarbonatemoleculesin their unit cells,and have
not beenobservedfor mineralsknown to possessonly identical molecules.
Therefore,they appear to furnish empirical evidenceof this condition.
Further interpretation of the spectrain Fig. 2 would require considerable detailed investigationof parametersbeyond the scopeof this paper.
CoNcr-usroN
Theoretically, infrared spectral analysis of minerals may be used to
establishthe symmetry of the carbonateion. The symmetry requirements
of the lattice provide a means of verifying the molecular symmetry suggested by the vibrational spectrum. The experimentaldata herein presented are in good agreementwith expectation,but some complications
arisefrom accidentalspectraldegeneracies.
The problemsand limitations
of the method are reflected in the alternative interpretations that are pos-
INFRARED SPECTRA OF CARBONATES
dJJ
sible. However, it is clear that the ideal symmetry of the carbonate ion,
which has a planar structure belongingto point group D31,is modifiedto
lower symmetry as a function of its extramolecularenvironment. This is
apparent in the spectra of aragonite, bastnaesiteand pirssonite which
preclude a highly symmetrical ionic form.
As suggestedby curves obtained from carbonate minerais known to
contain non-equivalent COr'z ions in the unit cell, it is possibleon the
basis of the presenceof spectral enrichment, i.e. fundamental bands in
excessof the number prescribedby symmetry requirementsfor a single
molecule,to predict the presenceof non-equivalentions in the unit cell.
In this application, the infrared method provides a technique that is
much simpler and faster than the classicalr-ray methods.
RnrnnnNcBS
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EvaNs, Howenn T., Jn. (1962) Personal communication.
Ga:rrnousn, B. M., S. E. LrvrNcsroNr,q.Nn R. S. Nvnor-u (1958) Theinfrared spectra of
some simple and complex carbonates Jour. Chem.Soc.Lond.on,3137-3142.
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Hrnzrrnc,
G. (1945) Molecular spectra and moleculoy structure. II. InJrored ond, Roman
slectra o.fpllyalomic molecules.D. Van Nostrand Co., Inc., New York.
Hrxrnn, R. M. (1958) High-resolution, temperature dependent spectra of calcite Spectr ochim. A cta. lO. 281-290.
HuNr, J M , M. P. Wrsnrnn a.NoL. C. BoNsau (1950) Infrared absorption spectra of
minerals and other inorganic compounds. Anal. Chent. 22, 1478-7497 .
Mrltnn, F A , G. L. CnnrsoN, F. F. BrNrr-rv aNo W. H. JoNrs (1960) Infrared spectra
of inorganic ions in the cesium bromide region (700-300 cm-r). Spectrocldm.Acta,16,
135-235.
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metallic complexes.Iour Am. Chem. 5oc.79,490+-4908.
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Manuscript recei.aed,December 18, 1962; acceptedfor publicoti.on, April 8, 1963.