American Mineralogist, Volume 64, pages I I9-126, 1979
NMR studyof micas,II. Distributionof Fe'*, F-, andoH- in theoctahedral
sheet
of phlogopites
J n s u sS e N z l
Groupe de Physico-Chimie Minbrale et de Catalyse, Uniuersity of Louuain
Place Croix du Sud I , B-1348Louuain-la-Neuue,Belgium
nNn WrLLr.q,uE. E. SroNr
Sectionde Physico-ChimieMinbrale ( M.R.A.C.-Teruuren)
Place Croix du Sud I , B-1348 Louuain-la-Neuue,Belgium
Abstract
By studyingtheNM R signalsof protonsandfluorinein a series
phlogopites
of crystalline
at
variousfrequencies
and angles,it hasbeenpossible
to differentiate
amongvariousoctahedral
associations.
In sampleswith continuously
variableamountsof fluorineand iron, we show
that the OH groups,ratherthan the fluorineions,are in directcoordinationwith the iron.
Thisanionicsegregation
relativeto Fe2+is alsoconfirmedby theobservation
of fluorinepairs,
whichmostprobablyform homogeneous
fluorinedomains.This localorderingof octahedral
ionsis an importantparameterin the studyof cohesionof layeredsilicates.
Introduction
Phlogopites
belongto the sheetsilicategroup;their
idealformulais KMgrSisAlOlo(OH)r.
The Mg,+ ions
Iocatedin the centraloctahedralsheetof the elementary layerare only slightlysubstitutedby Fe2+ions,
whereasthe OH groupsmay be replacedby F- ions
to variousdegrees.
It is well knownthat the natureof
the isomorphoussubstituents
within the octahedral
sheetof micasis a determinantfactor in the vermiculitizationprocessof theseminerals;in particular,
when the fluorine content increases,
the easewith
which K+ can be replacedby Na+ decreases.
The Fand H+ nuclearmagneticresonancespectraof ten
phlogopitesampleshave beenexaminedin order to
understandhow the nature of the octahedralsheet
influencesthis process.
In the octahedralsheet,two differentcation sites
are possible,dependingon whetherthe OH groups
are in czi (M2 site)or trans position (M1 site) (Fig.
I ). TheseOH groupsare locatedon the zr symmetry
plane and are coordinatedto one Ml cation in the
planeand to two M2 cationslocatedon both sidesof
the plane.
We will show that from the H+ and F- NMR
I P r e s e n ta d d r e s s C
: . S . I . C .S e r r a n o 1 1 5 D p d o , M a d r i d 6 , S p a i n
0003-004x/79/0102-01
19$02.00
signals,it is possibleto characterize
the distribution
of the Fe2+ion with respectto the anionsOH-, F- as
a result of the largeparamagnetic
influenceof the
Fe2+on the nuclearsignals.We will also showthat
the strong nucleardipolar interactionbetweenFions belongingto the sameoctahedraleadsto informationconcerning
the degreeof orderwhichexistsin
the octahedraldistributionof this ion. Theseresults
indicateionic segregation
within the octahedral
sheet
and thereforesuggestthe existenceof short-range
orderwhichsupplements
thepictureof average
structures given by X-ray diffractionstudies.
Experimental
The samplesstudied(for chemicalcompositionsee
Table I ) are part of a collectionof micas whose
weathering properties had been determined previously in our laboratory(Rousseaux
et al., 1973).
The sampleswerechosensuchthat the fluorinecontent variedcontinuouslybetween0.7 to 5.2 percent
(by weight),the paramagnetic
impurities,essentially
Fe'+ ions, beingin the range0.2 to 3 percent.The
samplesformed by superposingindividual platelets
(10 X l0 mm) were cut from large natural mica
plates,on which the a and D axis orientationshad
beendeterminedbeforehand.Altered or twinned regions werecarefullyavoided.
I 19
120
SANZ AND STONE: NMR STUDY OF MICAS
T a b l e l . S a m p l ec o m p o s i t i o n s '
!i*
2+
Ca-
Na
-
0.0r
0.05
0.06
0.85
2.4L
-
0.39
0.rr
0.01
0.0r
0.0r
si4+
At3+
A13+
ti4+
Fe3+
Fe2+
to,*
P-2
2.74
!.26
0.r8
0.05
0.0r
0.16
2,55
P-2t
2.74
r.26
0.r5
0.05
0.r0
0.17
P-I2
+
+
o-
OH
F
9.93
t.92
0.15
0.85
r0.22
r,48
0,30
0.07
0.88
10.35 r.r9
0.45
0.01
0.04
0.9r
r0,42
1.09
0.49
-
K
2.90
r.r0
0.16
0.08
0.04
0.r0
2.68
-
2.88
r.t2
0.27
0.05
0.02
0.02
2.-74
-
0.02
2,83
0.03
0.06
0.87
r0.16
r.21
0.63
0.05
2.86
0.04
0.05
0.90
r0.34
0.92
0.74
0.r5
2,64
-
0.0r
0.07
0.03
0.93
9.61
r.60
0.19
-
0.03
0.02
0.02
0.95
r0.24
L.02
0.74
0.01
0.03
0.93
r0.r4
0.89
0.97
0.03
0.04
0.95
L0.26 0.57
-
P-8
2.84
1.r6
0.r1
0.07
P-6
2.92
r. 08
0.12
0.06
P-13
)
r
0.0r
0.02
v-r5
2.88
r.L2
0.15
0.07
0.0r
0.04
2.73
2.83
r.17
0.05
0.03
0.04
0.07
2.90
3.00
r.00
0.06
0.02
0.03
0.17
2.73
P-r8
Mr2+
I
10
J.M.
,r
Rorr".aux
et
al.
0.0r
-
0.0r
0.03
l.t6
(1973).
NMR spectrawere obtained at room temperature
with a Varian continuous-wavecrossed-coiledspectrometer working at different fixed frequencies(14,
54.6,and 60 MHz). In order to obtain signal-to-noise
ratios around 30 to 50, the spectrawere signal-averaged. The paramagnetic influence on the nuclear
spectra increaseswith frequency.Therefore, experiments run at high frequencyallowed us to study the
influenceof varying Fe2+contentson the H+ and Fspectra.Theseexperimentswere also usefulin identifying the various interactionsresponsiblefor the observed spectra. Experiments at low frequency (14
Fig. l. Projection on the ab plane of the octahedral sheet of I M
micas showing the position of OH , F and MI and M2 sites.
MHz) werg run at various anglesaround the b axis,
becauseof better resolution in this plane, in order to
provide information concerning the diamagneticinteractlons.
Results
Proton signal-a axis
H+ gives rise to a large symmetricalGaussian line
accompanied, when the iron content is above one
percent,by small side lines,whose intensity increases
with the iron concentration.
Figure 2 showsthe secondmoment of the principal
line obtained from the peak-to-peak value of the
experimentalfirst derivativespectra,at 60 MHz, as a
function of iron content. The line-width is essentially
dominated by the paramagnetic interaction which
gives rise to the expectedparabolic shape.By working at lower frequencies,which decreasesthe paramagnetic contributions to the width of the line, the
dipolar diamagneticpart of the interaction remains
more or lessconstant for all samples.
The position of the side-lineswith respectto the
central line varies as a function of the orientation of
the samplewith respectto the applied magneticfield'
It also varieslinearly with the applied field, as is to be
expected for averaged electronic moments obeying
Curie's law. An analysisof theseside-lineshas clearly
shown (Sanz and Stone, 1977) that they are due to
OH- groups, which are first-nearestneighbors (i.e.
2 . 7 A ) t o t h e F e 2 +i o n s .
SANZ AND STONE: NMR STUDY OF MICAS
At high frequency,for sampleshaving a high iron
and F- content, weak lines closeto the main line can
be observed (see Fig. 3); these correspond to OH
groups which are second neighbors (r : 4.07A) to
Fe'?+.The intensity of these lines is lessthan that of
the first-neighborlines, although the multiplicity for
neighbors is the same. This observation could be
explained by a regrouping of certain Fe2+ions, with
the result that most of the OH- groups in these
regions will have, in addition to a Fe2+ as second
neighbor, another Fe2+as first neighbor. This therefore reducesthe total contribution of secondneighbors to the intensity of their respectiveside-line.This
regrouping of Fe2+ions would also explain why the
width of the first-neighborside-lineis larger than that
of the main line. By going to lower frequency, this
width decreasesand approachesthat of the main line
( s e eF i g . 3 ) . I n t h e p r e v i o u sa n a l y s i so f t h e H + s i d e l i n e s ,i t w a s a l s o s h o w n t h a t t h e s u b s t i t u t i o no f M g z +
by Fe'+ is random on the two available sites(this is
not in conflict with the above assumeddistribution of
Fe2+with respectto each other) and that as the iron
content increases,the associationof OH- with more
Fez+ also increases.
Fluorine signal-a
axis
In the F spectraobtained at high frequency(56.4
MHz) the side-linesbecomevisible,as in the casefor
H+, only when the iron content is above one percent.
In this case,however, the side-linesare much closer
to the central line (Fig. 4), and their intensity is much
less dependent on the Fe2+ concentration. At low
frequency,the side-linesdisappear,being included in
t2l
H (60MHz)
P - 1 8 H . (1 4 M H z )
F i g . 3 . E x a m p l e o f H + N M R s i g n a l sf o r s a m p l e p - 1 8 s h o w i n g
t h e p r i n c i p a l l i n e a n d s i d e - l i n e sa s f u n c t i o n o f o r i e n t a t i o n ( a a x i s )
and frequency. II corresponds to second-neighbor protons The
u p p e r c u r v e s r e p r e s e n t h e f i r s t d e r i v a t i v eo f t h e a b s o r p t i o n l i n e s .
the main central line. Using the samehypothesisas in
the previous work (Sanz and Stone, 1977), i.e., a
simple time-averagedpoint dipolar interaction between magnetic moments of iron and nuclei, it is
possible to calculate the angular variation of shift
that an iron placedeither on the M I or M2 site would
induce in surrounding F- nuclei. For this calculation
only Fe2+ ions are considered,as chemical analysis
and Mcissbauer spectra (Sanz et al., 1978) have
9=30'
shown that theseare the only abundant paramagnetic
2
speciespresent.Moreover, for this analysisthe struco
f
tural model obtained by Joswig (1972) on a phlogo9=60'
(,o
pite by neutron diffraction was utilized.
The calculation for the three neighbors (see Fig.
o1
5a) to Fe'+ at respectively2.03,3.68, and 4.54A has
been carried out, and the resultsfor secondand third
neighborsshown in Figure 5b. Experimentally,in no
case have lines correspondingto first-nearestneighbors been seen.The only lines observedat 56.4MHz
o123"o
are those corresponding to nuclei which are either
second
or third neighborsto Fe2+.Unfortunately the
Fig. 2. Experimental
secondmomentsof ttreprincipat/tiri."",
resolution is not sufficient to be able to distinguish
at 60 MHz, asfunctionof the iron contentand for two orientations
"r
of the C' axis relativeto the appliedmagneticfield.
between the two possiblesites.These resultstend to
122
SANZ AND STONE:N M R S T U D Y O F M I C A S
P 18,F ( 56.4 MHz),a axrs
( Ho// C')
OM?
Y(b*)
x (a+;
n
F i g 5 a P r o j e c t i o n i n t h e a b p l a n e s h o w i n g F - p o s i t i o n s( O )
w h i c h a r e a t f i r s t - n e i g h b o r( - ) , s e c o n d - n e i g h b o(r. - . - ) a n d t h i r d neighbor (--) distances with respect Lo Ml and M2 sites.
the paramagnetic contribution for certain orientations. In samplesof high fluorine content (such as Pl8), the paramagneticcontribution to the line is less
than half as important for the F- line as it is for the
Gauss
F L U O R I N E, a a x i s , 5 6 4 M H z
z
./1
Fig. 4. Example of F NMR signals for sample P-18 for
r o t a t i o n s a r o u n d t h e c a x i s . A t h i g h f r e q u e n c y , s i d e - l i n e sa r e
v i s i b l e .A t l o w f r e q u e n c y ,t h e p r i n c i p a l l i n e c o n s i s t sf o r 4 : 9 6 o o 6
a doublet.
show that unlike OH-, F- ions are not directly
coordinated to the Fe2+ions.
Moreover, an analysisof the width of the central
l i n e s h o w st h a t t h e l i n e - w i d t ho f t h e F - l i n e i s m u c h
more angular-dependentthan in the caseof H+. As
the paramagneticcontribution is practicallyrotationindependent,the interaction which is responsiblefor
the width variation must be of another nature. i.e.. of
diamagneticorigin. This interaction, betweennuclear
spins only, is observedto be very much larger than
-----\
n2
Itr1
+Q
+Q
+$
F i g . 5 b . C a l c u l a t e da n g u l a r v a r i a t i o n o f s h i f t s f o r F w h i c h a r e
s e c o n d n e i g h b o r s t o M l ( l l , ) a n d M 2 ( I l , ) o r t h i r d n e i g h b o r st o
M l ( l l l t ) a n d M 2 ( l l l r ) . l : E x p e r i m e n t a lp o i n t s f o r P - 1 8
SANZ AND STONE: NMR STTJDY OF MICAS
H+ line. This again shows that most of the fluorine
ions are more distant from the iron than are the
protons.
Finally, when the iron content is sufficientlylow, or
when the frequency is lowered (Fig. 4), a doublet is
observedfor certain orientations.A doublet in NMR
spectroscopyis the clear indication of a diamagnetic
interaction betweena pair of spins l/2, which is the
casefor H+ and F-. This pair interaction can only be
seenif the interaction that eachmember of the nuclei
pair has with its surroundings(i.e. electronsor other
nuclei such as Al) is less important than the interaction it has with its partner in the pair. The spectra
obtained around the a axis are poorly resolved,however, and thereforean exact evaluationofthe relative
importance of the two possiblediamagneticpair interactions,H-F and F-F, as function of the F- content is difficult. However, this identification will be
possible by obtaining spectra while rotating the
samplearound its D axis,as shown in the next section.
In summary, the experimentsrun around the a axis
have helped to distinguish betweenthe various interactionsinvolved in the H+ and F- spectra.They have
also shown that, for protons, the paramagneticcontribution is the dominant term at both short (i.e. sidelines) and long (central line) distance. For F- the
interactionsare on the whole more diamagnetic.
Fluorine signal-b
t23
Gauss
(H ila)
F i g . 6 C a l c u l a t e da n g u l a rv a r i a t i o n o f t h e F - d o u b l e t s e p a r a t i o n
for F-Fo and F-Ho (rotations around the b axis). f : Experimental
points for P- l8 with two clearly distinguishable doublet
s e p a r a t i o n sf r o m 9 0 o t o 1 8 0 o .
axis
We will discuss here the results obtained at low
frequency (14 MHz) for rotations around the b axis,
which is normal to the zr plane containing the F and
H atoms. In this case,becauseof geometricalreasons,
the doublet separation associatedwith these two ions
is now larger. This, coupled with the fact that at low
frequency the paramagnetic influence on the linewidth is smaller, results in better-resolvedspectra.
Therefore, it will be much easier to distinguish between the various octahedral associationsand also
easier to identify the F- distribution within the octahedral sheet.
The doublet separation,(Pake, 1948), is given by
the expression
h : + ( 3 c o s , -d l ) G a u s s
,"
wherea is a constantequal to 3 for two identicalspins
I : I/2 and equal to 2 for two differentspins1 : l/2;
p is the magneticmoment of the studied spin; r is the
internuclear distance; d the angle between r and the
applied magnetic field. With the above expression,it
is possibleto calculatethe doublet separationin the
case of an interaction between two ions located
within thesameoctahedron,
F-Fo andF-Hn (Fig. I );
this corresponds
to the smallestvaluefor r, i.e.,2.64
and 3.45A respectively.The geometricalfactorsare
taken from the structuresproposedby Joswig(1972)
for phlogopitesand McCauley et al. (1973) for a
fluor-phlogopite.
The resultsof the calculationaregivenin Figure6
by the broken lines for F-Fo and F-Ho respectively.
It can be seenthat around 130" the two doublet
separationsare quite different.This orientation is,
therefore,very convenientfor a quantitativeevaluation of the number of associations
of both types.
Figure6 alsoshowsthe experimentalresultsobtained
for sampleP-18.For the 90o to l80o region,two
doubletscan be distinguished
clearlywith the inner
doubletcorresponding
to F-Ha and the outer doublet to F-Fo (also seeFig. 7); herethe agreementis
quite good. Becauseof smallerdoublet separation
and non-negligible
line-widths,from 0" to 90", the
experimentalpoints only reflectan averagesituation.
From the relativelygood agreement,it can be concluded that (l) the accuracywith which we could
orient our samplesis around 5'; (2) the distance
124
SANZ AND STONE
NMR STUDY OF MICAS
F ( 14 MHz ), axe b, p =13O'
r-zl
r - tz
Fig. 7. F
s p e c t r af o r d i f f e r e n t p h l o g o p i t e sw i t h i n c r e a s i n gc o n t e n t i n F : P - 2 1 < P - 1 2 < P - 1 5 < P - 1 8
2.64A taken for F-Fo from a fluor-phlogopiteis quite
adequate in our calculation; and (3) complicated
polytypism is absent. Concerning this last point, in
two cases where frequent stacking faults had been
detectedby X-ray diffraction, the NMR doublet separation was badly resolvedand, becauseof the various possibleorientationsfor the F-Fo vector, did not
follow the expectedangular variation; thesesamples
were eliminated from this study.
Eight out of l0 sampleswere therefore examined
a r o u n d t h e a n g l eo f 1 3 0 o ,i n o r d e r t o s t u d y t h e d i s t r i bution of F betweenthe two types of association:FF o a n d F - H o . W e o b s e r v e d( F i g . 7 ) t h a t t h e i n t e n s i t y
of the .central doublet (F-Ho) decreasesrelative to
the outer doublet (F-Fo) as the fluorine content increases.After decomposing our spectra and taking
the area under the F-Fo doublet as a measureof the
number oi F-F pairs, it is possible to compare this
figure with the one obtained by taking the mineralogical formula and calculating the probability of
finding F-F pairs. This is done in Figure 8, where the
experimental and calculated numbers of F-F pairs
are given as a function of fluorine content. It can be
seen that the number of pairs found experimentally
increasesmuch faster at low concentration than is
expected from a statistical calculation (40 percent
compared to 15 percent for 0.25 F per site). The
number of pairs then levelsoff and finally tends towards the estimatedvalues.
Discussion: octahedral ionic-distribution model
4
rL
f
1
o02o.40.604
F / srte
F i g . 8 . F - F p e r c e n t a g ef o u n d f o r t h e s t u d i e d s a m p l e s ( O ) a s
function of the F content. The broken line curve (--) represents
the statistically-calculated values.
In this work, the identificationof the various magnetic interactions responsiblefor the NMR proton
and fluorine spectra has allowed us to differentiate
among various octahedralassociations.The distribution of elementswithin theseassociationswas determined by evaluation of the relative importance of
each component of the spectral lines. The ability to
optimize angular parametersthrough the use of crystalline sampleswas important in obtaining this information. Certain important points emergewhich will
be now discussed.
SANZ AND STONE: NMR STUDY OF MICAS
First, the observation of a rapid increase in the
number of fluorine pairs in the domain of very low
concentration in F- can be interpreted as evidence
for the existenceand prior growth of relatively large
fluorine-rich domains.The observationthat as the Fcontent increasesthe number of pairs levels off implies that the various domains gradually join together. Since fluorine is only a second-or third-nearest neighbor to Fe2+,thesetwo ions are not directly
coordinated to each other, in contrast with the hydroxyl groups, which are found to be locatedcloseto
the Fe2+ ions (nearest neighbor). The fluorine domains, therefore, consist of homogeneous regions
containing only Mg2+ ions, in which the F-F distance
is equal to what is found in a pure fluor-phlogopite.
The presence of F- second-neighborside-linesof
non-negligibleintensities,however, suggeststhat the
Fd+ ions are locatedon the fringe of the F- domains.
The importance of the latter could in principle be
estimatedby the relativearea of the F-OH doublet to
the F-F doublet, provided of coursethe geometryof
the domains were known.
The distortion ofthe octahedralsite and the nature
and relative distanceof surrounding ligands are factors which influencethe stabilizationenergyof a Fe2+
ion on its site. In connection with our work, it is
perhaps relevant to note that, on the basis of the
spectrochemicalseries(Burns, 1970) and for a given
metal and stereochemistry,the ligand field increases
when going from F- to OH-. This means that OHgroups form stronger covalent bonds with Fe2+,and
consequentlythat the crystal field energy is higher
than it would be for Fe2+sitessurrounded by F- ions
only. Note also that in minerals of the humite series,
the number of Fe2+-substitutedoctahedradecreases
as the number of F- ligands increases(Ribbe and
Gibbs,l97l).
Sanz and Stone (1977) showed by NMR that the
Fe2+ ions are distributed randomly on the two possible sites. However, the way the Fd+ ions are distributed relative to each other is also important. The
H+ spectraof the presentstudy show that when the
fluorine content is high (P-18), the Fe2+ions tend to
group together. Moreover, when the iron content is
high, such as in sample P-21 and particularly in biotites (Sanz and Stone, 1977),side-linesdue to OHgroups having several Fe2+ions as first neighbors are
observed. This observation provides an interesting
means of investigatingthe iron distribution and also
the influence of the F- content on this distribution.
Work in this field on a larger number of sampleswith
a wider range ofiron concentrationis being pursued.
It seemsinterestingto compareour approachwith
X-ray results.X-ray diffraction has clearly shown
at the unit-cellscale,of
(Bailey,1975)the existence,
minercationorderingin the caseof lM mica-related
als, in which cationsof differentsizesare locatedon
octahedralsitesof different symmetry.For phlogopites such as those studied here, X-ray diffraction
doesnot showsuchordering;a randomdistribution
is thereforeexpectedfor sampleshaving an octahecomposidral sheetof variableand heterogeneous
showsusthatOH- and F- ions
tion. NMR. however.
are highly differentiatedwith respectto cationicassofluorinedomainsare
ciationsand that homogeneous
preferred.The chemical homogeneityof these Fdomainsis preservedeven when the iron content
increasesdrastically (e.g. in biotites; Sanz, 1976).
This is not true for the OH- groups;their environwith the Fe2+content,and
ment variesconsiderably
associations
of one, two, or three Fe2+around the
sameOH- can be detected(Sanzand Stone,1977).
around
of cationicassociations
This heterogeneity
OH- is clearlyvisibleby IR (Vedder,1964;Rousseauxet al., 1972),which also showsthe association
of OH- groupswith highlychargedions(Al8+,Tin+,
sheet,this localredistribution
etc.).ln the octahedral
of cationsaround the anions does not necessarily
follow a regularperiodicpattern and thereforecannot be detectedby X-ray methods.An interesting
case investigatedby X-ray which is analogousto
phlogopiteis the recentstudy of zinnwaldite(Guggenheimand Bailey,1977).They found that on one
of the threesitesof the unit cell, an ordereddistribuAlexists.This distributionis sometion of octahedral
what favoredby the fact that in this mineralthereis a
quasi-stoichiometric
compositionof Al in the octahedra.It would be interestingto seeif the tendency
domainssuch as that
for F- to form homogeneous
shownherealsoexistsin thiscase,i.e.aroundcations
other than Mg. Only NMR, which directlydetects
the fluorinepairing,could providean unambiguous
answer.The two methodswould thereforebe complementary.
of fluorFinally,the mutualexclusionrelationship
ine and iron is an important parameterin the vermiculitizationof micas,as the role playedby thesetwo
ions is known to be quite different.The degreeof
order-disorderin the distribution of thesetwo octahedralelementscould considerablymodify the cohesionbetweenlayers,and thereforethe openingof
the latter during the exchangeprocess.This factor
addedto othersasrecentlyreviewedby Giese(1977),
i.e. structural distortions,polytypism,number of
t26
SANZ AND STONE: NMR STIJDY OF MICAS
vacant sites, e/c. shows that the alteration process
implies more than a simple point-like electrostatic
interaction betweenK+ and sheet.For a clear understanding of this process,the octahedralsheetshould
be consideredas a whole.
Pake, G. E. (1948) Nuclear resonance absorption in hydrated
crystals: fine structure of the proton line. J. Chem. Phys., 16,
327-336.
R i b b e , P . H . a n d G . V . G i b b s ( 1 9 7 1 ) C r y s t a l s t r u c t u r e so f t h e
h u m i t e m i n e r a l s :I I I . M g , / F e o r d e r i n g i n h u m i t e a n d i t s r e l a t i o n
t o o t h e r f e r r o m a g n e s i a ns i l i c a t e sA
. m . M i n e r a l . ,J 6 , I 1 5 5 - l 1 7 3 .
R o u s s e a u x J, . M . , C . G o m e z - L a v e r d e ,Y . N a t h a n a n d P . G . R o u x het (1972) Correlation between the hydroxyl stretching bands
Acknowledgments
and
the chemical composition of trioctahedral micas. ProWethankProfessor
(Universit6
H, Pezerat
Pierreet MarieCuceedings of the International Clay Conference, Madrid, Spain,
rie, Paris) and coworkers for their help in determining the poly89-98.
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